rustc_hir_typeck/method/

probe.rs

use std::assert_matches::debug_assert_matches;
use std::cell::{Cell, RefCell};
use std::cmp::max;
use std::ops::Deref;

use rustc_attr_parsing::is_doc_alias_attrs_contain_symbol;
use rustc_data_structures::fx::FxHashSet;
use rustc_data_structures::sso::SsoHashSet;
use rustc_errors::Applicability;
use rustc_hir as hir;
use rustc_hir::HirId;
use rustc_hir::def::DefKind;
use rustc_hir_analysis::autoderef::{self, Autoderef};
use rustc_infer::infer::canonical::{Canonical, OriginalQueryValues, QueryResponse};
use rustc_infer::infer::{BoundRegionConversionTime, DefineOpaqueTypes, InferOk, TyCtxtInferExt};
use rustc_infer::traits::ObligationCauseCode;
use rustc_middle::middle::stability;
use rustc_middle::ty::elaborate::supertrait_def_ids;
use rustc_middle::ty::fast_reject::{DeepRejectCtxt, TreatParams, simplify_type};
use rustc_middle::ty::{
    self, AssocItem, AssocItemContainer, GenericArgs, GenericArgsRef, GenericParamDefKind,
    ParamEnvAnd, Ty, TyCtxt, TypeVisitableExt, Upcast,
};
use rustc_middle::{bug, span_bug};
use rustc_session::lint;
use rustc_span::def_id::{DefId, LocalDefId};
use rustc_span::edit_distance::{
    edit_distance_with_substrings, find_best_match_for_name_with_substrings,
};
use rustc_span::{DUMMY_SP, Ident, Span, Symbol, sym};
use rustc_trait_selection::error_reporting::infer::need_type_info::TypeAnnotationNeeded;
use rustc_trait_selection::infer::InferCtxtExt as _;
use rustc_trait_selection::traits::query::CanonicalTyGoal;
use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
use rustc_trait_selection::traits::query::method_autoderef::{
    CandidateStep, MethodAutoderefBadTy, MethodAutoderefStepsResult,
};
use rustc_trait_selection::traits::{self, ObligationCause, ObligationCtxt};
use smallvec::{SmallVec, smallvec};
use tracing::{debug, instrument};

use self::CandidateKind::*;
pub(crate) use self::PickKind::*;
use super::{CandidateSource, MethodError, NoMatchData, suggest};
use crate::FnCtxt;

/// Boolean flag used to indicate if this search is for a suggestion
/// or not. If true, we can allow ambiguity and so forth.
#[derive(Clone, Copy, Debug)]
pub(crate) struct IsSuggestion(pub bool);

pub(crate) struct ProbeContext<'a, 'tcx> {
    fcx: &'a FnCtxt<'a, 'tcx>,
    span: Span,
    mode: Mode,
    method_name: Option<Ident>,
    return_type: Option<Ty<'tcx>>,

    /// This is the OriginalQueryValues for the steps queries
    /// that are answered in steps.
    orig_steps_var_values: &'a OriginalQueryValues<'tcx>,
    steps: &'tcx [CandidateStep<'tcx>],

    inherent_candidates: Vec<Candidate<'tcx>>,
    extension_candidates: Vec<Candidate<'tcx>>,
    impl_dups: FxHashSet<DefId>,

    /// When probing for names, include names that are close to the
    /// requested name (by edit distance)
    allow_similar_names: bool,

    /// List of potential private candidates. Will be trimmed to ones that
    /// actually apply and then the result inserted into `private_candidate`
    private_candidates: Vec<Candidate<'tcx>>,

    /// Some(candidate) if there is a private candidate
    private_candidate: Cell<Option<(DefKind, DefId)>>,

    /// Collects near misses when the candidate functions are missing a `self` keyword and is only
    /// used for error reporting
    static_candidates: RefCell<Vec<CandidateSource>>,

    scope_expr_id: HirId,

    /// Is this probe being done for a diagnostic? This will skip some error reporting
    /// machinery, since we don't particularly care about, for example, similarly named
    /// candidates if we're *reporting* similarly named candidates.
    is_suggestion: IsSuggestion,
}

impl<'a, 'tcx> Deref for ProbeContext<'a, 'tcx> {
    type Target = FnCtxt<'a, 'tcx>;
    fn deref(&self) -> &Self::Target {
        self.fcx
    }
}

#[derive(Debug, Clone)]
pub(crate) struct Candidate<'tcx> {
    pub(crate) item: ty::AssocItem,
    pub(crate) kind: CandidateKind<'tcx>,
    pub(crate) import_ids: SmallVec<[LocalDefId; 1]>,
}

#[derive(Debug, Clone)]
pub(crate) enum CandidateKind<'tcx> {
    InherentImplCandidate { impl_def_id: DefId, receiver_steps: usize },
    ObjectCandidate(ty::PolyTraitRef<'tcx>),
    TraitCandidate(ty::PolyTraitRef<'tcx>),
    WhereClauseCandidate(ty::PolyTraitRef<'tcx>),
}

#[derive(Debug, PartialEq, Eq, Copy, Clone)]
enum ProbeResult {
    NoMatch,
    BadReturnType,
    Match,
}

/// When adjusting a receiver we often want to do one of
///
/// - Add a `&` (or `&mut`), converting the receiver from `T` to `&T` (or `&mut T`)
/// - If the receiver has type `*mut T`, convert it to `*const T`
///
/// This type tells us which one to do.
///
/// Note that in principle we could do both at the same time. For example, when the receiver has
/// type `T`, we could autoref it to `&T`, then convert to `*const T`. Or, when it has type `*mut
/// T`, we could convert it to `*const T`, then autoref to `&*const T`. However, currently we do
/// (at most) one of these. Either the receiver has type `T` and we convert it to `&T` (or with
/// `mut`), or it has type `*mut T` and we convert it to `*const T`.
#[derive(Debug, PartialEq, Copy, Clone)]
pub(crate) enum AutorefOrPtrAdjustment {
    /// Receiver has type `T`, add `&` or `&mut` (if `T` is `mut`), and maybe also "unsize" it.
    /// Unsizing is used to convert a `[T; N]` to `[T]`, which only makes sense when autorefing.
    Autoref {
        mutbl: hir::Mutability,

        /// Indicates that the source expression should be "unsized" to a target type.
        /// This is special-cased for just arrays unsizing to slices.
        unsize: bool,
    },
    /// Receiver has type `*mut T`, convert to `*const T`
    ToConstPtr,

    /// Reborrow a `Pin<&mut T>` or `Pin<&T>`.
    ReborrowPin(hir::Mutability),
}

impl AutorefOrPtrAdjustment {
    fn get_unsize(&self) -> bool {
        match self {
            AutorefOrPtrAdjustment::Autoref { mutbl: _, unsize } => *unsize,
            AutorefOrPtrAdjustment::ToConstPtr => false,
            AutorefOrPtrAdjustment::ReborrowPin(_) => false,
        }
    }
}

/// Extra information required only for error reporting.
#[derive(Debug)]
struct PickDiagHints<'a, 'tcx> {
    /// Unstable candidates alongside the stable ones.
    unstable_candidates: Option<Vec<(Candidate<'tcx>, Symbol)>>,

    /// Collects near misses when trait bounds for type parameters are unsatisfied and is only used
    /// for error reporting
    unsatisfied_predicates: &'a mut Vec<(
        ty::Predicate<'tcx>,
        Option<ty::Predicate<'tcx>>,
        Option<ObligationCause<'tcx>>,
    )>,
}

/// Criteria to apply when searching for a given Pick. This is used during
/// the search for potentially shadowed methods to ensure we don't search
/// more candidates than strictly necessary.
#[derive(Debug)]
struct PickConstraintsForShadowed {
    autoderefs: usize,
    receiver_steps: Option<usize>,
    def_id: DefId,
}

impl PickConstraintsForShadowed {
    fn may_shadow_based_on_autoderefs(&self, autoderefs: usize) -> bool {
        autoderefs == self.autoderefs
    }

    fn candidate_may_shadow(&self, candidate: &Candidate<'_>) -> bool {
        // An item never shadows itself
        candidate.item.def_id != self.def_id
            // and we're only concerned about inherent impls doing the shadowing.
            // Shadowing can only occur if the shadowed is further along
            // the Receiver dereferencing chain than the shadowed.
            && match candidate.kind {
                CandidateKind::InherentImplCandidate { receiver_steps, .. } => match self.receiver_steps {
                    Some(shadowed_receiver_steps) => receiver_steps > shadowed_receiver_steps,
                    _ => false
                },
                _ => false
            }
    }
}

#[derive(Debug, Clone)]
pub(crate) struct Pick<'tcx> {
    pub item: ty::AssocItem,
    pub kind: PickKind<'tcx>,
    pub import_ids: SmallVec<[LocalDefId; 1]>,

    /// Indicates that the source expression should be autoderef'd N times
    /// ```ignore (not-rust)
    /// A = expr | *expr | **expr | ...
    /// ```
    pub autoderefs: usize,

    /// Indicates that we want to add an autoref (and maybe also unsize it), or if the receiver is
    /// `*mut T`, convert it to `*const T`.
    pub autoref_or_ptr_adjustment: Option<AutorefOrPtrAdjustment>,
    pub self_ty: Ty<'tcx>,

    /// Unstable candidates alongside the stable ones.
    unstable_candidates: Vec<(Candidate<'tcx>, Symbol)>,

    /// Number of jumps along the `Receiver::Target` chain we followed
    /// to identify this method. Used only for deshadowing errors.
    /// Only applies for inherent impls.
    pub receiver_steps: Option<usize>,

    /// Candidates that were shadowed by supertraits.
    pub shadowed_candidates: Vec<ty::AssocItem>,
}

#[derive(Clone, Debug, PartialEq, Eq)]
pub(crate) enum PickKind<'tcx> {
    InherentImplPick,
    ObjectPick,
    TraitPick,
    WhereClausePick(
        // Trait
        ty::PolyTraitRef<'tcx>,
    ),
}

pub(crate) type PickResult<'tcx> = Result<Pick<'tcx>, MethodError<'tcx>>;

#[derive(PartialEq, Eq, Copy, Clone, Debug)]
pub(crate) enum Mode {
    // An expression of the form `receiver.method_name(...)`.
    // Autoderefs are performed on `receiver`, lookup is done based on the
    // `self` argument of the method, and static methods aren't considered.
    MethodCall,
    // An expression of the form `Type::item` or `<T>::item`.
    // No autoderefs are performed, lookup is done based on the type each
    // implementation is for, and static methods are included.
    Path,
}

#[derive(PartialEq, Eq, Copy, Clone, Debug)]
pub(crate) enum ProbeScope {
    // Single candidate coming from pre-resolved delegation method.
    Single(DefId),

    // Assemble candidates coming only from traits in scope.
    TraitsInScope,

    // Assemble candidates coming from all traits.
    AllTraits,
}

impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
    /// This is used to offer suggestions to users. It returns methods
    /// that could have been called which have the desired return
    /// type. Some effort is made to rule out methods that, if called,
    /// would result in an error (basically, the same criteria we
    /// would use to decide if a method is a plausible fit for
    /// ambiguity purposes).
    #[instrument(level = "debug", skip(self, candidate_filter))]
    pub(crate) fn probe_for_return_type_for_diagnostic(
        &self,
        span: Span,
        mode: Mode,
        return_type: Ty<'tcx>,
        self_ty: Ty<'tcx>,
        scope_expr_id: HirId,
        candidate_filter: impl Fn(&ty::AssocItem) -> bool,
    ) -> Vec<ty::AssocItem> {
        let method_names = self
            .probe_op(
                span,
                mode,
                None,
                Some(return_type),
                IsSuggestion(true),
                self_ty,
                scope_expr_id,
                ProbeScope::AllTraits,
                |probe_cx| Ok(probe_cx.candidate_method_names(candidate_filter)),
            )
            .unwrap_or_default();
        method_names
            .iter()
            .flat_map(|&method_name| {
                self.probe_op(
                    span,
                    mode,
                    Some(method_name),
                    Some(return_type),
                    IsSuggestion(true),
                    self_ty,
                    scope_expr_id,
                    ProbeScope::AllTraits,
                    |probe_cx| probe_cx.pick(),
                )
                .ok()
                .map(|pick| pick.item)
            })
            .collect()
    }

    #[instrument(level = "debug", skip(self))]
    pub(crate) fn probe_for_name(
        &self,
        mode: Mode,
        item_name: Ident,
        return_type: Option<Ty<'tcx>>,
        is_suggestion: IsSuggestion,
        self_ty: Ty<'tcx>,
        scope_expr_id: HirId,
        scope: ProbeScope,
    ) -> PickResult<'tcx> {
        self.probe_op(
            item_name.span,
            mode,
            Some(item_name),
            return_type,
            is_suggestion,
            self_ty,
            scope_expr_id,
            scope,
            |probe_cx| probe_cx.pick(),
        )
    }

    #[instrument(level = "debug", skip(self))]
    pub(crate) fn probe_for_name_many(
        &self,
        mode: Mode,
        item_name: Ident,
        return_type: Option<Ty<'tcx>>,
        is_suggestion: IsSuggestion,
        self_ty: Ty<'tcx>,
        scope_expr_id: HirId,
        scope: ProbeScope,
    ) -> Result<Vec<Candidate<'tcx>>, MethodError<'tcx>> {
        self.probe_op(
            item_name.span,
            mode,
            Some(item_name),
            return_type,
            is_suggestion,
            self_ty,
            scope_expr_id,
            scope,
            |probe_cx| {
                Ok(probe_cx
                    .inherent_candidates
                    .into_iter()
                    .chain(probe_cx.extension_candidates)
                    .collect())
            },
        )
    }

    pub(crate) fn probe_op<OP, R>(
        &'a self,
        span: Span,
        mode: Mode,
        method_name: Option<Ident>,
        return_type: Option<Ty<'tcx>>,
        is_suggestion: IsSuggestion,
        self_ty: Ty<'tcx>,
        scope_expr_id: HirId,
        scope: ProbeScope,
        op: OP,
    ) -> Result<R, MethodError<'tcx>>
    where
        OP: FnOnce(ProbeContext<'_, 'tcx>) -> Result<R, MethodError<'tcx>>,
    {
        let mut orig_values = OriginalQueryValues::default();
        let query_input = self.canonicalize_query(
            ParamEnvAnd { param_env: self.param_env, value: self_ty },
            &mut orig_values,
        );

        let steps = match mode {
            Mode::MethodCall => self.tcx.method_autoderef_steps(query_input),
            Mode::Path => self.probe(|_| {
                // Mode::Path - the deref steps is "trivial". This turns
                // our CanonicalQuery into a "trivial" QueryResponse. This
                // is a bit inefficient, but I don't think that writing
                // special handling for this "trivial case" is a good idea.

                let infcx = &self.infcx;
                let (ParamEnvAnd { param_env: _, value: self_ty }, canonical_inference_vars) =
                    infcx.instantiate_canonical(span, &query_input.canonical);
                debug!(?self_ty, ?query_input, "probe_op: Mode::Path");
                MethodAutoderefStepsResult {
                    steps: infcx.tcx.arena.alloc_from_iter([CandidateStep {
                        self_ty: self.make_query_response_ignoring_pending_obligations(
                            canonical_inference_vars,
                            self_ty,
                        ),
                        autoderefs: 0,
                        from_unsafe_deref: false,
                        unsize: false,
                        reachable_via_deref: true,
                    }]),
                    opt_bad_ty: None,
                    reached_recursion_limit: false,
                }
            }),
        };

        // If our autoderef loop had reached the recursion limit,
        // report an overflow error, but continue going on with
        // the truncated autoderef list.
        if steps.reached_recursion_limit && !is_suggestion.0 {
            self.probe(|_| {
                let ty = &steps
                    .steps
                    .last()
                    .unwrap_or_else(|| span_bug!(span, "reached the recursion limit in 0 steps?"))
                    .self_ty;
                let ty = self
                    .probe_instantiate_query_response(span, &orig_values, ty)
                    .unwrap_or_else(|_| span_bug!(span, "instantiating {:?} failed?", ty));
                autoderef::report_autoderef_recursion_limit_error(self.tcx, span, ty.value);
            });
        }

        // If we encountered an `_` type or an error type during autoderef, this is
        // ambiguous.
        if let Some(bad_ty) = &steps.opt_bad_ty {
            if is_suggestion.0 {
                // Ambiguity was encountered during a suggestion. There's really
                // not much use in suggesting methods in this case.
                return Err(MethodError::NoMatch(NoMatchData {
                    static_candidates: Vec::new(),
                    unsatisfied_predicates: Vec::new(),
                    out_of_scope_traits: Vec::new(),
                    similar_candidate: None,
                    mode,
                }));
            } else if bad_ty.reached_raw_pointer
                && !self.tcx.features().arbitrary_self_types_pointers()
                && !self.tcx.sess.at_least_rust_2018()
            {
                // this case used to be allowed by the compiler,
                // so we do a future-compat lint here for the 2015 edition
                // (see https://github.com/rust-lang/rust/issues/46906)
                self.tcx.node_span_lint(
                    lint::builtin::TYVAR_BEHIND_RAW_POINTER,
                    scope_expr_id,
                    span,
                    |lint| {
                        lint.primary_message("type annotations needed");
                    },
                );
            } else {
                // Ended up encountering a type variable when doing autoderef,
                // but it may not be a type variable after processing obligations
                // in our local `FnCtxt`, so don't call `structurally_resolve_type`.
                let ty = &bad_ty.ty;
                let ty = self
                    .probe_instantiate_query_response(span, &orig_values, ty)
                    .unwrap_or_else(|_| span_bug!(span, "instantiating {:?} failed?", ty));
                let ty = self.resolve_vars_if_possible(ty.value);
                let guar = match *ty.kind() {
                    ty::Infer(ty::TyVar(_)) => {
                        let raw_ptr_call = bad_ty.reached_raw_pointer
                            && !self.tcx.features().arbitrary_self_types();
                        let mut err = self.err_ctxt().emit_inference_failure_err(
                            self.body_id,
                            span,
                            ty.into(),
                            TypeAnnotationNeeded::E0282,
                            !raw_ptr_call,
                        );
                        if raw_ptr_call {
                            err.span_label(span, "cannot call a method on a raw pointer with an unknown pointee type");
                        }
                        err.emit()
                    }
                    ty::Error(guar) => guar,
                    _ => bug!("unexpected bad final type in method autoderef"),
                };
                self.demand_eqtype(span, ty, Ty::new_error(self.tcx, guar));
                return Err(MethodError::ErrorReported(guar));
            }
        }

        debug!("ProbeContext: steps for self_ty={:?} are {:?}", self_ty, steps);

        // this creates one big transaction so that all type variables etc
        // that we create during the probe process are removed later
        self.probe(|_| {
            let mut probe_cx = ProbeContext::new(
                self,
                span,
                mode,
                method_name,
                return_type,
                &orig_values,
                steps.steps,
                scope_expr_id,
                is_suggestion,
            );

            match scope {
                ProbeScope::TraitsInScope => {
                    probe_cx.assemble_inherent_candidates();
                    probe_cx.assemble_extension_candidates_for_traits_in_scope();
                }
                ProbeScope::AllTraits => {
                    probe_cx.assemble_inherent_candidates();
                    probe_cx.assemble_extension_candidates_for_all_traits();
                }
                ProbeScope::Single(def_id) => {
                    let item = self.tcx.associated_item(def_id);
                    // FIXME(fn_delegation): Delegation to inherent methods is not yet supported.
                    assert_eq!(item.container, AssocItemContainer::Trait);

                    let trait_def_id = self.tcx.parent(def_id);
                    let trait_span = self.tcx.def_span(trait_def_id);

                    let trait_args = self.fresh_args_for_item(trait_span, trait_def_id);
                    let trait_ref = ty::TraitRef::new_from_args(self.tcx, trait_def_id, trait_args);

                    probe_cx.push_candidate(
                        Candidate {
                            item,
                            kind: CandidateKind::TraitCandidate(ty::Binder::dummy(trait_ref)),
                            import_ids: smallvec![],
                        },
                        false,
                    );
                }
            };
            op(probe_cx)
        })
    }
}

pub(crate) fn method_autoderef_steps<'tcx>(
    tcx: TyCtxt<'tcx>,
    goal: CanonicalTyGoal<'tcx>,
) -> MethodAutoderefStepsResult<'tcx> {
    debug!("method_autoderef_steps({:?})", goal);

    let (ref infcx, goal, inference_vars) = tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &goal);
    let ParamEnvAnd { param_env, value: self_ty } = goal;

    // If arbitrary self types is not enabled, we follow the chain of
    // `Deref<Target=T>`. If arbitrary self types is enabled, we instead
    // follow the chain of `Receiver<Target=T>`, but we also record whether
    // such types are reachable by following the (potentially shorter)
    // chain of `Deref<Target=T>`. We will use the first list when finding
    // potentially relevant function implementations (e.g. relevant impl blocks)
    // but the second list when determining types that the receiver may be
    // converted to, in order to find out which of those methods might actually
    // be callable.
    let mut autoderef_via_deref =
        Autoderef::new(infcx, param_env, hir::def_id::CRATE_DEF_ID, DUMMY_SP, self_ty)
            .include_raw_pointers()
            .silence_errors();

    let mut reached_raw_pointer = false;
    let arbitrary_self_types_enabled =
        tcx.features().arbitrary_self_types() || tcx.features().arbitrary_self_types_pointers();
    let (mut steps, reached_recursion_limit): (Vec<_>, bool) = if arbitrary_self_types_enabled {
        let reachable_via_deref =
            autoderef_via_deref.by_ref().map(|_| true).chain(std::iter::repeat(false));

        let mut autoderef_via_receiver =
            Autoderef::new(infcx, param_env, hir::def_id::CRATE_DEF_ID, DUMMY_SP, self_ty)
                .include_raw_pointers()
                .use_receiver_trait()
                .silence_errors();
        let steps = autoderef_via_receiver
            .by_ref()
            .zip(reachable_via_deref)
            .map(|((ty, d), reachable_via_deref)| {
                let step = CandidateStep {
                    self_ty: infcx
                        .make_query_response_ignoring_pending_obligations(inference_vars, ty),
                    autoderefs: d,
                    from_unsafe_deref: reached_raw_pointer,
                    unsize: false,
                    reachable_via_deref,
                };
                if ty.is_raw_ptr() {
                    // all the subsequent steps will be from_unsafe_deref
                    reached_raw_pointer = true;
                }
                step
            })
            .collect();
        (steps, autoderef_via_receiver.reached_recursion_limit())
    } else {
        let steps = autoderef_via_deref
            .by_ref()
            .map(|(ty, d)| {
                let step = CandidateStep {
                    self_ty: infcx
                        .make_query_response_ignoring_pending_obligations(inference_vars, ty),
                    autoderefs: d,
                    from_unsafe_deref: reached_raw_pointer,
                    unsize: false,
                    reachable_via_deref: true,
                };
                if ty.is_raw_ptr() {
                    // all the subsequent steps will be from_unsafe_deref
                    reached_raw_pointer = true;
                }
                step
            })
            .collect();
        (steps, autoderef_via_deref.reached_recursion_limit())
    };
    let final_ty = autoderef_via_deref.final_ty(true);
    let opt_bad_ty = match final_ty.kind() {
        ty::Infer(ty::TyVar(_)) | ty::Error(_) => Some(MethodAutoderefBadTy {
            reached_raw_pointer,
            ty: infcx.make_query_response_ignoring_pending_obligations(inference_vars, final_ty),
        }),
        ty::Array(elem_ty, _) => {
            let autoderefs = steps.iter().filter(|s| s.reachable_via_deref).count() - 1;
            steps.push(CandidateStep {
                self_ty: infcx.make_query_response_ignoring_pending_obligations(
                    inference_vars,
                    Ty::new_slice(infcx.tcx, *elem_ty),
                ),
                autoderefs,
                // this could be from an unsafe deref if we had
                // a *mut/const [T; N]
                from_unsafe_deref: reached_raw_pointer,
                unsize: true,
                reachable_via_deref: true, // this is always the final type from
                                           // autoderef_via_deref
            });

            None
        }
        _ => None,
    };

    debug!("method_autoderef_steps: steps={:?} opt_bad_ty={:?}", steps, opt_bad_ty);

    MethodAutoderefStepsResult {
        steps: tcx.arena.alloc_from_iter(steps),
        opt_bad_ty: opt_bad_ty.map(|ty| &*tcx.arena.alloc(ty)),
        reached_recursion_limit,
    }
}

impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
    fn new(
        fcx: &'a FnCtxt<'a, 'tcx>,
        span: Span,
        mode: Mode,
        method_name: Option<Ident>,
        return_type: Option<Ty<'tcx>>,
        orig_steps_var_values: &'a OriginalQueryValues<'tcx>,
        steps: &'tcx [CandidateStep<'tcx>],
        scope_expr_id: HirId,
        is_suggestion: IsSuggestion,
    ) -> ProbeContext<'a, 'tcx> {
        ProbeContext {
            fcx,
            span,
            mode,
            method_name,
            return_type,
            inherent_candidates: Vec::new(),
            extension_candidates: Vec::new(),
            impl_dups: FxHashSet::default(),
            orig_steps_var_values,
            steps,
            allow_similar_names: false,
            private_candidates: Vec::new(),
            private_candidate: Cell::new(None),
            static_candidates: RefCell::new(Vec::new()),
            scope_expr_id,
            is_suggestion,
        }
    }

    fn reset(&mut self) {
        self.inherent_candidates.clear();
        self.extension_candidates.clear();
        self.impl_dups.clear();
        self.private_candidates.clear();
        self.private_candidate.set(None);
        self.static_candidates.borrow_mut().clear();
    }

    /// When we're looking up a method by path (UFCS), we relate the receiver
    /// types invariantly. When we are looking up a method by the `.` operator,
    /// we relate them covariantly.
    fn variance(&self) -> ty::Variance {
        match self.mode {
            Mode::MethodCall => ty::Covariant,
            Mode::Path => ty::Invariant,
        }
    }

    ///////////////////////////////////////////////////////////////////////////
    // CANDIDATE ASSEMBLY

    fn push_candidate(&mut self, candidate: Candidate<'tcx>, is_inherent: bool) {
        let is_accessible = if let Some(name) = self.method_name {
            let item = candidate.item;
            let hir_id = self.tcx.local_def_id_to_hir_id(self.body_id);
            let def_scope =
                self.tcx.adjust_ident_and_get_scope(name, item.container_id(self.tcx), hir_id).1;
            item.visibility(self.tcx).is_accessible_from(def_scope, self.tcx)
        } else {
            true
        };
        if is_accessible {
            if is_inherent {
                self.inherent_candidates.push(candidate);
            } else {
                self.extension_candidates.push(candidate);
            }
        } else {
            self.private_candidates.push(candidate);
        }
    }

    fn assemble_inherent_candidates(&mut self) {
        for step in self.steps.iter() {
            self.assemble_probe(&step.self_ty, step.autoderefs);
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn assemble_probe(
        &mut self,
        self_ty: &Canonical<'tcx, QueryResponse<'tcx, Ty<'tcx>>>,
        receiver_steps: usize,
    ) {
        let raw_self_ty = self_ty.value.value;
        match *raw_self_ty.kind() {
            ty::Dynamic(data, ..) if let Some(p) = data.principal() => {
                // Subtle: we can't use `instantiate_query_response` here: using it will
                // commit to all of the type equalities assumed by inference going through
                // autoderef (see the `method-probe-no-guessing` test).
                //
                // However, in this code, it is OK if we end up with an object type that is
                // "more general" than the object type that we are evaluating. For *every*
                // object type `MY_OBJECT`, a function call that goes through a trait-ref
                // of the form `<MY_OBJECT as SuperTraitOf(MY_OBJECT)>::func` is a valid
                // `ObjectCandidate`, and it should be discoverable "exactly" through one
                // of the iterations in the autoderef loop, so there is no problem with it
                // being discoverable in another one of these iterations.
                //
                // Using `instantiate_canonical` on our
                // `Canonical<QueryResponse<Ty<'tcx>>>` and then *throwing away* the
                // `CanonicalVarValues` will exactly give us such a generalization - it
                // will still match the original object type, but it won't pollute our
                // type variables in any form, so just do that!
                let (QueryResponse { value: generalized_self_ty, .. }, _ignored_var_values) =
                    self.fcx.instantiate_canonical(self.span, self_ty);

                self.assemble_inherent_candidates_from_object(generalized_self_ty);
                self.assemble_inherent_impl_candidates_for_type(p.def_id(), receiver_steps);
                if self.tcx.has_attr(p.def_id(), sym::rustc_has_incoherent_inherent_impls) {
                    self.assemble_inherent_candidates_for_incoherent_ty(
                        raw_self_ty,
                        receiver_steps,
                    );
                }
            }
            ty::Adt(def, _) => {
                let def_id = def.did();
                self.assemble_inherent_impl_candidates_for_type(def_id, receiver_steps);
                if self.tcx.has_attr(def_id, sym::rustc_has_incoherent_inherent_impls) {
                    self.assemble_inherent_candidates_for_incoherent_ty(
                        raw_self_ty,
                        receiver_steps,
                    );
                }
            }
            ty::Foreign(did) => {
                self.assemble_inherent_impl_candidates_for_type(did, receiver_steps);
                if self.tcx.has_attr(did, sym::rustc_has_incoherent_inherent_impls) {
                    self.assemble_inherent_candidates_for_incoherent_ty(
                        raw_self_ty,
                        receiver_steps,
                    );
                }
            }
            ty::Param(_) => {
                self.assemble_inherent_candidates_from_param(raw_self_ty);
            }
            ty::Bool
            | ty::Char
            | ty::Int(_)
            | ty::Uint(_)
            | ty::Float(_)
            | ty::Str
            | ty::Array(..)
            | ty::Slice(_)
            | ty::RawPtr(_, _)
            | ty::Ref(..)
            | ty::Never
            | ty::Tuple(..) => {
                self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty, receiver_steps)
            }
            _ => {}
        }
    }

    fn assemble_inherent_candidates_for_incoherent_ty(
        &mut self,
        self_ty: Ty<'tcx>,
        receiver_steps: usize,
    ) {
        let Some(simp) = simplify_type(self.tcx, self_ty, TreatParams::InstantiateWithInfer) else {
            bug!("unexpected incoherent type: {:?}", self_ty)
        };
        for &impl_def_id in self.tcx.incoherent_impls(simp).into_iter() {
            self.assemble_inherent_impl_probe(impl_def_id, receiver_steps);
        }
    }

    fn assemble_inherent_impl_candidates_for_type(&mut self, def_id: DefId, receiver_steps: usize) {
        let impl_def_ids = self.tcx.at(self.span).inherent_impls(def_id).into_iter();
        for &impl_def_id in impl_def_ids {
            self.assemble_inherent_impl_probe(impl_def_id, receiver_steps);
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn assemble_inherent_impl_probe(&mut self, impl_def_id: DefId, receiver_steps: usize) {
        if !self.impl_dups.insert(impl_def_id) {
            return; // already visited
        }

        for item in self.impl_or_trait_item(impl_def_id) {
            if !self.has_applicable_self(&item) {
                // No receiver declared. Not a candidate.
                self.record_static_candidate(CandidateSource::Impl(impl_def_id));
                continue;
            }
            self.push_candidate(
                Candidate {
                    item,
                    kind: InherentImplCandidate { impl_def_id, receiver_steps },
                    import_ids: smallvec![],
                },
                true,
            );
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn assemble_inherent_candidates_from_object(&mut self, self_ty: Ty<'tcx>) {
        let principal = match self_ty.kind() {
            ty::Dynamic(data, ..) => Some(data),
            _ => None,
        }
        .and_then(|data| data.principal())
        .unwrap_or_else(|| {
            span_bug!(
                self.span,
                "non-object {:?} in assemble_inherent_candidates_from_object",
                self_ty
            )
        });

        // It is illegal to invoke a method on a trait instance that refers to
        // the `Self` type. An [`DynCompatibilityViolation::SupertraitSelf`] error
        // will be reported by `dyn_compatibility.rs` if the method refers to the
        // `Self` type anywhere other than the receiver. Here, we use a
        // instantiation that replaces `Self` with the object type itself. Hence,
        // a `&self` method will wind up with an argument type like `&dyn Trait`.
        let trait_ref = principal.with_self_ty(self.tcx, self_ty);
        self.assemble_candidates_for_bounds(
            traits::supertraits(self.tcx, trait_ref),
            |this, new_trait_ref, item| {
                this.push_candidate(
                    Candidate {
                        item,
                        kind: ObjectCandidate(new_trait_ref),
                        import_ids: smallvec![],
                    },
                    true,
                );
            },
        );
    }

    #[instrument(level = "debug", skip(self))]
    fn assemble_inherent_candidates_from_param(&mut self, param_ty: Ty<'tcx>) {
        debug_assert_matches!(param_ty.kind(), ty::Param(_));

        let tcx = self.tcx;
        let bounds = self.param_env.caller_bounds().iter().filter_map(|predicate| {
            let bound_predicate = predicate.kind();
            match bound_predicate.skip_binder() {
                ty::ClauseKind::Trait(trait_predicate) => DeepRejectCtxt::relate_rigid_rigid(tcx)
                    .types_may_unify(param_ty, trait_predicate.trait_ref.self_ty())
                    .then(|| bound_predicate.rebind(trait_predicate.trait_ref)),
                ty::ClauseKind::RegionOutlives(_)
                | ty::ClauseKind::TypeOutlives(_)
                | ty::ClauseKind::Projection(_)
                | ty::ClauseKind::ConstArgHasType(_, _)
                | ty::ClauseKind::WellFormed(_)
                | ty::ClauseKind::ConstEvaluatable(_)
                | ty::ClauseKind::UnstableFeature(_)
                | ty::ClauseKind::HostEffect(..) => None,
            }
        });

        self.assemble_candidates_for_bounds(bounds, |this, poly_trait_ref, item| {
            this.push_candidate(
                Candidate {
                    item,
                    kind: WhereClauseCandidate(poly_trait_ref),
                    import_ids: smallvec![],
                },
                true,
            );
        });
    }

    // Do a search through a list of bounds, using a callback to actually
    // create the candidates.
    fn assemble_candidates_for_bounds<F>(
        &mut self,
        bounds: impl Iterator<Item = ty::PolyTraitRef<'tcx>>,
        mut mk_cand: F,
    ) where
        F: for<'b> FnMut(&mut ProbeContext<'b, 'tcx>, ty::PolyTraitRef<'tcx>, ty::AssocItem),
    {
        for bound_trait_ref in bounds {
            debug!("elaborate_bounds(bound_trait_ref={:?})", bound_trait_ref);
            for item in self.impl_or_trait_item(bound_trait_ref.def_id()) {
                if !self.has_applicable_self(&item) {
                    self.record_static_candidate(CandidateSource::Trait(bound_trait_ref.def_id()));
                } else {
                    mk_cand(self, bound_trait_ref, item);
                }
            }
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn assemble_extension_candidates_for_traits_in_scope(&mut self) {
        let mut duplicates = FxHashSet::default();
        let opt_applicable_traits = self.tcx.in_scope_traits(self.scope_expr_id);
        if let Some(applicable_traits) = opt_applicable_traits {
            for trait_candidate in applicable_traits.iter() {
                let trait_did = trait_candidate.def_id;
                if duplicates.insert(trait_did) {
                    self.assemble_extension_candidates_for_trait(
                        &trait_candidate.import_ids,
                        trait_did,
                    );
                }
            }
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn assemble_extension_candidates_for_all_traits(&mut self) {
        let mut duplicates = FxHashSet::default();
        for trait_info in suggest::all_traits(self.tcx) {
            if duplicates.insert(trait_info.def_id) {
                self.assemble_extension_candidates_for_trait(&smallvec![], trait_info.def_id);
            }
        }
    }

    fn matches_return_type(&self, method: ty::AssocItem, expected: Ty<'tcx>) -> bool {
        match method.kind {
            ty::AssocKind::Fn { .. } => self.probe(|_| {
                let args = self.fresh_args_for_item(self.span, method.def_id);
                let fty = self.tcx.fn_sig(method.def_id).instantiate(self.tcx, args);
                let fty = self.instantiate_binder_with_fresh_vars(
                    self.span,
                    BoundRegionConversionTime::FnCall,
                    fty,
                );
                self.can_eq(self.param_env, fty.output(), expected)
            }),
            _ => false,
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn assemble_extension_candidates_for_trait(
        &mut self,
        import_ids: &SmallVec<[LocalDefId; 1]>,
        trait_def_id: DefId,
    ) {
        let trait_args = self.fresh_args_for_item(self.span, trait_def_id);
        let trait_ref = ty::TraitRef::new_from_args(self.tcx, trait_def_id, trait_args);

        if self.tcx.is_trait_alias(trait_def_id) {
            // For trait aliases, recursively assume all explicitly named traits are relevant
            for (bound_trait_pred, _) in
                traits::expand_trait_aliases(self.tcx, [(trait_ref.upcast(self.tcx), self.span)]).0
            {
                assert_eq!(bound_trait_pred.polarity(), ty::PredicatePolarity::Positive);
                let bound_trait_ref = bound_trait_pred.map_bound(|pred| pred.trait_ref);
                for item in self.impl_or_trait_item(bound_trait_ref.def_id()) {
                    if !self.has_applicable_self(&item) {
                        self.record_static_candidate(CandidateSource::Trait(
                            bound_trait_ref.def_id(),
                        ));
                    } else {
                        self.push_candidate(
                            Candidate {
                                item,
                                import_ids: import_ids.clone(),
                                kind: TraitCandidate(bound_trait_ref),
                            },
                            false,
                        );
                    }
                }
            }
        } else {
            debug_assert!(self.tcx.is_trait(trait_def_id));
            if self.tcx.trait_is_auto(trait_def_id) {
                return;
            }
            for item in self.impl_or_trait_item(trait_def_id) {
                // Check whether `trait_def_id` defines a method with suitable name.
                if !self.has_applicable_self(&item) {
                    debug!("method has inapplicable self");
                    self.record_static_candidate(CandidateSource::Trait(trait_def_id));
                    continue;
                }
                self.push_candidate(
                    Candidate {
                        item,
                        import_ids: import_ids.clone(),
                        kind: TraitCandidate(ty::Binder::dummy(trait_ref)),
                    },
                    false,
                );
            }
        }
    }

    fn candidate_method_names(
        &self,
        candidate_filter: impl Fn(&ty::AssocItem) -> bool,
    ) -> Vec<Ident> {
        let mut set = FxHashSet::default();
        let mut names: Vec<_> = self
            .inherent_candidates
            .iter()
            .chain(&self.extension_candidates)
            .filter(|candidate| candidate_filter(&candidate.item))
            .filter(|candidate| {
                if let Some(return_ty) = self.return_type {
                    self.matches_return_type(candidate.item, return_ty)
                } else {
                    true
                }
            })
            // ensure that we don't suggest unstable methods
            .filter(|candidate| {
                // note that `DUMMY_SP` is ok here because it is only used for
                // suggestions and macro stuff which isn't applicable here.
                !matches!(
                    self.tcx.eval_stability(candidate.item.def_id, None, DUMMY_SP, None),
                    stability::EvalResult::Deny { .. }
                )
            })
            .map(|candidate| candidate.item.ident(self.tcx))
            .filter(|&name| set.insert(name))
            .collect();

        // Sort them by the name so we have a stable result.
        names.sort_by(|a, b| a.as_str().cmp(b.as_str()));
        names
    }

    ///////////////////////////////////////////////////////////////////////////
    // THE ACTUAL SEARCH

    #[instrument(level = "debug", skip(self))]
    fn pick(mut self) -> PickResult<'tcx> {
        assert!(self.method_name.is_some());

        let mut unsatisfied_predicates = Vec::new();

        if let Some(r) = self.pick_core(&mut unsatisfied_predicates) {
            return r;
        }

        // If it's a `lookup_probe_for_diagnostic`, then quit early. No need to
        // probe for other candidates.
        if self.is_suggestion.0 {
            return Err(MethodError::NoMatch(NoMatchData {
                static_candidates: vec![],
                unsatisfied_predicates: vec![],
                out_of_scope_traits: vec![],
                similar_candidate: None,
                mode: self.mode,
            }));
        }

        debug!("pick: actual search failed, assemble diagnostics");

        let static_candidates = std::mem::take(self.static_candidates.get_mut());
        let private_candidate = self.private_candidate.take();

        // things failed, so lets look at all traits, for diagnostic purposes now:
        self.reset();

        let span = self.span;
        let tcx = self.tcx;

        self.assemble_extension_candidates_for_all_traits();

        let out_of_scope_traits = match self.pick_core(&mut Vec::new()) {
            Some(Ok(p)) => vec![p.item.container_id(self.tcx)],
            Some(Err(MethodError::Ambiguity(v))) => v
                .into_iter()
                .map(|source| match source {
                    CandidateSource::Trait(id) => id,
                    CandidateSource::Impl(impl_id) => match tcx.trait_id_of_impl(impl_id) {
                        Some(id) => id,
                        None => span_bug!(span, "found inherent method when looking at traits"),
                    },
                })
                .collect(),
            Some(Err(MethodError::NoMatch(NoMatchData {
                out_of_scope_traits: others, ..
            }))) => {
                assert!(others.is_empty());
                vec![]
            }
            _ => vec![],
        };

        if let Some((kind, def_id)) = private_candidate {
            return Err(MethodError::PrivateMatch(kind, def_id, out_of_scope_traits));
        }
        let similar_candidate = self.probe_for_similar_candidate()?;

        Err(MethodError::NoMatch(NoMatchData {
            static_candidates,
            unsatisfied_predicates,
            out_of_scope_traits,
            similar_candidate,
            mode: self.mode,
        }))
    }

    fn pick_core(
        &self,
        unsatisfied_predicates: &mut Vec<(
            ty::Predicate<'tcx>,
            Option<ty::Predicate<'tcx>>,
            Option<ObligationCause<'tcx>>,
        )>,
    ) -> Option<PickResult<'tcx>> {
        // Pick stable methods only first, and consider unstable candidates if not found.
        self.pick_all_method(&mut PickDiagHints {
            // This first cycle, maintain a list of unstable candidates which
            // we encounter. This will end up in the Pick for diagnostics.
            unstable_candidates: Some(Vec::new()),
            // Contribute to the list of unsatisfied predicates which may
            // also be used for diagnostics.
            unsatisfied_predicates,
        })
        .or_else(|| {
            self.pick_all_method(&mut PickDiagHints {
                // On the second search, don't provide a special list of unstable
                // candidates. This indicates to the picking code that it should
                // in fact include such unstable candidates in the actual
                // search.
                unstable_candidates: None,
                // And there's no need to duplicate ourselves in the
                // unsatisifed predicates list. Provide a throwaway list.
                unsatisfied_predicates: &mut Vec::new(),
            })
        })
    }

    fn pick_all_method<'b>(
        &self,
        pick_diag_hints: &mut PickDiagHints<'b, 'tcx>,
    ) -> Option<PickResult<'tcx>> {
        let track_unstable_candidates = pick_diag_hints.unstable_candidates.is_some();
        self.steps
            .iter()
            // At this point we're considering the types to which the receiver can be converted,
            // so we want to follow the `Deref` chain not the `Receiver` chain. Filter out
            // steps which can only be reached by following the (longer) `Receiver` chain.
            .filter(|step| step.reachable_via_deref)
            .filter(|step| {
                debug!("pick_all_method: step={:?}", step);
                // skip types that are from a type error or that would require dereferencing
                // a raw pointer
                !step.self_ty.value.references_error() && !step.from_unsafe_deref
            })
            .find_map(|step| {
                let InferOk { value: self_ty, obligations: _ } = self
                    .fcx
                    .probe_instantiate_query_response(
                        self.span,
                        self.orig_steps_var_values,
                        &step.self_ty,
                    )
                    .unwrap_or_else(|_| {
                        span_bug!(self.span, "{:?} was applicable but now isn't?", step.self_ty)
                    });

                let by_value_pick = self.pick_by_value_method(step, self_ty, pick_diag_hints);

                // Check for shadowing of a by-reference method by a by-value method (see comments on check_for_shadowing)
                if let Some(by_value_pick) = by_value_pick {
                    if let Ok(by_value_pick) = by_value_pick.as_ref() {
                        if by_value_pick.kind == PickKind::InherentImplPick {
                            for mutbl in [hir::Mutability::Not, hir::Mutability::Mut] {
                                if let Err(e) = self.check_for_shadowed_autorefd_method(
                                    by_value_pick,
                                    step,
                                    self_ty,
                                    mutbl,
                                    track_unstable_candidates,
                                ) {
                                    return Some(Err(e));
                                }
                            }
                        }
                    }
                    return Some(by_value_pick);
                }

                let autoref_pick = self.pick_autorefd_method(
                    step,
                    self_ty,
                    hir::Mutability::Not,
                    pick_diag_hints,
                    None,
                );
                // Check for shadowing of a by-mut-ref method by a by-reference method (see comments on check_for_shadowing)
                if let Some(autoref_pick) = autoref_pick {
                    if let Ok(autoref_pick) = autoref_pick.as_ref() {
                        // Check we're not shadowing others
                        if autoref_pick.kind == PickKind::InherentImplPick {
                            if let Err(e) = self.check_for_shadowed_autorefd_method(
                                autoref_pick,
                                step,
                                self_ty,
                                hir::Mutability::Mut,
                                track_unstable_candidates,
                            ) {
                                return Some(Err(e));
                            }
                        }
                    }
                    return Some(autoref_pick);
                }

                // Note that no shadowing errors are produced from here on,
                // as we consider const ptr methods.
                // We allow new methods that take *mut T to shadow
                // methods which took *const T, so there is no entry in
                // this list for the results of `pick_const_ptr_method`.
                // The reason is that the standard pointer cast method
                // (on a mutable pointer) always already shadows the
                // cast method (on a const pointer). So, if we added
                // `pick_const_ptr_method` to this method, the anti-
                // shadowing algorithm would always complain about
                // the conflict between *const::cast and *mut::cast.
                // In practice therefore this does constrain us:
                // we cannot add new
                //   self: *mut Self
                // methods to types such as NonNull or anything else
                // which implements Receiver, because this might in future
                // shadow existing methods taking
                //   self: *const NonNull<Self>
                // in the pointee. In practice, methods taking raw pointers
                // are rare, and it seems that it should be easily possible
                // to avoid such compatibility breaks.
                // We also don't check for reborrowed pin methods which
                // may be shadowed; these also seem unlikely to occur.
                self.pick_autorefd_method(
                    step,
                    self_ty,
                    hir::Mutability::Mut,
                    pick_diag_hints,
                    None,
                )
                .or_else(|| self.pick_const_ptr_method(step, self_ty, pick_diag_hints))
                .or_else(|| self.pick_reborrow_pin_method(step, self_ty, pick_diag_hints))
            })
    }

    /// Check for cases where arbitrary self types allows shadowing
    /// of methods that might be a compatibility break. Specifically,
    /// we have something like:
    /// ```ignore (illustrative)
    /// struct A;
    /// impl A {
    ///   fn foo(self: &NonNull<A>) {}
    ///      // note this is by reference
    /// }
    /// ```
    /// then we've come along and added this method to `NonNull`:
    /// ```ignore (illustrative)
    ///   fn foo(self)  // note this is by value
    /// ```
    /// Report an error in this case.
    fn check_for_shadowed_autorefd_method(
        &self,
        possible_shadower: &Pick<'tcx>,
        step: &CandidateStep<'tcx>,
        self_ty: Ty<'tcx>,
        mutbl: hir::Mutability,
        track_unstable_candidates: bool,
    ) -> Result<(), MethodError<'tcx>> {
        // The errors emitted by this function are part of
        // the arbitrary self types work, and should not impact
        // other users.
        if !self.tcx.features().arbitrary_self_types()
            && !self.tcx.features().arbitrary_self_types_pointers()
        {
            return Ok(());
        }

        // We don't want to remember any of the diagnostic hints from this
        // shadow search, but we do need to provide Some/None for the
        // unstable_candidates in order to reflect the behavior of the
        // main search.
        let mut pick_diag_hints = PickDiagHints {
            unstable_candidates: if track_unstable_candidates { Some(Vec::new()) } else { None },
            unsatisfied_predicates: &mut Vec::new(),
        };
        // Set criteria for how we find methods possibly shadowed by 'possible_shadower'
        let pick_constraints = PickConstraintsForShadowed {
            // It's the same `self` type...
            autoderefs: possible_shadower.autoderefs,
            // ... but the method was found in an impl block determined
            // by searching further along the Receiver chain than the other,
            // showing that it's a smart pointer type causing the problem...
            receiver_steps: possible_shadower.receiver_steps,
            // ... and they don't end up pointing to the same item in the
            // first place (could happen with things like blanket impls for T)
            def_id: possible_shadower.item.def_id,
        };
        // A note on the autoderefs above. Within pick_by_value_method, an extra
        // autoderef may be applied in order to reborrow a reference with
        // a different lifetime. That seems as though it would break the
        // logic of these constraints, since the number of autoderefs could
        // no longer be used to identify the fundamental type of the receiver.
        // However, this extra autoderef is applied only to by-value calls
        // where the receiver is already a reference. So this situation would
        // only occur in cases where the shadowing looks like this:
        // ```
        // struct A;
        // impl A {
        //   fn foo(self: &&NonNull<A>) {}
        //      // note this is by DOUBLE reference
        // }
        // ```
        // then we've come along and added this method to `NonNull`:
        // ```
        //   fn foo(&self)  // note this is by single reference
        // ```
        // and the call is:
        // ```
        // let bar = NonNull<Foo>;
        // let bar = &foo;
        // bar.foo();
        // ```
        // In these circumstances, the logic is wrong, and we wouldn't spot
        // the shadowing, because the autoderef-based maths wouldn't line up.
        // This is a niche case and we can live without generating an error
        // in the case of such shadowing.
        let potentially_shadowed_pick = self.pick_autorefd_method(
            step,
            self_ty,
            mutbl,
            &mut pick_diag_hints,
            Some(&pick_constraints),
        );
        // Look for actual pairs of shadower/shadowed which are
        // the sort of shadowing case we want to avoid. Specifically...
        if let Some(Ok(possible_shadowed)) = potentially_shadowed_pick.as_ref() {
            let sources = [possible_shadower, possible_shadowed]
                .into_iter()
                .map(|p| self.candidate_source_from_pick(p))
                .collect();
            return Err(MethodError::Ambiguity(sources));
        }
        Ok(())
    }

    /// For each type `T` in the step list, this attempts to find a method where
    /// the (transformed) self type is exactly `T`. We do however do one
    /// transformation on the adjustment: if we are passing a region pointer in,
    /// we will potentially *reborrow* it to a shorter lifetime. This allows us
    /// to transparently pass `&mut` pointers, in particular, without consuming
    /// them for their entire lifetime.
    fn pick_by_value_method(
        &self,
        step: &CandidateStep<'tcx>,
        self_ty: Ty<'tcx>,
        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
    ) -> Option<PickResult<'tcx>> {
        if step.unsize {
            return None;
        }

        self.pick_method(self_ty, pick_diag_hints, None).map(|r| {
            r.map(|mut pick| {
                pick.autoderefs = step.autoderefs;

                match *step.self_ty.value.value.kind() {
                    // Insert a `&*` or `&mut *` if this is a reference type:
                    ty::Ref(_, _, mutbl) => {
                        pick.autoderefs += 1;
                        pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::Autoref {
                            mutbl,
                            unsize: pick.autoref_or_ptr_adjustment.is_some_and(|a| a.get_unsize()),
                        })
                    }

                    ty::Adt(def, args)
                        if self.tcx.features().pin_ergonomics()
                            && self.tcx.is_lang_item(def.did(), hir::LangItem::Pin) =>
                    {
                        // make sure this is a pinned reference (and not a `Pin<Box>` or something)
                        if let ty::Ref(_, _, mutbl) = args[0].expect_ty().kind() {
                            pick.autoref_or_ptr_adjustment =
                                Some(AutorefOrPtrAdjustment::ReborrowPin(*mutbl));
                        }
                    }

                    _ => (),
                }

                pick
            })
        })
    }

    fn pick_autorefd_method(
        &self,
        step: &CandidateStep<'tcx>,
        self_ty: Ty<'tcx>,
        mutbl: hir::Mutability,
        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
        pick_constraints: Option<&PickConstraintsForShadowed>,
    ) -> Option<PickResult<'tcx>> {
        let tcx = self.tcx;

        if let Some(pick_constraints) = pick_constraints {
            if !pick_constraints.may_shadow_based_on_autoderefs(step.autoderefs) {
                return None;
            }
        }

        // In general, during probing we erase regions.
        let region = tcx.lifetimes.re_erased;

        let autoref_ty = Ty::new_ref(tcx, region, self_ty, mutbl);
        self.pick_method(autoref_ty, pick_diag_hints, pick_constraints).map(|r| {
            r.map(|mut pick| {
                pick.autoderefs = step.autoderefs;
                pick.autoref_or_ptr_adjustment =
                    Some(AutorefOrPtrAdjustment::Autoref { mutbl, unsize: step.unsize });
                pick
            })
        })
    }

    /// Looks for applicable methods if we reborrow a `Pin<&mut T>` as a `Pin<&T>`.
    #[instrument(level = "debug", skip(self, step, pick_diag_hints))]
    fn pick_reborrow_pin_method(
        &self,
        step: &CandidateStep<'tcx>,
        self_ty: Ty<'tcx>,
        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
    ) -> Option<PickResult<'tcx>> {
        if !self.tcx.features().pin_ergonomics() {
            return None;
        }

        // make sure self is a Pin<&mut T>
        let inner_ty = match self_ty.kind() {
            ty::Adt(def, args) if self.tcx.is_lang_item(def.did(), hir::LangItem::Pin) => {
                match args[0].expect_ty().kind() {
                    ty::Ref(_, ty, hir::Mutability::Mut) => *ty,
                    _ => {
                        return None;
                    }
                }
            }
            _ => return None,
        };

        let region = self.tcx.lifetimes.re_erased;
        let autopin_ty = Ty::new_pinned_ref(self.tcx, region, inner_ty, hir::Mutability::Not);
        self.pick_method(autopin_ty, pick_diag_hints, None).map(|r| {
            r.map(|mut pick| {
                pick.autoderefs = step.autoderefs;
                pick.autoref_or_ptr_adjustment =
                    Some(AutorefOrPtrAdjustment::ReborrowPin(hir::Mutability::Not));
                pick
            })
        })
    }

    /// If `self_ty` is `*mut T` then this picks `*const T` methods. The reason why we have a
    /// special case for this is because going from `*mut T` to `*const T` with autoderefs and
    /// autorefs would require dereferencing the pointer, which is not safe.
    fn pick_const_ptr_method(
        &self,
        step: &CandidateStep<'tcx>,
        self_ty: Ty<'tcx>,
        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
    ) -> Option<PickResult<'tcx>> {
        // Don't convert an unsized reference to ptr
        if step.unsize {
            return None;
        }

        let &ty::RawPtr(ty, hir::Mutability::Mut) = self_ty.kind() else {
            return None;
        };

        let const_ptr_ty = Ty::new_imm_ptr(self.tcx, ty);
        self.pick_method(const_ptr_ty, pick_diag_hints, None).map(|r| {
            r.map(|mut pick| {
                pick.autoderefs = step.autoderefs;
                pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::ToConstPtr);
                pick
            })
        })
    }

    fn pick_method(
        &self,
        self_ty: Ty<'tcx>,
        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
        pick_constraints: Option<&PickConstraintsForShadowed>,
    ) -> Option<PickResult<'tcx>> {
        debug!("pick_method(self_ty={})", self.ty_to_string(self_ty));

        for (kind, candidates) in
            [("inherent", &self.inherent_candidates), ("extension", &self.extension_candidates)]
        {
            debug!("searching {} candidates", kind);
            let res =
                self.consider_candidates(self_ty, candidates, pick_diag_hints, pick_constraints);
            if let Some(pick) = res {
                return Some(pick);
            }
        }

        if self.private_candidate.get().is_none() {
            if let Some(Ok(pick)) = self.consider_candidates(
                self_ty,
                &self.private_candidates,
                &mut PickDiagHints {
                    unstable_candidates: None,
                    unsatisfied_predicates: &mut vec![],
                },
                None,
            ) {
                self.private_candidate.set(Some((pick.item.as_def_kind(), pick.item.def_id)));
            }
        }
        None
    }

    fn consider_candidates(
        &self,
        self_ty: Ty<'tcx>,
        candidates: &[Candidate<'tcx>],
        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
        pick_constraints: Option<&PickConstraintsForShadowed>,
    ) -> Option<PickResult<'tcx>> {
        let mut applicable_candidates: Vec<_> = candidates
            .iter()
            .filter(|candidate| {
                pick_constraints
                    .map(|pick_constraints| pick_constraints.candidate_may_shadow(&candidate))
                    .unwrap_or(true)
            })
            .map(|probe| {
                (
                    probe,
                    self.consider_probe(
                        self_ty,
                        probe,
                        &mut pick_diag_hints.unsatisfied_predicates,
                    ),
                )
            })
            .filter(|&(_, status)| status != ProbeResult::NoMatch)
            .collect();

        debug!("applicable_candidates: {:?}", applicable_candidates);

        if applicable_candidates.len() > 1 {
            if let Some(pick) =
                self.collapse_candidates_to_trait_pick(self_ty, &applicable_candidates)
            {
                return Some(Ok(pick));
            }
        }

        if let Some(uc) = &mut pick_diag_hints.unstable_candidates {
            applicable_candidates.retain(|&(candidate, _)| {
                if let stability::EvalResult::Deny { feature, .. } =
                    self.tcx.eval_stability(candidate.item.def_id, None, self.span, None)
                {
                    uc.push((candidate.clone(), feature));
                    return false;
                }
                true
            });
        }

        if applicable_candidates.len() > 1 {
            // We collapse to a subtrait pick *after* filtering unstable candidates
            // to make sure we don't prefer a unstable subtrait method over a stable
            // supertrait method.
            if self.tcx.features().supertrait_item_shadowing() {
                if let Some(pick) =
                    self.collapse_candidates_to_subtrait_pick(self_ty, &applicable_candidates)
                {
                    return Some(Ok(pick));
                }
            }

            let sources =
                applicable_candidates.iter().map(|p| self.candidate_source(p.0, self_ty)).collect();
            return Some(Err(MethodError::Ambiguity(sources)));
        }

        applicable_candidates.pop().map(|(probe, status)| match status {
            ProbeResult::Match => Ok(probe.to_unadjusted_pick(
                self_ty,
                pick_diag_hints.unstable_candidates.clone().unwrap_or_default(),
            )),
            ProbeResult::NoMatch | ProbeResult::BadReturnType => Err(MethodError::BadReturnType),
        })
    }
}

impl<'tcx> Pick<'tcx> {
    /// In case there were unstable name collisions, emit them as a lint.
    /// Checks whether two picks do not refer to the same trait item for the same `Self` type.
    /// Only useful for comparisons of picks in order to improve diagnostics.
    /// Do not use for type checking.
    pub(crate) fn differs_from(&self, other: &Self) -> bool {
        let Self {
            item: AssocItem { def_id, kind: _, container: _, trait_item_def_id: _ },
            kind: _,
            import_ids: _,
            autoderefs: _,
            autoref_or_ptr_adjustment: _,
            self_ty,
            unstable_candidates: _,
            receiver_steps: _,
            shadowed_candidates: _,
        } = *self;
        self_ty != other.self_ty || def_id != other.item.def_id
    }

    /// In case there were unstable name collisions, emit them as a lint.
    pub(crate) fn maybe_emit_unstable_name_collision_hint(
        &self,
        tcx: TyCtxt<'tcx>,
        span: Span,
        scope_expr_id: HirId,
    ) {
        if self.unstable_candidates.is_empty() {
            return;
        }
        let def_kind = self.item.as_def_kind();
        tcx.node_span_lint(lint::builtin::UNSTABLE_NAME_COLLISIONS, scope_expr_id, span, |lint| {
            lint.primary_message(format!(
                "{} {} with this name may be added to the standard library in the future",
                tcx.def_kind_descr_article(def_kind, self.item.def_id),
                tcx.def_kind_descr(def_kind, self.item.def_id),
            ));

            match (self.item.kind, self.item.container) {
                (ty::AssocKind::Fn { .. }, _) => {
                    // FIXME: This should be a `span_suggestion` instead of `help`
                    // However `self.span` only
                    // highlights the method name, so we can't use it. Also consider reusing
                    // the code from `report_method_error()`.
                    lint.help(format!(
                        "call with fully qualified syntax `{}(...)` to keep using the current \
                             method",
                        tcx.def_path_str(self.item.def_id),
                    ));
                }
                (ty::AssocKind::Const { name }, ty::AssocItemContainer::Trait) => {
                    let def_id = self.item.container_id(tcx);
                    lint.span_suggestion(
                        span,
                        "use the fully qualified path to the associated const",
                        format!("<{} as {}>::{}", self.self_ty, tcx.def_path_str(def_id), name),
                        Applicability::MachineApplicable,
                    );
                }
                _ => {}
            }
            tcx.disabled_nightly_features(
                lint,
                self.unstable_candidates.iter().map(|(candidate, feature)| {
                    (format!(" `{}`", tcx.def_path_str(candidate.item.def_id)), *feature)
                }),
            );
        });
    }
}

impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
    fn select_trait_candidate(
        &self,
        trait_ref: ty::TraitRef<'tcx>,
    ) -> traits::SelectionResult<'tcx, traits::Selection<'tcx>> {
        let obligation =
            traits::Obligation::new(self.tcx, self.misc(self.span), self.param_env, trait_ref);
        traits::SelectionContext::new(self).select(&obligation)
    }

    /// Used for ambiguous method call error reporting. Uses probing that throws away the result internally,
    /// so do not use to make a decision that may lead to a successful compilation.
    fn candidate_source(&self, candidate: &Candidate<'tcx>, self_ty: Ty<'tcx>) -> CandidateSource {
        match candidate.kind {
            InherentImplCandidate { .. } => {
                CandidateSource::Impl(candidate.item.container_id(self.tcx))
            }
            ObjectCandidate(_) | WhereClauseCandidate(_) => {
                CandidateSource::Trait(candidate.item.container_id(self.tcx))
            }
            TraitCandidate(trait_ref) => self.probe(|_| {
                let trait_ref = self.instantiate_binder_with_fresh_vars(
                    self.span,
                    BoundRegionConversionTime::FnCall,
                    trait_ref,
                );
                let (xform_self_ty, _) =
                    self.xform_self_ty(candidate.item, trait_ref.self_ty(), trait_ref.args);
                // Guide the trait selection to show impls that have methods whose type matches
                // up with the `self` parameter of the method.
                let _ = self.at(&ObligationCause::dummy(), self.param_env).sup(
                    DefineOpaqueTypes::Yes,
                    xform_self_ty,
                    self_ty,
                );
                match self.select_trait_candidate(trait_ref) {
                    Ok(Some(traits::ImplSource::UserDefined(ref impl_data))) => {
                        // If only a single impl matches, make the error message point
                        // to that impl.
                        CandidateSource::Impl(impl_data.impl_def_id)
                    }
                    _ => CandidateSource::Trait(candidate.item.container_id(self.tcx)),
                }
            }),
        }
    }

    fn candidate_source_from_pick(&self, pick: &Pick<'tcx>) -> CandidateSource {
        match pick.kind {
            InherentImplPick => CandidateSource::Impl(pick.item.container_id(self.tcx)),
            ObjectPick | WhereClausePick(_) | TraitPick => {
                CandidateSource::Trait(pick.item.container_id(self.tcx))
            }
        }
    }

    #[instrument(level = "trace", skip(self, possibly_unsatisfied_predicates), ret)]
    fn consider_probe(
        &self,
        self_ty: Ty<'tcx>,
        probe: &Candidate<'tcx>,
        possibly_unsatisfied_predicates: &mut Vec<(
            ty::Predicate<'tcx>,
            Option<ty::Predicate<'tcx>>,
            Option<ObligationCause<'tcx>>,
        )>,
    ) -> ProbeResult {
        debug!("consider_probe: self_ty={:?} probe={:?}", self_ty, probe);

        self.probe(|snapshot| {
            let outer_universe = self.universe();

            let mut result = ProbeResult::Match;
            let cause = &self.misc(self.span);
            let ocx = ObligationCtxt::new_with_diagnostics(self);

            let mut trait_predicate = None;
            let (mut xform_self_ty, mut xform_ret_ty);

            match probe.kind {
                InherentImplCandidate { impl_def_id, .. } => {
                    let impl_args = self.fresh_args_for_item(self.span, impl_def_id);
                    let impl_ty = self.tcx.type_of(impl_def_id).instantiate(self.tcx, impl_args);
                    (xform_self_ty, xform_ret_ty) =
                        self.xform_self_ty(probe.item, impl_ty, impl_args);
                    xform_self_ty = ocx.normalize(cause, self.param_env, xform_self_ty);
                    match ocx.relate(cause, self.param_env, self.variance(), self_ty, xform_self_ty)
                    {
                        Ok(()) => {}
                        Err(err) => {
                            debug!("--> cannot relate self-types {:?}", err);
                            return ProbeResult::NoMatch;
                        }
                    }
                    // FIXME: Weirdly, we normalize the ret ty in this candidate, but no other candidates.
                    xform_ret_ty = ocx.normalize(cause, self.param_env, xform_ret_ty);
                    // Check whether the impl imposes obligations we have to worry about.
                    let impl_def_id = probe.item.container_id(self.tcx);
                    let impl_bounds =
                        self.tcx.predicates_of(impl_def_id).instantiate(self.tcx, impl_args);
                    let impl_bounds = ocx.normalize(cause, self.param_env, impl_bounds);
                    // Convert the bounds into obligations.
                    ocx.register_obligations(traits::predicates_for_generics(
                        |idx, span| {
                            let code = ObligationCauseCode::WhereClauseInExpr(
                                impl_def_id,
                                span,
                                self.scope_expr_id,
                                idx,
                            );
                            self.cause(self.span, code)
                        },
                        self.param_env,
                        impl_bounds,
                    ));
                }
                TraitCandidate(poly_trait_ref) => {
                    // Some trait methods are excluded for arrays before 2021.
                    // (`array.into_iter()` wants a slice iterator for compatibility.)
                    if let Some(method_name) = self.method_name {
                        if self_ty.is_array() && !method_name.span.at_least_rust_2021() {
                            let trait_def = self.tcx.trait_def(poly_trait_ref.def_id());
                            if trait_def.skip_array_during_method_dispatch {
                                return ProbeResult::NoMatch;
                            }
                        }

                        // Some trait methods are excluded for boxed slices before 2024.
                        // (`boxed_slice.into_iter()` wants a slice iterator for compatibility.)
                        if self_ty.boxed_ty().is_some_and(Ty::is_slice)
                            && !method_name.span.at_least_rust_2024()
                        {
                            let trait_def = self.tcx.trait_def(poly_trait_ref.def_id());
                            if trait_def.skip_boxed_slice_during_method_dispatch {
                                return ProbeResult::NoMatch;
                            }
                        }
                    }

                    let trait_ref = self.instantiate_binder_with_fresh_vars(
                        self.span,
                        BoundRegionConversionTime::FnCall,
                        poly_trait_ref,
                    );
                    let trait_ref = ocx.normalize(cause, self.param_env, trait_ref);
                    (xform_self_ty, xform_ret_ty) =
                        self.xform_self_ty(probe.item, trait_ref.self_ty(), trait_ref.args);
                    xform_self_ty = ocx.normalize(cause, self.param_env, xform_self_ty);
                    match self_ty.kind() {
                        // HACK: opaque types will match anything for which their bounds hold.
                        // Thus we need to prevent them from trying to match the `&_` autoref
                        // candidates that get created for `&self` trait methods.
                        ty::Alias(ty::Opaque, alias_ty)
                            if !self.next_trait_solver()
                                && self.infcx.can_define_opaque_ty(alias_ty.def_id)
                                && !xform_self_ty.is_ty_var() =>
                        {
                            return ProbeResult::NoMatch;
                        }
                        _ => match ocx.relate(
                            cause,
                            self.param_env,
                            self.variance(),
                            self_ty,
                            xform_self_ty,
                        ) {
                            Ok(()) => {}
                            Err(err) => {
                                debug!("--> cannot relate self-types {:?}", err);
                                return ProbeResult::NoMatch;
                            }
                        },
                    }
                    let obligation = traits::Obligation::new(
                        self.tcx,
                        cause.clone(),
                        self.param_env,
                        ty::Binder::dummy(trait_ref),
                    );

                    // We only need this hack to deal with fatal overflow in the old solver.
                    if self.infcx.next_trait_solver() || self.infcx.predicate_may_hold(&obligation)
                    {
                        ocx.register_obligation(obligation);
                    } else {
                        result = ProbeResult::NoMatch;
                        if let Ok(Some(candidate)) = self.select_trait_candidate(trait_ref) {
                            for nested_obligation in candidate.nested_obligations() {
                                if !self.infcx.predicate_may_hold(&nested_obligation) {
                                    possibly_unsatisfied_predicates.push((
                                        self.resolve_vars_if_possible(nested_obligation.predicate),
                                        Some(self.resolve_vars_if_possible(obligation.predicate)),
                                        Some(nested_obligation.cause),
                                    ));
                                }
                            }
                        }
                    }

                    trait_predicate = Some(trait_ref.upcast(self.tcx));
                }
                ObjectCandidate(poly_trait_ref) | WhereClauseCandidate(poly_trait_ref) => {
                    let trait_ref = self.instantiate_binder_with_fresh_vars(
                        self.span,
                        BoundRegionConversionTime::FnCall,
                        poly_trait_ref,
                    );
                    (xform_self_ty, xform_ret_ty) =
                        self.xform_self_ty(probe.item, trait_ref.self_ty(), trait_ref.args);
                    xform_self_ty = ocx.normalize(cause, self.param_env, xform_self_ty);
                    match ocx.relate(cause, self.param_env, self.variance(), self_ty, xform_self_ty)
                    {
                        Ok(()) => {}
                        Err(err) => {
                            debug!("--> cannot relate self-types {:?}", err);
                            return ProbeResult::NoMatch;
                        }
                    }
                }
            }

            // See <https://github.com/rust-lang/trait-system-refactor-initiative/issues/134>.
            //
            // In the new solver, check the well-formedness of the return type.
            // This emulates, in a way, the predicates that fall out of
            // normalizing the return type in the old solver.
            //
            // FIXME(-Znext-solver): We alternatively could check the predicates of
            // the method itself hold, but we intentionally do not do this in the old
            // solver b/c of cycles, and doing it in the new solver would be stronger.
            // This should be fixed in the future, since it likely leads to much better
            // method winnowing.
            if let Some(xform_ret_ty) = xform_ret_ty
                && self.infcx.next_trait_solver()
            {
                ocx.register_obligation(traits::Obligation::new(
                    self.tcx,
                    cause.clone(),
                    self.param_env,
                    ty::ClauseKind::WellFormed(xform_ret_ty.into()),
                ));
            }

            // Evaluate those obligations to see if they might possibly hold.
            for error in ocx.select_where_possible() {
                result = ProbeResult::NoMatch;
                let nested_predicate = self.resolve_vars_if_possible(error.obligation.predicate);
                if let Some(trait_predicate) = trait_predicate
                    && nested_predicate == self.resolve_vars_if_possible(trait_predicate)
                {
                    // Don't report possibly unsatisfied predicates if the root
                    // trait obligation from a `TraitCandidate` is unsatisfied.
                    // That just means the candidate doesn't hold.
                } else {
                    possibly_unsatisfied_predicates.push((
                        nested_predicate,
                        Some(self.resolve_vars_if_possible(error.root_obligation.predicate))
                            .filter(|root_predicate| *root_predicate != nested_predicate),
                        Some(error.obligation.cause),
                    ));
                }
            }

            if let ProbeResult::Match = result
                && let Some(return_ty) = self.return_type
                && let Some(mut xform_ret_ty) = xform_ret_ty
            {
                // `xform_ret_ty` has only been normalized for `InherentImplCandidate`.
                // We don't normalize the other candidates for perf/backwards-compat reasons...
                // but `self.return_type` is only set on the diagnostic-path, so we
                // should be okay doing it here.
                if !matches!(probe.kind, InherentImplCandidate { .. }) {
                    xform_ret_ty = ocx.normalize(&cause, self.param_env, xform_ret_ty);
                }

                debug!("comparing return_ty {:?} with xform ret ty {:?}", return_ty, xform_ret_ty);
                match ocx.relate(cause, self.param_env, self.variance(), xform_ret_ty, return_ty) {
                    Ok(()) => {}
                    Err(_) => {
                        result = ProbeResult::BadReturnType;
                    }
                }

                // Evaluate those obligations to see if they might possibly hold.
                for error in ocx.select_where_possible() {
                    result = ProbeResult::NoMatch;
                    possibly_unsatisfied_predicates.push((
                        error.obligation.predicate,
                        Some(error.root_obligation.predicate)
                            .filter(|predicate| *predicate != error.obligation.predicate),
                        Some(error.root_obligation.cause),
                    ));
                }
            }

            // Previously, method probe used `evaluate_predicate` to determine if a predicate
            // was impossible to satisfy. This did a leak check, so we must also do a leak
            // check here to prevent backwards-incompatible ambiguity being introduced. See
            // `tests/ui/methods/leak-check-disquality.rs` for a simple example of when this
            // may happen.
            if let Err(_) = self.leak_check(outer_universe, Some(snapshot)) {
                result = ProbeResult::NoMatch;
            }

            result
        })
    }

    /// Sometimes we get in a situation where we have multiple probes that are all impls of the
    /// same trait, but we don't know which impl to use. In this case, since in all cases the
    /// external interface of the method can be determined from the trait, it's ok not to decide.
    /// We can basically just collapse all of the probes for various impls into one where-clause
    /// probe. This will result in a pending obligation so when more type-info is available we can
    /// make the final decision.
    ///
    /// Example (`tests/ui/methods/method-two-trait-defer-resolution-1.rs`):
    ///
    /// ```ignore (illustrative)
    /// trait Foo { ... }
    /// impl Foo for Vec<i32> { ... }
    /// impl Foo for Vec<usize> { ... }
    /// ```
    ///
    /// Now imagine the receiver is `Vec<_>`. It doesn't really matter at this time which impl we
    /// use, so it's ok to just commit to "using the method from the trait Foo".
    fn collapse_candidates_to_trait_pick(
        &self,
        self_ty: Ty<'tcx>,
        probes: &[(&Candidate<'tcx>, ProbeResult)],
    ) -> Option<Pick<'tcx>> {
        // Do all probes correspond to the same trait?
        let container = probes[0].0.item.trait_container(self.tcx)?;
        for (p, _) in &probes[1..] {
            let p_container = p.item.trait_container(self.tcx)?;
            if p_container != container {
                return None;
            }
        }

        // FIXME: check the return type here somehow.
        // If so, just use this trait and call it a day.
        Some(Pick {
            item: probes[0].0.item,
            kind: TraitPick,
            import_ids: probes[0].0.import_ids.clone(),
            autoderefs: 0,
            autoref_or_ptr_adjustment: None,
            self_ty,
            unstable_candidates: vec![],
            receiver_steps: None,
            shadowed_candidates: vec![],
        })
    }

    /// Much like `collapse_candidates_to_trait_pick`, this method allows us to collapse
    /// multiple conflicting picks if there is one pick whose trait container is a subtrait
    /// of the trait containers of all of the other picks.
    ///
    /// This implements RFC #3624.
    fn collapse_candidates_to_subtrait_pick(
        &self,
        self_ty: Ty<'tcx>,
        probes: &[(&Candidate<'tcx>, ProbeResult)],
    ) -> Option<Pick<'tcx>> {
        let mut child_candidate = probes[0].0;
        let mut child_trait = child_candidate.item.trait_container(self.tcx)?;
        let mut supertraits: SsoHashSet<_> = supertrait_def_ids(self.tcx, child_trait).collect();

        let mut remaining_candidates: Vec<_> = probes[1..].iter().map(|&(p, _)| p).collect();
        while !remaining_candidates.is_empty() {
            let mut made_progress = false;
            let mut next_round = vec![];

            for remaining_candidate in remaining_candidates {
                let remaining_trait = remaining_candidate.item.trait_container(self.tcx)?;
                if supertraits.contains(&remaining_trait) {
                    made_progress = true;
                    continue;
                }

                // This pick is not a supertrait of the `child_pick`.
                // Check if it's a subtrait of the `child_pick`, instead.
                // If it is, then it must have been a subtrait of every
                // other pick we've eliminated at this point. It will
                // take over at this point.
                let remaining_trait_supertraits: SsoHashSet<_> =
                    supertrait_def_ids(self.tcx, remaining_trait).collect();
                if remaining_trait_supertraits.contains(&child_trait) {
                    child_candidate = remaining_candidate;
                    child_trait = remaining_trait;
                    supertraits = remaining_trait_supertraits;
                    made_progress = true;
                    continue;
                }

                // `child_pick` is not a supertrait of this pick.
                // Don't bail here, since we may be comparing two supertraits
                // of a common subtrait. These two supertraits won't be related
                // at all, but we will pick them up next round when we find their
                // child as we continue iterating in this round.
                next_round.push(remaining_candidate);
            }

            if made_progress {
                // If we've made progress, iterate again.
                remaining_candidates = next_round;
            } else {
                // Otherwise, we must have at least two candidates which
                // are not related to each other at all.
                return None;
            }
        }

        Some(Pick {
            item: child_candidate.item,
            kind: TraitPick,
            import_ids: child_candidate.import_ids.clone(),
            autoderefs: 0,
            autoref_or_ptr_adjustment: None,
            self_ty,
            unstable_candidates: vec![],
            shadowed_candidates: probes
                .iter()
                .map(|(c, _)| c.item)
                .filter(|item| item.def_id != child_candidate.item.def_id)
                .collect(),
            receiver_steps: None,
        })
    }

    /// Similarly to `probe_for_return_type`, this method attempts to find the best matching
    /// candidate method where the method name may have been misspelled. Similarly to other
    /// edit distance based suggestions, we provide at most one such suggestion.
    #[instrument(level = "debug", skip(self))]
    pub(crate) fn probe_for_similar_candidate(
        &mut self,
    ) -> Result<Option<ty::AssocItem>, MethodError<'tcx>> {
        debug!("probing for method names similar to {:?}", self.method_name);

        self.probe(|_| {
            let mut pcx = ProbeContext::new(
                self.fcx,
                self.span,
                self.mode,
                self.method_name,
                self.return_type,
                self.orig_steps_var_values,
                self.steps,
                self.scope_expr_id,
                IsSuggestion(true),
            );
            pcx.allow_similar_names = true;
            pcx.assemble_inherent_candidates();
            pcx.assemble_extension_candidates_for_all_traits();

            let method_names = pcx.candidate_method_names(|_| true);
            pcx.allow_similar_names = false;
            let applicable_close_candidates: Vec<ty::AssocItem> = method_names
                .iter()
                .filter_map(|&method_name| {
                    pcx.reset();
                    pcx.method_name = Some(method_name);
                    pcx.assemble_inherent_candidates();
                    pcx.assemble_extension_candidates_for_all_traits();
                    pcx.pick_core(&mut Vec::new()).and_then(|pick| pick.ok()).map(|pick| pick.item)
                })
                .collect();

            if applicable_close_candidates.is_empty() {
                Ok(None)
            } else {
                let best_name = {
                    let names = applicable_close_candidates
                        .iter()
                        .map(|cand| cand.name())
                        .collect::<Vec<Symbol>>();
                    find_best_match_for_name_with_substrings(
                        &names,
                        self.method_name.unwrap().name,
                        None,
                    )
                }
                .or_else(|| {
                    applicable_close_candidates
                        .iter()
                        .find(|cand| self.matches_by_doc_alias(cand.def_id))
                        .map(|cand| cand.name())
                });
                Ok(best_name.and_then(|best_name| {
                    applicable_close_candidates
                        .into_iter()
                        .find(|method| method.name() == best_name)
                }))
            }
        })
    }

    ///////////////////////////////////////////////////////////////////////////
    // MISCELLANY
    fn has_applicable_self(&self, item: &ty::AssocItem) -> bool {
        // "Fast track" -- check for usage of sugar when in method call
        // mode.
        //
        // In Path mode (i.e., resolving a value like `T::next`), consider any
        // associated value (i.e., methods, constants) but not types.
        match self.mode {
            Mode::MethodCall => item.is_method(),
            Mode::Path => match item.kind {
                ty::AssocKind::Type { .. } => false,
                ty::AssocKind::Fn { .. } | ty::AssocKind::Const { .. } => true,
            },
        }
        // FIXME -- check for types that deref to `Self`,
        // like `Rc<Self>` and so on.
        //
        // Note also that the current code will break if this type
        // includes any of the type parameters defined on the method
        // -- but this could be overcome.
    }

    fn record_static_candidate(&self, source: CandidateSource) {
        self.static_candidates.borrow_mut().push(source);
    }

    #[instrument(level = "debug", skip(self))]
    fn xform_self_ty(
        &self,
        item: ty::AssocItem,
        impl_ty: Ty<'tcx>,
        args: GenericArgsRef<'tcx>,
    ) -> (Ty<'tcx>, Option<Ty<'tcx>>) {
        if item.is_fn() && self.mode == Mode::MethodCall {
            let sig = self.xform_method_sig(item.def_id, args);
            (sig.inputs()[0], Some(sig.output()))
        } else {
            (impl_ty, None)
        }
    }

    #[instrument(level = "debug", skip(self))]
    fn xform_method_sig(&self, method: DefId, args: GenericArgsRef<'tcx>) -> ty::FnSig<'tcx> {
        let fn_sig = self.tcx.fn_sig(method);
        debug!(?fn_sig);

        assert!(!args.has_escaping_bound_vars());

        // It is possible for type parameters or early-bound lifetimes
        // to appear in the signature of `self`. The generic parameters
        // we are given do not include type/lifetime parameters for the
        // method yet. So create fresh variables here for those too,
        // if there are any.
        let generics = self.tcx.generics_of(method);
        assert_eq!(args.len(), generics.parent_count);

        let xform_fn_sig = if generics.is_own_empty() {
            fn_sig.instantiate(self.tcx, args)
        } else {
            let args = GenericArgs::for_item(self.tcx, method, |param, _| {
                let i = param.index as usize;
                if i < args.len() {
                    args[i]
                } else {
                    match param.kind {
                        GenericParamDefKind::Lifetime => {
                            // In general, during probe we erase regions.
                            self.tcx.lifetimes.re_erased.into()
                        }
                        GenericParamDefKind::Type { .. } | GenericParamDefKind::Const { .. } => {
                            self.var_for_def(self.span, param)
                        }
                    }
                }
            });
            fn_sig.instantiate(self.tcx, args)
        };

        self.tcx.instantiate_bound_regions_with_erased(xform_fn_sig)
    }

    /// Determine if the given associated item type is relevant in the current context.
    fn is_relevant_kind_for_mode(&self, kind: ty::AssocKind) -> bool {
        match (self.mode, kind) {
            (Mode::MethodCall, ty::AssocKind::Fn { .. }) => true,
            (Mode::Path, ty::AssocKind::Const { .. } | ty::AssocKind::Fn { .. }) => true,
            _ => false,
        }
    }

    /// Determine if the associated item with the given DefId matches
    /// the desired name via a doc alias.
    fn matches_by_doc_alias(&self, def_id: DefId) -> bool {
        let Some(method) = self.method_name else {
            return false;
        };
        let Some(local_def_id) = def_id.as_local() else {
            return false;
        };
        let hir_id = self.fcx.tcx.local_def_id_to_hir_id(local_def_id);
        let attrs = self.fcx.tcx.hir_attrs(hir_id);

        if is_doc_alias_attrs_contain_symbol(attrs.into_iter(), method.name) {
            return true;
        }

        for attr in attrs {
            if attr.has_name(sym::rustc_confusables) {
                let Some(confusables) = attr.meta_item_list() else {
                    continue;
                };
                // #[rustc_confusables("foo", "bar"))]
                for n in confusables {
                    if let Some(lit) = n.lit()
                        && method.name == lit.symbol
                    {
                        return true;
                    }
                }
            }
        }
        false
    }

    /// Finds the method with the appropriate name (or return type, as the case may be). If
    /// `allow_similar_names` is set, find methods with close-matching names.
    // The length of the returned iterator is nearly always 0 or 1 and this
    // method is fairly hot.
    fn impl_or_trait_item(&self, def_id: DefId) -> SmallVec<[ty::AssocItem; 1]> {
        if let Some(name) = self.method_name {
            if self.allow_similar_names {
                let max_dist = max(name.as_str().len(), 3) / 3;
                self.tcx
                    .associated_items(def_id)
                    .in_definition_order()
                    .filter(|x| {
                        if !self.is_relevant_kind_for_mode(x.kind) {
                            return false;
                        }
                        if self.matches_by_doc_alias(x.def_id) {
                            return true;
                        }
                        match edit_distance_with_substrings(
                            name.as_str(),
                            x.name().as_str(),
                            max_dist,
                        ) {
                            Some(d) => d > 0,
                            None => false,
                        }
                    })
                    .copied()
                    .collect()
            } else {
                self.fcx
                    .associated_value(def_id, name)
                    .filter(|x| self.is_relevant_kind_for_mode(x.kind))
                    .map_or_else(SmallVec::new, |x| SmallVec::from_buf([x]))
            }
        } else {
            self.tcx
                .associated_items(def_id)
                .in_definition_order()
                .filter(|x| self.is_relevant_kind_for_mode(x.kind))
                .copied()
                .collect()
        }
    }
}

impl<'tcx> Candidate<'tcx> {
    fn to_unadjusted_pick(
        &self,
        self_ty: Ty<'tcx>,
        unstable_candidates: Vec<(Candidate<'tcx>, Symbol)>,
    ) -> Pick<'tcx> {
        Pick {
            item: self.item,
            kind: match self.kind {
                InherentImplCandidate { .. } => InherentImplPick,
                ObjectCandidate(_) => ObjectPick,
                TraitCandidate(_) => TraitPick,
                WhereClauseCandidate(trait_ref) => {
                    // Only trait derived from where-clauses should
                    // appear here, so they should not contain any
                    // inference variables or other artifacts. This
                    // means they are safe to put into the
                    // `WhereClausePick`.
                    assert!(
                        !trait_ref.skip_binder().args.has_infer()
                            && !trait_ref.skip_binder().args.has_placeholders()
                    );

                    WhereClausePick(trait_ref)
                }
            },
            import_ids: self.import_ids.clone(),
            autoderefs: 0,
            autoref_or_ptr_adjustment: None,
            self_ty,
            unstable_candidates,
            receiver_steps: match self.kind {
                InherentImplCandidate { receiver_steps, .. } => Some(receiver_steps),
                _ => None,
            },
            shadowed_candidates: vec![],
        }
    }
}