rustc_ty_utils/
ty.rs

1use rustc_data_structures::fx::FxHashSet;
2use rustc_hir as hir;
3use rustc_hir::LangItem;
4use rustc_hir::def::DefKind;
5use rustc_index::bit_set::DenseBitSet;
6use rustc_infer::infer::TyCtxtInferExt;
7use rustc_middle::bug;
8use rustc_middle::query::Providers;
9use rustc_middle::ty::{
10    self, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable, TypeVisitable, TypeVisitor, Upcast,
11    fold_regions,
12};
13use rustc_span::DUMMY_SP;
14use rustc_span::def_id::{CRATE_DEF_ID, DefId, LocalDefId};
15use rustc_trait_selection::traits;
16use tracing::instrument;
17
18#[instrument(level = "debug", skip(tcx), ret)]
19fn sized_constraint_for_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
20    match ty.kind() {
21        // these are always sized
22        ty::Bool
23        | ty::Char
24        | ty::Int(..)
25        | ty::Uint(..)
26        | ty::Float(..)
27        | ty::RawPtr(..)
28        | ty::Ref(..)
29        | ty::FnDef(..)
30        | ty::FnPtr(..)
31        | ty::Array(..)
32        | ty::Closure(..)
33        | ty::CoroutineClosure(..)
34        | ty::Coroutine(..)
35        | ty::CoroutineWitness(..)
36        | ty::Never
37        | ty::Dynamic(_, _, ty::DynStar) => None,
38
39        // these are never sized
40        ty::Str | ty::Slice(..) | ty::Dynamic(_, _, ty::Dyn) | ty::Foreign(..) => Some(ty),
41
42        ty::Pat(ty, _) => sized_constraint_for_ty(tcx, *ty),
43
44        ty::Tuple(tys) => tys.last().and_then(|&ty| sized_constraint_for_ty(tcx, ty)),
45
46        // recursive case
47        ty::Adt(adt, args) => adt.sized_constraint(tcx).and_then(|intermediate| {
48            let ty = intermediate.instantiate(tcx, args);
49            sized_constraint_for_ty(tcx, ty)
50        }),
51
52        // these can be sized or unsized.
53        ty::Param(..) | ty::Alias(..) | ty::Error(_) => Some(ty),
54
55        // We cannot instantiate the binder, so just return the *original* type back,
56        // but only if the inner type has a sized constraint. Thus we skip the binder,
57        // but don't actually use the result from `sized_constraint_for_ty`.
58        ty::UnsafeBinder(inner_ty) => {
59            sized_constraint_for_ty(tcx, inner_ty.skip_binder()).map(|_| ty)
60        }
61
62        ty::Placeholder(..) | ty::Bound(..) | ty::Infer(..) => {
63            bug!("unexpected type `{ty:?}` in sized_constraint_for_ty")
64        }
65    }
66}
67
68fn defaultness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Defaultness {
69    match tcx.hir_node_by_def_id(def_id) {
70        hir::Node::Item(hir::Item { kind: hir::ItemKind::Impl(impl_), .. }) => impl_.defaultness,
71        hir::Node::ImplItem(hir::ImplItem { defaultness, .. })
72        | hir::Node::TraitItem(hir::TraitItem { defaultness, .. }) => *defaultness,
73        node => {
74            bug!("`defaultness` called on {:?}", node);
75        }
76    }
77}
78
79/// Calculates the `Sized` constraint.
80///
81/// In fact, there are only a few options for the types in the constraint:
82///     - an obviously-unsized type
83///     - a type parameter or projection whose sizedness can't be known
84#[instrument(level = "debug", skip(tcx), ret)]
85fn adt_sized_constraint<'tcx>(
86    tcx: TyCtxt<'tcx>,
87    def_id: DefId,
88) -> Option<ty::EarlyBinder<'tcx, Ty<'tcx>>> {
89    if let Some(def_id) = def_id.as_local() {
90        if let ty::Representability::Infinite(_) = tcx.representability(def_id) {
91            return None;
92        }
93    }
94    let def = tcx.adt_def(def_id);
95
96    if !def.is_struct() {
97        bug!("`adt_sized_constraint` called on non-struct type: {def:?}");
98    }
99
100    let tail_def = def.non_enum_variant().tail_opt()?;
101    let tail_ty = tcx.type_of(tail_def.did).instantiate_identity();
102
103    let constraint_ty = sized_constraint_for_ty(tcx, tail_ty)?;
104
105    // perf hack: if there is a `constraint_ty: Sized` bound, then we know
106    // that the type is sized and do not need to check it on the impl.
107    let sized_trait_def_id = tcx.require_lang_item(LangItem::Sized, None);
108    let predicates = tcx.predicates_of(def.did()).predicates;
109    if predicates.iter().any(|(p, _)| {
110        p.as_trait_clause().is_some_and(|trait_pred| {
111            trait_pred.def_id() == sized_trait_def_id
112                && trait_pred.self_ty().skip_binder() == constraint_ty
113        })
114    }) {
115        return None;
116    }
117
118    Some(ty::EarlyBinder::bind(constraint_ty))
119}
120
121/// See `ParamEnv` struct definition for details.
122fn param_env(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
123    // Compute the bounds on Self and the type parameters.
124    let ty::InstantiatedPredicates { mut predicates, .. } =
125        tcx.predicates_of(def_id).instantiate_identity(tcx);
126
127    // Finally, we have to normalize the bounds in the environment, in
128    // case they contain any associated type projections. This process
129    // can yield errors if the put in illegal associated types, like
130    // `<i32 as Foo>::Bar` where `i32` does not implement `Foo`. We
131    // report these errors right here; this doesn't actually feel
132    // right to me, because constructing the environment feels like a
133    // kind of an "idempotent" action, but I'm not sure where would be
134    // a better place. In practice, we construct environments for
135    // every fn once during type checking, and we'll abort if there
136    // are any errors at that point, so outside of type inference you can be
137    // sure that this will succeed without errors anyway.
138
139    if tcx.def_kind(def_id) == DefKind::AssocFn
140        && let assoc_item = tcx.associated_item(def_id)
141        && assoc_item.container == ty::AssocItemContainer::Trait
142        && assoc_item.defaultness(tcx).has_value()
143    {
144        let sig = tcx.fn_sig(def_id).instantiate_identity();
145        // We accounted for the binder of the fn sig, so skip the binder.
146        sig.skip_binder().visit_with(&mut ImplTraitInTraitFinder {
147            tcx,
148            fn_def_id: def_id,
149            bound_vars: sig.bound_vars(),
150            predicates: &mut predicates,
151            seen: FxHashSet::default(),
152            depth: ty::INNERMOST,
153        });
154    }
155
156    // We extend the param-env of our item with the const conditions of the item,
157    // since we're allowed to assume `~const` bounds hold within the item itself.
158    if tcx.is_conditionally_const(def_id) {
159        predicates.extend(
160            tcx.const_conditions(def_id).instantiate_identity(tcx).into_iter().map(
161                |(trait_ref, _)| trait_ref.to_host_effect_clause(tcx, ty::BoundConstness::Maybe),
162            ),
163        );
164    }
165
166    let local_did = def_id.as_local();
167
168    let unnormalized_env = ty::ParamEnv::new(tcx.mk_clauses(&predicates));
169
170    let body_id = local_did.unwrap_or(CRATE_DEF_ID);
171    let cause = traits::ObligationCause::misc(tcx.def_span(def_id), body_id);
172    traits::normalize_param_env_or_error(tcx, unnormalized_env, cause)
173}
174
175/// Walk through a function type, gathering all RPITITs and installing a
176/// `NormalizesTo(Projection(RPITIT) -> Opaque(RPITIT))` predicate into the
177/// predicates list. This allows us to observe that an RPITIT projects to
178/// its corresponding opaque within the body of a default-body trait method.
179struct ImplTraitInTraitFinder<'a, 'tcx> {
180    tcx: TyCtxt<'tcx>,
181    predicates: &'a mut Vec<ty::Clause<'tcx>>,
182    fn_def_id: DefId,
183    bound_vars: &'tcx ty::List<ty::BoundVariableKind>,
184    seen: FxHashSet<DefId>,
185    depth: ty::DebruijnIndex,
186}
187
188impl<'tcx> TypeVisitor<TyCtxt<'tcx>> for ImplTraitInTraitFinder<'_, 'tcx> {
189    fn visit_binder<T: TypeFoldable<TyCtxt<'tcx>>>(&mut self, binder: &ty::Binder<'tcx, T>) {
190        self.depth.shift_in(1);
191        binder.super_visit_with(self);
192        self.depth.shift_out(1);
193    }
194
195    fn visit_ty(&mut self, ty: Ty<'tcx>) {
196        if let ty::Alias(ty::Projection, unshifted_alias_ty) = *ty.kind()
197            && let Some(
198                ty::ImplTraitInTraitData::Trait { fn_def_id, .. }
199                | ty::ImplTraitInTraitData::Impl { fn_def_id, .. },
200            ) = self.tcx.opt_rpitit_info(unshifted_alias_ty.def_id)
201            && fn_def_id == self.fn_def_id
202            && self.seen.insert(unshifted_alias_ty.def_id)
203        {
204            // We have entered some binders as we've walked into the
205            // bounds of the RPITIT. Shift these binders back out when
206            // constructing the top-level projection predicate.
207            let shifted_alias_ty = fold_regions(self.tcx, unshifted_alias_ty, |re, depth| {
208                if let ty::ReBound(index, bv) = re.kind() {
209                    if depth != ty::INNERMOST {
210                        return ty::Region::new_error_with_message(
211                            self.tcx,
212                            DUMMY_SP,
213                            "we shouldn't walk non-predicate binders with `impl Trait`...",
214                        );
215                    }
216                    ty::Region::new_bound(self.tcx, index.shifted_out_to_binder(self.depth), bv)
217                } else {
218                    re
219                }
220            });
221
222            // If we're lowering to associated item, install the opaque type which is just
223            // the `type_of` of the trait's associated item. If we're using the old lowering
224            // strategy, then just reinterpret the associated type like an opaque :^)
225            let default_ty = self
226                .tcx
227                .type_of(shifted_alias_ty.def_id)
228                .instantiate(self.tcx, shifted_alias_ty.args);
229
230            self.predicates.push(
231                ty::Binder::bind_with_vars(
232                    ty::ProjectionPredicate {
233                        projection_term: shifted_alias_ty.into(),
234                        term: default_ty.into(),
235                    },
236                    self.bound_vars,
237                )
238                .upcast(self.tcx),
239            );
240
241            // We walk the *un-shifted* alias ty, because we're tracking the de bruijn
242            // binder depth, and if we were to walk `shifted_alias_ty` instead, we'd
243            // have to reset `self.depth` back to `ty::INNERMOST` or something. It's
244            // easier to just do this.
245            for bound in self
246                .tcx
247                .item_bounds(unshifted_alias_ty.def_id)
248                .iter_instantiated(self.tcx, unshifted_alias_ty.args)
249            {
250                bound.visit_with(self);
251            }
252        }
253
254        ty.super_visit_with(self)
255    }
256}
257
258fn param_env_normalized_for_post_analysis(tcx: TyCtxt<'_>, def_id: DefId) -> ty::ParamEnv<'_> {
259    // This is a bit ugly but the easiest way to avoid code duplication.
260    let typing_env = ty::TypingEnv::non_body_analysis(tcx, def_id);
261    typing_env.with_post_analysis_normalized(tcx).param_env
262}
263
264/// Check if a function is async.
265fn asyncness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Asyncness {
266    let node = tcx.hir_node_by_def_id(def_id);
267    node.fn_sig().map_or(ty::Asyncness::No, |sig| match sig.header.asyncness {
268        hir::IsAsync::Async(_) => ty::Asyncness::Yes,
269        hir::IsAsync::NotAsync => ty::Asyncness::No,
270    })
271}
272
273fn unsizing_params_for_adt<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> DenseBitSet<u32> {
274    let def = tcx.adt_def(def_id);
275    let num_params = tcx.generics_of(def_id).count();
276
277    let maybe_unsizing_param_idx = |arg: ty::GenericArg<'tcx>| match arg.kind() {
278        ty::GenericArgKind::Type(ty) => match ty.kind() {
279            ty::Param(p) => Some(p.index),
280            _ => None,
281        },
282
283        // We can't unsize a lifetime
284        ty::GenericArgKind::Lifetime(_) => None,
285
286        ty::GenericArgKind::Const(ct) => match ct.kind() {
287            ty::ConstKind::Param(p) => Some(p.index),
288            _ => None,
289        },
290    };
291
292    // The last field of the structure has to exist and contain type/const parameters.
293    let Some((tail_field, prefix_fields)) = def.non_enum_variant().fields.raw.split_last() else {
294        return DenseBitSet::new_empty(num_params);
295    };
296
297    let mut unsizing_params = DenseBitSet::new_empty(num_params);
298    for arg in tcx.type_of(tail_field.did).instantiate_identity().walk() {
299        if let Some(i) = maybe_unsizing_param_idx(arg) {
300            unsizing_params.insert(i);
301        }
302    }
303
304    // Ensure none of the other fields mention the parameters used
305    // in unsizing.
306    for field in prefix_fields {
307        for arg in tcx.type_of(field.did).instantiate_identity().walk() {
308            if let Some(i) = maybe_unsizing_param_idx(arg) {
309                unsizing_params.remove(i);
310            }
311        }
312    }
313
314    unsizing_params
315}
316
317fn impl_self_is_guaranteed_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_def_id: DefId) -> bool {
318    debug_assert_eq!(tcx.def_kind(impl_def_id), DefKind::Impl { of_trait: true });
319
320    let infcx = tcx.infer_ctxt().ignoring_regions().build(ty::TypingMode::non_body_analysis());
321
322    let ocx = traits::ObligationCtxt::new_with_diagnostics(&infcx);
323    let cause = traits::ObligationCause::dummy();
324    let param_env = tcx.param_env(impl_def_id);
325
326    let tail = tcx.struct_tail_raw(
327        tcx.type_of(impl_def_id).instantiate_identity(),
328        |ty| {
329            ocx.structurally_normalize_ty(&cause, param_env, ty).unwrap_or_else(|_| {
330                Ty::new_error_with_message(
331                    tcx,
332                    tcx.def_span(impl_def_id),
333                    "struct tail should be computable",
334                )
335            })
336        },
337        || (),
338    );
339
340    match tail.kind() {
341        ty::Dynamic(_, _, ty::Dyn) | ty::Slice(_) | ty::Str => true,
342        ty::Bool
343        | ty::Char
344        | ty::Int(_)
345        | ty::Uint(_)
346        | ty::Float(_)
347        | ty::Adt(_, _)
348        | ty::Foreign(_)
349        | ty::Array(_, _)
350        | ty::Pat(_, _)
351        | ty::RawPtr(_, _)
352        | ty::Ref(_, _, _)
353        | ty::FnDef(_, _)
354        | ty::FnPtr(_, _)
355        | ty::UnsafeBinder(_)
356        | ty::Closure(_, _)
357        | ty::CoroutineClosure(_, _)
358        | ty::Coroutine(_, _)
359        | ty::CoroutineWitness(_, _)
360        | ty::Never
361        | ty::Tuple(_)
362        | ty::Alias(_, _)
363        | ty::Param(_)
364        | ty::Bound(_, _)
365        | ty::Placeholder(_)
366        | ty::Infer(_)
367        | ty::Error(_)
368        | ty::Dynamic(_, _, ty::DynStar) => false,
369    }
370}
371
372pub(crate) fn provide(providers: &mut Providers) {
373    *providers = Providers {
374        asyncness,
375        adt_sized_constraint,
376        param_env,
377        param_env_normalized_for_post_analysis,
378        defaultness,
379        unsizing_params_for_adt,
380        impl_self_is_guaranteed_unsized,
381        ..*providers
382    };
383}
rustc_ty_utils/ty.rs

rustc_ty_utils/
ty.rs
