rustc_hir_typeck/
place_op.rs

1use rustc_errors::Applicability;
2use rustc_hir_analysis::autoderef::Autoderef;
3use rustc_infer::infer::InferOk;
4use rustc_infer::traits::{Obligation, ObligationCauseCode};
5use rustc_middle::span_bug;
6use rustc_middle::ty::adjustment::{
7    Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, OverloadedDeref,
8    PointerCoercion,
9};
10use rustc_middle::ty::{self, Ty};
11use rustc_span::{Span, sym};
12use tracing::debug;
13use {rustc_ast as ast, rustc_hir as hir};
14
15use crate::method::MethodCallee;
16use crate::{FnCtxt, PlaceOp};
17
18impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
19    /// Type-check `*oprnd_expr` with `oprnd_expr` type-checked already.
20    pub(super) fn lookup_derefing(
21        &self,
22        expr: &hir::Expr<'_>,
23        oprnd_expr: &'tcx hir::Expr<'tcx>,
24        oprnd_ty: Ty<'tcx>,
25    ) -> Option<Ty<'tcx>> {
26        if let Some(ty) = oprnd_ty.builtin_deref(true) {
27            return Some(ty);
28        }
29
30        let ok = self.try_overloaded_deref(expr.span, oprnd_ty)?;
31        let method = self.register_infer_ok_obligations(ok);
32        if let ty::Ref(_, _, hir::Mutability::Not) = method.sig.inputs()[0].kind() {
33            self.apply_adjustments(
34                oprnd_expr,
35                vec![Adjustment {
36                    kind: Adjust::Borrow(AutoBorrow::Ref(AutoBorrowMutability::Not)),
37                    target: method.sig.inputs()[0],
38                }],
39            );
40        } else {
41            span_bug!(expr.span, "input to deref is not a ref?");
42        }
43        let ty = self.make_overloaded_place_return_type(method);
44        self.write_method_call_and_enforce_effects(expr.hir_id, expr.span, method);
45        Some(ty)
46    }
47
48    /// Type-check `*base_expr[index_expr]` with `base_expr` and `index_expr` type-checked already.
49    pub(super) fn lookup_indexing(
50        &self,
51        expr: &hir::Expr<'_>,
52        base_expr: &'tcx hir::Expr<'tcx>,
53        base_ty: Ty<'tcx>,
54        index_expr: &'tcx hir::Expr<'tcx>,
55        idx_ty: Ty<'tcx>,
56    ) -> Option<(/*index type*/ Ty<'tcx>, /*element type*/ Ty<'tcx>)> {
57        // FIXME(#18741) -- this is almost but not quite the same as the
58        // autoderef that normal method probing does. They could likely be
59        // consolidated.
60
61        let mut autoderef = self.autoderef(base_expr.span, base_ty);
62        let mut result = None;
63        while result.is_none() && autoderef.next().is_some() {
64            result = self.try_index_step(expr, base_expr, &autoderef, idx_ty, index_expr);
65        }
66        self.register_predicates(autoderef.into_obligations());
67        result
68    }
69
70    fn negative_index(
71        &self,
72        ty: Ty<'tcx>,
73        span: Span,
74        base_expr: &hir::Expr<'_>,
75    ) -> Option<(Ty<'tcx>, Ty<'tcx>)> {
76        let ty = self.resolve_vars_if_possible(ty);
77        let mut err = self.dcx().struct_span_err(
78            span,
79            format!("negative integers cannot be used to index on a `{ty}`"),
80        );
81        err.span_label(span, format!("cannot use a negative integer for indexing on `{ty}`"));
82        if let (hir::ExprKind::Path(..), Ok(snippet)) =
83            (&base_expr.kind, self.tcx.sess.source_map().span_to_snippet(base_expr.span))
84        {
85            // `foo[-1]` to `foo[foo.len() - 1]`
86            err.span_suggestion_verbose(
87                span.shrink_to_lo(),
88                format!(
89                    "to access an element starting from the end of the `{ty}`, compute the index",
90                ),
91                format!("{snippet}.len() "),
92                Applicability::MachineApplicable,
93            );
94        }
95        let reported = err.emit();
96        Some((Ty::new_error(self.tcx, reported), Ty::new_error(self.tcx, reported)))
97    }
98
99    /// To type-check `base_expr[index_expr]`, we progressively autoderef
100    /// (and otherwise adjust) `base_expr`, looking for a type which either
101    /// supports builtin indexing or overloaded indexing.
102    /// This loop implements one step in that search; the autoderef loop
103    /// is implemented by `lookup_indexing`.
104    fn try_index_step(
105        &self,
106        expr: &hir::Expr<'_>,
107        base_expr: &hir::Expr<'_>,
108        autoderef: &Autoderef<'a, 'tcx>,
109        index_ty: Ty<'tcx>,
110        index_expr: &hir::Expr<'_>,
111    ) -> Option<(/*index type*/ Ty<'tcx>, /*element type*/ Ty<'tcx>)> {
112        let adjusted_ty =
113            self.structurally_resolve_type(autoderef.span(), autoderef.final_ty(false));
114        debug!(
115            "try_index_step(expr={:?}, base_expr={:?}, adjusted_ty={:?}, \
116             index_ty={:?})",
117            expr, base_expr, adjusted_ty, index_ty
118        );
119
120        if let hir::ExprKind::Unary(
121            hir::UnOp::Neg,
122            hir::Expr {
123                kind: hir::ExprKind::Lit(hir::Lit { node: ast::LitKind::Int(..), .. }),
124                ..
125            },
126        ) = index_expr.kind
127        {
128            match adjusted_ty.kind() {
129                ty::Adt(def, _) if self.tcx.is_diagnostic_item(sym::Vec, def.did()) => {
130                    return self.negative_index(adjusted_ty, index_expr.span, base_expr);
131                }
132                ty::Slice(_) | ty::Array(_, _) => {
133                    return self.negative_index(adjusted_ty, index_expr.span, base_expr);
134                }
135                _ => {}
136            }
137        }
138
139        for unsize in [false, true] {
140            let mut self_ty = adjusted_ty;
141            if unsize {
142                // We only unsize arrays here.
143                if let ty::Array(element_ty, ct) = *adjusted_ty.kind() {
144                    self.register_predicate(Obligation::new(
145                        self.tcx,
146                        self.cause(base_expr.span, ObligationCauseCode::ArrayLen(adjusted_ty)),
147                        self.param_env,
148                        ty::ClauseKind::ConstArgHasType(ct, self.tcx.types.usize),
149                    ));
150                    self_ty = Ty::new_slice(self.tcx, element_ty);
151                } else {
152                    continue;
153                }
154            }
155
156            // If some lookup succeeds, write callee into table and extract index/element
157            // type from the method signature.
158            // If some lookup succeeded, install method in table
159            let input_ty = self.next_ty_var(base_expr.span);
160            let method =
161                self.try_overloaded_place_op(expr.span, self_ty, Some(input_ty), PlaceOp::Index);
162
163            if let Some(result) = method {
164                debug!("try_index_step: success, using overloaded indexing");
165                let method = self.register_infer_ok_obligations(result);
166
167                let mut adjustments = self.adjust_steps(autoderef);
168                if let ty::Ref(region, _, hir::Mutability::Not) = method.sig.inputs()[0].kind() {
169                    adjustments.push(Adjustment {
170                        kind: Adjust::Borrow(AutoBorrow::Ref(AutoBorrowMutability::Not)),
171                        target: Ty::new_imm_ref(self.tcx, *region, adjusted_ty),
172                    });
173                } else {
174                    span_bug!(expr.span, "input to index is not a ref?");
175                }
176                if unsize {
177                    adjustments.push(Adjustment {
178                        kind: Adjust::Pointer(PointerCoercion::Unsize),
179                        target: method.sig.inputs()[0],
180                    });
181                }
182                self.apply_adjustments(base_expr, adjustments);
183
184                self.write_method_call_and_enforce_effects(expr.hir_id, expr.span, method);
185
186                return Some((input_ty, self.make_overloaded_place_return_type(method)));
187            }
188        }
189
190        None
191    }
192
193    /// Try to resolve an overloaded place op. We only deal with the immutable
194    /// variant here (Deref/Index). In some contexts we would need the mutable
195    /// variant (DerefMut/IndexMut); those would be later converted by
196    /// `convert_place_derefs_to_mutable`.
197    pub(super) fn try_overloaded_place_op(
198        &self,
199        span: Span,
200        base_ty: Ty<'tcx>,
201        opt_rhs_ty: Option<Ty<'tcx>>,
202        op: PlaceOp,
203    ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
204        debug!("try_overloaded_place_op({:?},{:?},{:?})", span, base_ty, op);
205
206        let (Some(imm_tr), imm_op) = (match op {
207            PlaceOp::Deref => (self.tcx.lang_items().deref_trait(), sym::deref),
208            PlaceOp::Index => (self.tcx.lang_items().index_trait(), sym::index),
209        }) else {
210            // Bail if `Deref` or `Index` isn't defined.
211            return None;
212        };
213
214        self.lookup_method_for_operator(self.misc(span), imm_op, imm_tr, base_ty, opt_rhs_ty)
215    }
216
217    fn try_mutable_overloaded_place_op(
218        &self,
219        span: Span,
220        base_ty: Ty<'tcx>,
221        opt_rhs_ty: Option<Ty<'tcx>>,
222        op: PlaceOp,
223    ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
224        debug!("try_mutable_overloaded_place_op({:?},{:?},{:?})", span, base_ty, op);
225
226        let (Some(mut_tr), mut_op) = (match op {
227            PlaceOp::Deref => (self.tcx.lang_items().deref_mut_trait(), sym::deref_mut),
228            PlaceOp::Index => (self.tcx.lang_items().index_mut_trait(), sym::index_mut),
229        }) else {
230            // Bail if `DerefMut` or `IndexMut` isn't defined.
231            return None;
232        };
233
234        self.lookup_method_for_operator(self.misc(span), mut_op, mut_tr, base_ty, opt_rhs_ty)
235    }
236
237    /// Convert auto-derefs, indices, etc of an expression from `Deref` and `Index`
238    /// into `DerefMut` and `IndexMut` respectively.
239    ///
240    /// This is a second pass of typechecking derefs/indices. We need this because we do not
241    /// always know whether a place needs to be mutable or not in the first pass.
242    /// This happens whether there is an implicit mutable reborrow, e.g. when the type
243    /// is used as the receiver of a method call.
244    pub(crate) fn convert_place_derefs_to_mutable(&self, expr: &hir::Expr<'_>) {
245        // Gather up expressions we want to munge.
246        let mut exprs = vec![expr];
247
248        while let hir::ExprKind::Field(expr, _)
249        | hir::ExprKind::Index(expr, _, _)
250        | hir::ExprKind::Unary(hir::UnOp::Deref, expr) = exprs.last().unwrap().kind
251        {
252            exprs.push(expr);
253        }
254
255        debug!("convert_place_derefs_to_mutable: exprs={:?}", exprs);
256
257        // Fix up autoderefs and derefs.
258        let mut inside_union = false;
259        for (i, &expr) in exprs.iter().rev().enumerate() {
260            debug!("convert_place_derefs_to_mutable: i={} expr={:?}", i, expr);
261
262            let mut source = self.node_ty(expr.hir_id);
263            if matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Deref, _)) {
264                // Clear previous flag; after a pointer indirection it does not apply any more.
265                inside_union = false;
266            }
267            if source.is_union() {
268                inside_union = true;
269            }
270            // Fix up the autoderefs. Autorefs can only occur immediately preceding
271            // overloaded place ops, and will be fixed by them in order to get
272            // the correct region.
273            // Do not mutate adjustments in place, but rather take them,
274            // and replace them after mutating them, to avoid having the
275            // typeck results borrowed during (`deref_mut`) method resolution.
276            let previous_adjustments =
277                self.typeck_results.borrow_mut().adjustments_mut().remove(expr.hir_id);
278            if let Some(mut adjustments) = previous_adjustments {
279                for adjustment in &mut adjustments {
280                    if let Adjust::Deref(Some(ref mut deref)) = adjustment.kind
281                        && let Some(ok) = self.try_mutable_overloaded_place_op(
282                            expr.span,
283                            source,
284                            None,
285                            PlaceOp::Deref,
286                        )
287                    {
288                        let method = self.register_infer_ok_obligations(ok);
289                        let ty::Ref(_, _, mutbl) = *method.sig.output().kind() else {
290                            span_bug!(
291                                self.tcx.def_span(method.def_id),
292                                "expected DerefMut to return a &mut"
293                            );
294                        };
295                        *deref = OverloadedDeref { mutbl, span: deref.span };
296                        self.enforce_context_effects(None, expr.span, method.def_id, method.args);
297                        // If this is a union field, also throw an error for `DerefMut` of `ManuallyDrop` (see RFC 2514).
298                        // This helps avoid accidental drops.
299                        if inside_union
300                            && source.ty_adt_def().is_some_and(|adt| adt.is_manually_drop())
301                        {
302                            self.dcx().struct_span_err(
303                                expr.span,
304                                "not automatically applying `DerefMut` on `ManuallyDrop` union field",
305                            )
306                            .with_help(
307                                "writing to this reference calls the destructor for the old value",
308                            )
309                            .with_help("add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor")
310                            .emit();
311                        }
312                    }
313                    source = adjustment.target;
314                }
315                self.typeck_results.borrow_mut().adjustments_mut().insert(expr.hir_id, adjustments);
316            }
317
318            match expr.kind {
319                hir::ExprKind::Index(base_expr, ..) => {
320                    self.convert_place_op_to_mutable(PlaceOp::Index, expr, base_expr);
321                }
322                hir::ExprKind::Unary(hir::UnOp::Deref, base_expr) => {
323                    self.convert_place_op_to_mutable(PlaceOp::Deref, expr, base_expr);
324                }
325                _ => {}
326            }
327        }
328    }
329
330    fn convert_place_op_to_mutable(
331        &self,
332        op: PlaceOp,
333        expr: &hir::Expr<'_>,
334        base_expr: &hir::Expr<'_>,
335    ) {
336        debug!("convert_place_op_to_mutable({:?}, {:?}, {:?})", op, expr, base_expr);
337        if !self.typeck_results.borrow().is_method_call(expr) {
338            debug!("convert_place_op_to_mutable - builtin, nothing to do");
339            return;
340        }
341
342        // Need to deref because overloaded place ops take self by-reference.
343        let base_ty = self
344            .typeck_results
345            .borrow()
346            .expr_ty_adjusted(base_expr)
347            .builtin_deref(false)
348            .expect("place op takes something that is not a ref");
349
350        let arg_ty = match op {
351            PlaceOp::Deref => None,
352            PlaceOp::Index => {
353                // We would need to recover the `T` used when we resolve `<_ as Index<T>>::index`
354                // in try_index_step. This is the arg at index 1.
355                //
356                // Note: we should *not* use `expr_ty` of index_expr here because autoderef
357                // during coercions can cause type of index_expr to differ from `T` (#72002).
358                // We also could not use `expr_ty_adjusted` of index_expr because reborrowing
359                // during coercions can also cause type of index_expr to differ from `T`,
360                // which can potentially cause regionck failure (#74933).
361                Some(self.typeck_results.borrow().node_args(expr.hir_id).type_at(1))
362            }
363        };
364        let method = self.try_mutable_overloaded_place_op(expr.span, base_ty, arg_ty, op);
365        let method = match method {
366            Some(ok) => self.register_infer_ok_obligations(ok),
367            // Couldn't find the mutable variant of the place op, keep the
368            // current, immutable version.
369            None => return,
370        };
371        debug!("convert_place_op_to_mutable: method={:?}", method);
372        self.write_method_call_and_enforce_effects(expr.hir_id, expr.span, method);
373
374        let ty::Ref(region, _, hir::Mutability::Mut) = method.sig.inputs()[0].kind() else {
375            span_bug!(expr.span, "input to mutable place op is not a mut ref?");
376        };
377
378        // Convert the autoref in the base expr to mutable with the correct
379        // region and mutability.
380        let base_expr_ty = self.node_ty(base_expr.hir_id);
381        if let Some(adjustments) =
382            self.typeck_results.borrow_mut().adjustments_mut().get_mut(base_expr.hir_id)
383        {
384            let mut source = base_expr_ty;
385            for adjustment in &mut adjustments[..] {
386                if let Adjust::Borrow(AutoBorrow::Ref(..)) = adjustment.kind {
387                    debug!("convert_place_op_to_mutable: converting autoref {:?}", adjustment);
388                    let mutbl = AutoBorrowMutability::Mut {
389                        // Deref/indexing can be desugared to a method call,
390                        // so maybe we could use two-phase here.
391                        // See the documentation of AllowTwoPhase for why that's
392                        // not the case today.
393                        allow_two_phase_borrow: AllowTwoPhase::No,
394                    };
395                    adjustment.kind = Adjust::Borrow(AutoBorrow::Ref(mutbl));
396                    adjustment.target = Ty::new_ref(self.tcx, *region, source, mutbl.into());
397                }
398                source = adjustment.target;
399            }
400
401            // If we have an autoref followed by unsizing at the end, fix the unsize target.
402            if let [
403                ..,
404                Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(..)), .. },
405                Adjustment { kind: Adjust::Pointer(PointerCoercion::Unsize), ref mut target },
406            ] = adjustments[..]
407            {
408                *target = method.sig.inputs()[0];
409            }
410        }
411    }
412}
rustc_hir_typeck/place_op.rs

rustc_hir_typeck/
place_op.rs
