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 = self.structurally_resolve_type(autoderef.span(), autoderef.final_ty());
113        debug!(
114            "try_index_step(expr={:?}, base_expr={:?}, adjusted_ty={:?}, \
115             index_ty={:?})",
116            expr, base_expr, adjusted_ty, index_ty
117        );
118
119        if let hir::ExprKind::Unary(
120            hir::UnOp::Neg,
121            hir::Expr {
122                kind: hir::ExprKind::Lit(hir::Lit { node: ast::LitKind::Int(..), .. }),
123                ..
124            },
125        ) = index_expr.kind
126        {
127            match adjusted_ty.kind() {
128                ty::Adt(def, _) if self.tcx.is_diagnostic_item(sym::Vec, def.did()) => {
129                    return self.negative_index(adjusted_ty, index_expr.span, base_expr);
130                }
131                ty::Slice(_) | ty::Array(_, _) => {
132                    return self.negative_index(adjusted_ty, index_expr.span, base_expr);
133                }
134                _ => {}
135            }
136        }
137
138        for unsize in [false, true] {
139            let mut self_ty = adjusted_ty;
140            if unsize {
141                // We only unsize arrays here.
142                if let ty::Array(element_ty, ct) = *adjusted_ty.kind() {
143                    self.register_predicate(Obligation::new(
144                        self.tcx,
145                        self.cause(base_expr.span, ObligationCauseCode::ArrayLen(adjusted_ty)),
146                        self.param_env,
147                        ty::ClauseKind::ConstArgHasType(ct, self.tcx.types.usize),
148                    ));
149                    self_ty = Ty::new_slice(self.tcx, element_ty);
150                } else {
151                    continue;
152                }
153            }
154
155            // If some lookup succeeds, write callee into table and extract index/element
156            // type from the method signature.
157            // If some lookup succeeded, install method in table
158            let input_ty = self.next_ty_var(base_expr.span);
159            let method =
160                self.try_overloaded_place_op(expr.span, self_ty, Some(input_ty), PlaceOp::Index);
161
162            if let Some(result) = method {
163                debug!("try_index_step: success, using overloaded indexing");
164                let method = self.register_infer_ok_obligations(result);
165
166                let mut adjustments = self.adjust_steps(autoderef);
167                if let ty::Ref(region, _, hir::Mutability::Not) = method.sig.inputs()[0].kind() {
168                    adjustments.push(Adjustment {
169                        kind: Adjust::Borrow(AutoBorrow::Ref(AutoBorrowMutability::Not)),
170                        target: Ty::new_imm_ref(self.tcx, *region, adjusted_ty),
171                    });
172                } else {
173                    span_bug!(expr.span, "input to index is not a ref?");
174                }
175                if unsize {
176                    adjustments.push(Adjustment {
177                        kind: Adjust::Pointer(PointerCoercion::Unsize),
178                        target: method.sig.inputs()[0],
179                    });
180                }
181                self.apply_adjustments(base_expr, adjustments);
182
183                self.write_method_call_and_enforce_effects(expr.hir_id, expr.span, method);
184
185                return Some((input_ty, self.make_overloaded_place_return_type(method)));
186            }
187        }
188
189        None
190    }
191
192    /// Try to resolve an overloaded place op. We only deal with the immutable
193    /// variant here (Deref/Index). In some contexts we would need the mutable
194    /// variant (DerefMut/IndexMut); those would be later converted by
195    /// `convert_place_derefs_to_mutable`.
196    pub(super) fn try_overloaded_place_op(
197        &self,
198        span: Span,
199        base_ty: Ty<'tcx>,
200        opt_rhs_ty: Option<Ty<'tcx>>,
201        op: PlaceOp,
202    ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
203        debug!("try_overloaded_place_op({:?},{:?},{:?})", span, base_ty, op);
204
205        let (Some(imm_tr), imm_op) = (match op {
206            PlaceOp::Deref => (self.tcx.lang_items().deref_trait(), sym::deref),
207            PlaceOp::Index => (self.tcx.lang_items().index_trait(), sym::index),
208        }) else {
209            // Bail if `Deref` or `Index` isn't defined.
210            return None;
211        };
212
213        self.lookup_method_for_operator(self.misc(span), imm_op, imm_tr, base_ty, opt_rhs_ty)
214    }
215
216    fn try_mutable_overloaded_place_op(
217        &self,
218        span: Span,
219        base_ty: Ty<'tcx>,
220        opt_rhs_ty: Option<Ty<'tcx>>,
221        op: PlaceOp,
222    ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
223        debug!("try_mutable_overloaded_place_op({:?},{:?},{:?})", span, base_ty, op);
224
225        let (Some(mut_tr), mut_op) = (match op {
226            PlaceOp::Deref => (self.tcx.lang_items().deref_mut_trait(), sym::deref_mut),
227            PlaceOp::Index => (self.tcx.lang_items().index_mut_trait(), sym::index_mut),
228        }) else {
229            // Bail if `DerefMut` or `IndexMut` isn't defined.
230            return None;
231        };
232
233        self.lookup_method_for_operator(self.misc(span), mut_op, mut_tr, base_ty, opt_rhs_ty)
234    }
235
236    /// Convert auto-derefs, indices, etc of an expression from `Deref` and `Index`
237    /// into `DerefMut` and `IndexMut` respectively.
238    ///
239    /// This is a second pass of typechecking derefs/indices. We need this because we do not
240    /// always know whether a place needs to be mutable or not in the first pass.
241    /// This happens whether there is an implicit mutable reborrow, e.g. when the type
242    /// is used as the receiver of a method call.
243    pub(crate) fn convert_place_derefs_to_mutable(&self, expr: &hir::Expr<'_>) {
244        // Gather up expressions we want to munge.
245        let mut exprs = vec![expr];
246
247        while let hir::ExprKind::Field(expr, _)
248        | hir::ExprKind::Index(expr, _, _)
249        | hir::ExprKind::Unary(hir::UnOp::Deref, expr) = exprs.last().unwrap().kind
250        {
251            exprs.push(expr);
252        }
253
254        debug!("convert_place_derefs_to_mutable: exprs={:?}", exprs);
255
256        // Fix up autoderefs and derefs.
257        let mut inside_union = false;
258        for (i, &expr) in exprs.iter().rev().enumerate() {
259            debug!("convert_place_derefs_to_mutable: i={} expr={:?}", i, expr);
260
261            let mut source = self.node_ty(expr.hir_id);
262            if matches!(expr.kind, hir::ExprKind::Unary(hir::UnOp::Deref, _)) {
263                // Clear previous flag; after a pointer indirection it does not apply any more.
264                inside_union = false;
265            }
266            if source.is_union() {
267                inside_union = true;
268            }
269            // Fix up the autoderefs. Autorefs can only occur immediately preceding
270            // overloaded place ops, and will be fixed by them in order to get
271            // the correct region.
272            // Do not mutate adjustments in place, but rather take them,
273            // and replace them after mutating them, to avoid having the
274            // typeck results borrowed during (`deref_mut`) method resolution.
275            let previous_adjustments =
276                self.typeck_results.borrow_mut().adjustments_mut().remove(expr.hir_id);
277            if let Some(mut adjustments) = previous_adjustments {
278                for adjustment in &mut adjustments {
279                    if let Adjust::Deref(Some(ref mut deref)) = adjustment.kind
280                        && let Some(ok) = self.try_mutable_overloaded_place_op(
281                            expr.span,
282                            source,
283                            None,
284                            PlaceOp::Deref,
285                        )
286                    {
287                        let method = self.register_infer_ok_obligations(ok);
288                        let ty::Ref(_, _, mutbl) = *method.sig.output().kind() else {
289                            span_bug!(
290                                self.tcx.def_span(method.def_id),
291                                "expected DerefMut to return a &mut"
292                            );
293                        };
294                        *deref = OverloadedDeref { mutbl, span: deref.span };
295                        self.enforce_context_effects(None, expr.span, method.def_id, method.args);
296                        // If this is a union field, also throw an error for `DerefMut` of `ManuallyDrop` (see RFC 2514).
297                        // This helps avoid accidental drops.
298                        if inside_union
299                            && source.ty_adt_def().is_some_and(|adt| adt.is_manually_drop())
300                        {
301                            self.dcx().struct_span_err(
302                                expr.span,
303                                "not automatically applying `DerefMut` on `ManuallyDrop` union field",
304                            )
305                            .with_help(
306                                "writing to this reference calls the destructor for the old value",
307                            )
308                            .with_help("add an explicit `*` if that is desired, or call `ptr::write` to not run the destructor")
309                            .emit();
310                        }
311                    }
312                    source = adjustment.target;
313                }
314                self.typeck_results.borrow_mut().adjustments_mut().insert(expr.hir_id, adjustments);
315            }
316
317            match expr.kind {
318                hir::ExprKind::Index(base_expr, ..) => {
319                    self.convert_place_op_to_mutable(PlaceOp::Index, expr, base_expr);
320                }
321                hir::ExprKind::Unary(hir::UnOp::Deref, base_expr) => {
322                    self.convert_place_op_to_mutable(PlaceOp::Deref, expr, base_expr);
323                }
324                _ => {}
325            }
326        }
327    }
328
329    fn convert_place_op_to_mutable(
330        &self,
331        op: PlaceOp,
332        expr: &hir::Expr<'_>,
333        base_expr: &hir::Expr<'_>,
334    ) {
335        debug!("convert_place_op_to_mutable({:?}, {:?}, {:?})", op, expr, base_expr);
336        if !self.typeck_results.borrow().is_method_call(expr) {
337            debug!("convert_place_op_to_mutable - builtin, nothing to do");
338            return;
339        }
340
341        // Need to deref because overloaded place ops take self by-reference.
342        let base_ty = self
343            .typeck_results
344            .borrow()
345            .expr_ty_adjusted(base_expr)
346            .builtin_deref(false)
347            .expect("place op takes something that is not a ref");
348
349        let arg_ty = match op {
350            PlaceOp::Deref => None,
351            PlaceOp::Index => {
352                // We would need to recover the `T` used when we resolve `<_ as Index<T>>::index`
353                // in try_index_step. This is the arg at index 1.
354                //
355                // Note: we should *not* use `expr_ty` of index_expr here because autoderef
356                // during coercions can cause type of index_expr to differ from `T` (#72002).
357                // We also could not use `expr_ty_adjusted` of index_expr because reborrowing
358                // during coercions can also cause type of index_expr to differ from `T`,
359                // which can potentially cause regionck failure (#74933).
360                Some(self.typeck_results.borrow().node_args(expr.hir_id).type_at(1))
361            }
362        };
363        let method = self.try_mutable_overloaded_place_op(expr.span, base_ty, arg_ty, op);
364        let method = match method {
365            Some(ok) => self.register_infer_ok_obligations(ok),
366            // Couldn't find the mutable variant of the place op, keep the
367            // current, immutable version.
368            None => return,
369        };
370        debug!("convert_place_op_to_mutable: method={:?}", method);
371        self.write_method_call_and_enforce_effects(expr.hir_id, expr.span, method);
372
373        let ty::Ref(region, _, hir::Mutability::Mut) = method.sig.inputs()[0].kind() else {
374            span_bug!(expr.span, "input to mutable place op is not a mut ref?");
375        };
376
377        // Convert the autoref in the base expr to mutable with the correct
378        // region and mutability.
379        let base_expr_ty = self.node_ty(base_expr.hir_id);
380        if let Some(adjustments) =
381            self.typeck_results.borrow_mut().adjustments_mut().get_mut(base_expr.hir_id)
382        {
383            let mut source = base_expr_ty;
384            for adjustment in &mut adjustments[..] {
385                if let Adjust::Borrow(AutoBorrow::Ref(..)) = adjustment.kind {
386                    debug!("convert_place_op_to_mutable: converting autoref {:?}", adjustment);
387                    let mutbl = AutoBorrowMutability::Mut {
388                        // Deref/indexing can be desugared to a method call,
389                        // so maybe we could use two-phase here.
390                        // See the documentation of AllowTwoPhase for why that's
391                        // not the case today.
392                        allow_two_phase_borrow: AllowTwoPhase::No,
393                    };
394                    adjustment.kind = Adjust::Borrow(AutoBorrow::Ref(mutbl));
395                    adjustment.target = Ty::new_ref(self.tcx, *region, source, mutbl.into());
396                }
397                source = adjustment.target;
398            }
399
400            // If we have an autoref followed by unsizing at the end, fix the unsize target.
401            if let [
402                ..,
403                Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(..)), .. },
404                Adjustment { kind: Adjust::Pointer(PointerCoercion::Unsize), ref mut target },
405            ] = adjustments[..]
406            {
407                *target = method.sig.inputs()[0];
408            }
409        }
410    }
411}
rustc_hir_typeck/place_op.rs

rustc_hir_typeck/
place_op.rs
