rustc_hir_typeck/
coercion.rs

1//! # Type Coercion
2//!
3//! Under certain circumstances we will coerce from one type to another,
4//! for example by auto-borrowing. This occurs in situations where the
5//! compiler has a firm 'expected type' that was supplied from the user,
6//! and where the actual type is similar to that expected type in purpose
7//! but not in representation (so actual subtyping is inappropriate).
8//!
9//! ## Reborrowing
10//!
11//! Note that if we are expecting a reference, we will *reborrow*
12//! even if the argument provided was already a reference. This is
13//! useful for freezing mut things (that is, when the expected type is &T
14//! but you have &mut T) and also for avoiding the linearity
15//! of mut things (when the expected is &mut T and you have &mut T). See
16//! the various `tests/ui/coerce/*.rs` tests for
17//! examples of where this is useful.
18//!
19//! ## Subtle note
20//!
21//! When inferring the generic arguments of functions, the argument
22//! order is relevant, which can lead to the following edge case:
23//!
24//! ```ignore (illustrative)
25//! fn foo<T>(a: T, b: T) {
26//!     // ...
27//! }
28//!
29//! foo(&7i32, &mut 7i32);
30//! // This compiles, as we first infer `T` to be `&i32`,
31//! // and then coerce `&mut 7i32` to `&7i32`.
32//!
33//! foo(&mut 7i32, &7i32);
34//! // This does not compile, as we first infer `T` to be `&mut i32`
35//! // and are then unable to coerce `&7i32` to `&mut i32`.
36//! ```
37
38use std::ops::Deref;
39
40use rustc_attr_data_structures::InlineAttr;
41use rustc_errors::codes::*;
42use rustc_errors::{Applicability, Diag, struct_span_code_err};
43use rustc_hir as hir;
44use rustc_hir::def_id::{DefId, LocalDefId};
45use rustc_hir_analysis::hir_ty_lowering::HirTyLowerer;
46use rustc_infer::infer::relate::RelateResult;
47use rustc_infer::infer::{DefineOpaqueTypes, InferOk, InferResult, RegionVariableOrigin};
48use rustc_infer::traits::{
49    MatchExpressionArmCause, Obligation, PredicateObligation, PredicateObligations, SelectionError,
50};
51use rustc_middle::span_bug;
52use rustc_middle::ty::adjustment::{
53    Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCoercion,
54};
55use rustc_middle::ty::error::TypeError;
56use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, TypeVisitableExt};
57use rustc_span::{BytePos, DUMMY_SP, DesugaringKind, Span};
58use rustc_trait_selection::infer::InferCtxtExt as _;
59use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
60use rustc_trait_selection::traits::{
61    self, ImplSource, NormalizeExt, ObligationCause, ObligationCauseCode, ObligationCtxt,
62};
63use smallvec::{SmallVec, smallvec};
64use tracing::{debug, instrument};
65
66use crate::FnCtxt;
67use crate::errors::SuggestBoxingForReturnImplTrait;
68
69struct Coerce<'a, 'tcx> {
70    fcx: &'a FnCtxt<'a, 'tcx>,
71    cause: ObligationCause<'tcx>,
72    use_lub: bool,
73    /// Determines whether or not allow_two_phase_borrow is set on any
74    /// autoref adjustments we create while coercing. We don't want to
75    /// allow deref coercions to create two-phase borrows, at least initially,
76    /// but we do need two-phase borrows for function argument reborrows.
77    /// See #47489 and #48598
78    /// See docs on the "AllowTwoPhase" type for a more detailed discussion
79    allow_two_phase: AllowTwoPhase,
80    /// Whether we allow `NeverToAny` coercions. This is unsound if we're
81    /// coercing a place expression without it counting as a read in the MIR.
82    /// This is a side-effect of HIR not really having a great distinction
83    /// between places and values.
84    coerce_never: bool,
85}
86
87impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
88    type Target = FnCtxt<'a, 'tcx>;
89    fn deref(&self) -> &Self::Target {
90        self.fcx
91    }
92}
93
94type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
95
96/// Coercing a mutable reference to an immutable works, while
97/// coercing `&T` to `&mut T` should be forbidden.
98fn coerce_mutbls<'tcx>(
99    from_mutbl: hir::Mutability,
100    to_mutbl: hir::Mutability,
101) -> RelateResult<'tcx, ()> {
102    if from_mutbl >= to_mutbl { Ok(()) } else { Err(TypeError::Mutability) }
103}
104
105/// This always returns `Ok(...)`.
106fn success<'tcx>(
107    adj: Vec<Adjustment<'tcx>>,
108    target: Ty<'tcx>,
109    obligations: PredicateObligations<'tcx>,
110) -> CoerceResult<'tcx> {
111    Ok(InferOk { value: (adj, target), obligations })
112}
113
114impl<'f, 'tcx> Coerce<'f, 'tcx> {
115    fn new(
116        fcx: &'f FnCtxt<'f, 'tcx>,
117        cause: ObligationCause<'tcx>,
118        allow_two_phase: AllowTwoPhase,
119        coerce_never: bool,
120    ) -> Self {
121        Coerce { fcx, cause, allow_two_phase, use_lub: false, coerce_never }
122    }
123
124    fn unify_raw(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
125        debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
126        self.commit_if_ok(|_| {
127            let at = self.at(&self.cause, self.fcx.param_env);
128
129            let res = if self.use_lub {
130                at.lub(b, a)
131            } else {
132                at.sup(DefineOpaqueTypes::Yes, b, a)
133                    .map(|InferOk { value: (), obligations }| InferOk { value: b, obligations })
134            };
135
136            // In the new solver, lazy norm may allow us to shallowly equate
137            // more types, but we emit possibly impossible-to-satisfy obligations.
138            // Filter these cases out to make sure our coercion is more accurate.
139            match res {
140                Ok(InferOk { value, obligations }) if self.next_trait_solver() => {
141                    let ocx = ObligationCtxt::new(self);
142                    ocx.register_obligations(obligations);
143                    if ocx.select_where_possible().is_empty() {
144                        Ok(InferOk { value, obligations: ocx.into_pending_obligations() })
145                    } else {
146                        Err(TypeError::Mismatch)
147                    }
148                }
149                res => res,
150            }
151        })
152    }
153
154    /// Unify two types (using sub or lub).
155    fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
156        self.unify_raw(a, b)
157            .and_then(|InferOk { value: ty, obligations }| success(vec![], ty, obligations))
158    }
159
160    /// Unify two types (using sub or lub) and produce a specific coercion.
161    fn unify_and(
162        &self,
163        a: Ty<'tcx>,
164        b: Ty<'tcx>,
165        adjustments: impl IntoIterator<Item = Adjustment<'tcx>>,
166        final_adjustment: Adjust,
167    ) -> CoerceResult<'tcx> {
168        self.unify_raw(a, b).and_then(|InferOk { value: ty, obligations }| {
169            success(
170                adjustments
171                    .into_iter()
172                    .chain(std::iter::once(Adjustment { target: ty, kind: final_adjustment }))
173                    .collect(),
174                ty,
175                obligations,
176            )
177        })
178    }
179
180    #[instrument(skip(self))]
181    fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
182        // First, remove any resolved type variables (at the top level, at least):
183        let a = self.shallow_resolve(a);
184        let b = self.shallow_resolve(b);
185        debug!("Coerce.tys({:?} => {:?})", a, b);
186
187        // Coercing from `!` to any type is allowed:
188        if a.is_never() {
189            if self.coerce_never {
190                return success(
191                    vec![Adjustment { kind: Adjust::NeverToAny, target: b }],
192                    b,
193                    PredicateObligations::new(),
194                );
195            } else {
196                // Otherwise the only coercion we can do is unification.
197                return self.unify(a, b);
198            }
199        }
200
201        // Coercing *from* an unresolved inference variable means that
202        // we have no information about the source type. This will always
203        // ultimately fall back to some form of subtyping.
204        if a.is_ty_var() {
205            return self.coerce_from_inference_variable(a, b);
206        }
207
208        // Consider coercing the subtype to a DST
209        //
210        // NOTE: this is wrapped in a `commit_if_ok` because it creates
211        // a "spurious" type variable, and we don't want to have that
212        // type variable in memory if the coercion fails.
213        let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
214        match unsize {
215            Ok(_) => {
216                debug!("coerce: unsize successful");
217                return unsize;
218            }
219            Err(error) => {
220                debug!(?error, "coerce: unsize failed");
221            }
222        }
223
224        // Examine the supertype and consider type-specific coercions, such
225        // as auto-borrowing, coercing pointer mutability, a `dyn*` coercion,
226        // or pin-ergonomics.
227        match *b.kind() {
228            ty::RawPtr(_, b_mutbl) => {
229                return self.coerce_raw_ptr(a, b, b_mutbl);
230            }
231            ty::Ref(r_b, _, mutbl_b) => {
232                return self.coerce_borrowed_pointer(a, b, r_b, mutbl_b);
233            }
234            ty::Dynamic(predicates, region, ty::DynStar) if self.tcx.features().dyn_star() => {
235                return self.coerce_dyn_star(a, b, predicates, region);
236            }
237            ty::Adt(pin, _)
238                if self.tcx.features().pin_ergonomics()
239                    && self.tcx.is_lang_item(pin.did(), hir::LangItem::Pin) =>
240            {
241                let pin_coerce = self.commit_if_ok(|_| self.coerce_pin_ref(a, b));
242                if pin_coerce.is_ok() {
243                    return pin_coerce;
244                }
245            }
246            _ => {}
247        }
248
249        match *a.kind() {
250            ty::FnDef(..) => {
251                // Function items are coercible to any closure
252                // type; function pointers are not (that would
253                // require double indirection).
254                // Additionally, we permit coercion of function
255                // items to drop the unsafe qualifier.
256                self.coerce_from_fn_item(a, b)
257            }
258            ty::FnPtr(a_sig_tys, a_hdr) => {
259                // We permit coercion of fn pointers to drop the
260                // unsafe qualifier.
261                self.coerce_from_fn_pointer(a_sig_tys.with(a_hdr), b)
262            }
263            ty::Closure(closure_def_id_a, args_a) => {
264                // Non-capturing closures are coercible to
265                // function pointers or unsafe function pointers.
266                // It cannot convert closures that require unsafe.
267                self.coerce_closure_to_fn(a, closure_def_id_a, args_a, b)
268            }
269            _ => {
270                // Otherwise, just use unification rules.
271                self.unify(a, b)
272            }
273        }
274    }
275
276    /// Coercing *from* an inference variable. In this case, we have no information
277    /// about the source type, so we can't really do a true coercion and we always
278    /// fall back to subtyping (`unify_and`).
279    fn coerce_from_inference_variable(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
280        debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
281        assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
282        assert!(self.shallow_resolve(b) == b);
283
284        if b.is_ty_var() {
285            // Two unresolved type variables: create a `Coerce` predicate.
286            let target_ty = if self.use_lub { self.next_ty_var(self.cause.span) } else { b };
287
288            let mut obligations = PredicateObligations::with_capacity(2);
289            for &source_ty in &[a, b] {
290                if source_ty != target_ty {
291                    obligations.push(Obligation::new(
292                        self.tcx(),
293                        self.cause.clone(),
294                        self.param_env,
295                        ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate {
296                            a: source_ty,
297                            b: target_ty,
298                        })),
299                    ));
300                }
301            }
302
303            debug!(
304                "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
305                target_ty, obligations
306            );
307            success(vec![], target_ty, obligations)
308        } else {
309            // One unresolved type variable: just apply subtyping, we may be able
310            // to do something useful.
311            self.unify(a, b)
312        }
313    }
314
315    /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
316    /// To match `A` with `B`, autoderef will be performed,
317    /// calling `deref`/`deref_mut` where necessary.
318    fn coerce_borrowed_pointer(
319        &self,
320        a: Ty<'tcx>,
321        b: Ty<'tcx>,
322        r_b: ty::Region<'tcx>,
323        mutbl_b: hir::Mutability,
324    ) -> CoerceResult<'tcx> {
325        debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
326
327        // If we have a parameter of type `&M T_a` and the value
328        // provided is `expr`, we will be adding an implicit borrow,
329        // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
330        // to type check, we will construct the type that `&M*expr` would
331        // yield.
332
333        let (r_a, mt_a) = match *a.kind() {
334            ty::Ref(r_a, ty, mutbl) => {
335                let mt_a = ty::TypeAndMut { ty, mutbl };
336                coerce_mutbls(mt_a.mutbl, mutbl_b)?;
337                (r_a, mt_a)
338            }
339            _ => return self.unify(a, b),
340        };
341
342        let span = self.cause.span;
343
344        let mut first_error = None;
345        let mut r_borrow_var = None;
346        let mut autoderef = self.autoderef(span, a);
347        let mut found = None;
348
349        for (referent_ty, autoderefs) in autoderef.by_ref() {
350            if autoderefs == 0 {
351                // Don't let this pass, otherwise it would cause
352                // &T to autoref to &&T.
353                continue;
354            }
355
356            // At this point, we have deref'd `a` to `referent_ty`. So
357            // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
358            // In the autoderef loop for `&'a mut Vec<T>`, we would get
359            // three callbacks:
360            //
361            // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
362            // - `Vec<T>` -- 1 deref
363            // - `[T]` -- 2 deref
364            //
365            // At each point after the first callback, we want to
366            // check to see whether this would match out target type
367            // (`&'b mut [T]`) if we autoref'd it. We can't just
368            // compare the referent types, though, because we still
369            // have to consider the mutability. E.g., in the case
370            // we've been considering, we have an `&mut` reference, so
371            // the `T` in `[T]` needs to be unified with equality.
372            //
373            // Therefore, we construct reference types reflecting what
374            // the types will be after we do the final auto-ref and
375            // compare those. Note that this means we use the target
376            // mutability [1], since it may be that we are coercing
377            // from `&mut T` to `&U`.
378            //
379            // One fine point concerns the region that we use. We
380            // choose the region such that the region of the final
381            // type that results from `unify` will be the region we
382            // want for the autoref:
383            //
384            // - if in sub mode, that means we want to use `'b` (the
385            //   region from the target reference) for both
386            //   pointers [2]. This is because sub mode (somewhat
387            //   arbitrarily) returns the subtype region. In the case
388            //   where we are coercing to a target type, we know we
389            //   want to use that target type region (`'b`) because --
390            //   for the program to type-check -- it must be the
391            //   smaller of the two.
392            //   - One fine point. It may be surprising that we can
393            //     use `'b` without relating `'a` and `'b`. The reason
394            //     that this is ok is that what we produce is
395            //     effectively a `&'b *x` expression (if you could
396            //     annotate the region of a borrow), and regionck has
397            //     code that adds edges from the region of a borrow
398            //     (`'b`, here) into the regions in the borrowed
399            //     expression (`*x`, here). (Search for "link".)
400            // - if in lub mode, things can get fairly complicated. The
401            //   easiest thing is just to make a fresh
402            //   region variable [4], which effectively means we defer
403            //   the decision to region inference (and regionck, which will add
404            //   some more edges to this variable). However, this can wind up
405            //   creating a crippling number of variables in some cases --
406            //   e.g., #32278 -- so we optimize one particular case [3].
407            //   Let me try to explain with some examples:
408            //   - The "running example" above represents the simple case,
409            //     where we have one `&` reference at the outer level and
410            //     ownership all the rest of the way down. In this case,
411            //     we want `LUB('a, 'b)` as the resulting region.
412            //   - However, if there are nested borrows, that region is
413            //     too strong. Consider a coercion from `&'a &'x Rc<T>` to
414            //     `&'b T`. In this case, `'a` is actually irrelevant.
415            //     The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
416            //     we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
417            //     (The errors actually show up in borrowck, typically, because
418            //     this extra edge causes the region `'a` to be inferred to something
419            //     too big, which then results in borrowck errors.)
420            //   - We could track the innermost shared reference, but there is already
421            //     code in regionck that has the job of creating links between
422            //     the region of a borrow and the regions in the thing being
423            //     borrowed (here, `'a` and `'x`), and it knows how to handle
424            //     all the various cases. So instead we just make a region variable
425            //     and let regionck figure it out.
426            let r = if !self.use_lub {
427                r_b // [2] above
428            } else if autoderefs == 1 {
429                r_a // [3] above
430            } else {
431                if r_borrow_var.is_none() {
432                    // create var lazily, at most once
433                    let coercion = RegionVariableOrigin::Coercion(span);
434                    let r = self.next_region_var(coercion);
435                    r_borrow_var = Some(r); // [4] above
436                }
437                r_borrow_var.unwrap()
438            };
439            let derefd_ty_a = Ty::new_ref(
440                self.tcx,
441                r,
442                referent_ty,
443                mutbl_b, // [1] above
444            );
445            match self.unify_raw(derefd_ty_a, b) {
446                Ok(ok) => {
447                    found = Some(ok);
448                    break;
449                }
450                Err(err) => {
451                    if first_error.is_none() {
452                        first_error = Some(err);
453                    }
454                }
455            }
456        }
457
458        // Extract type or return an error. We return the first error
459        // we got, which should be from relating the "base" type
460        // (e.g., in example above, the failure from relating `Vec<T>`
461        // to the target type), since that should be the least
462        // confusing.
463        let Some(InferOk { value: ty, mut obligations }) = found else {
464            if let Some(first_error) = first_error {
465                debug!("coerce_borrowed_pointer: failed with err = {:?}", first_error);
466                return Err(first_error);
467            } else {
468                // This may happen in the new trait solver since autoderef requires
469                // the pointee to be structurally normalizable, or else it'll just bail.
470                // So when we have a type like `&<not well formed>`, then we get no
471                // autoderef steps (even though there should be at least one). That means
472                // we get no type mismatches, since the loop above just exits early.
473                return Err(TypeError::Mismatch);
474            }
475        };
476
477        if ty == a && mt_a.mutbl.is_not() && autoderef.step_count() == 1 {
478            // As a special case, if we would produce `&'a *x`, that's
479            // a total no-op. We end up with the type `&'a T` just as
480            // we started with. In that case, just skip it
481            // altogether. This is just an optimization.
482            //
483            // Note that for `&mut`, we DO want to reborrow --
484            // otherwise, this would be a move, which might be an
485            // error. For example `foo(self.x)` where `self` and
486            // `self.x` both have `&mut `type would be a move of
487            // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
488            // which is a borrow.
489            assert!(mutbl_b.is_not()); // can only coerce &T -> &U
490            return success(vec![], ty, obligations);
491        }
492
493        let InferOk { value: mut adjustments, obligations: o } =
494            self.adjust_steps_as_infer_ok(&autoderef);
495        obligations.extend(o);
496        obligations.extend(autoderef.into_obligations());
497
498        // Now apply the autoref. We have to extract the region out of
499        // the final ref type we got.
500        let ty::Ref(..) = ty.kind() else {
501            span_bug!(span, "expected a ref type, got {:?}", ty);
502        };
503        let mutbl = AutoBorrowMutability::new(mutbl_b, self.allow_two_phase);
504        adjustments.push(Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(mutbl)), target: ty });
505
506        debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
507
508        success(adjustments, ty, obligations)
509    }
510
511    /// Performs [unsized coercion] by emulating a fulfillment loop on a
512    /// `CoerceUnsized` goal until all `CoerceUnsized` and `Unsize` goals
513    /// are successfully selected.
514    ///
515    /// [unsized coercion](https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions)
516    #[instrument(skip(self), level = "debug")]
517    fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
518        source = self.shallow_resolve(source);
519        target = self.shallow_resolve(target);
520        debug!(?source, ?target);
521
522        // We don't apply any coercions incase either the source or target
523        // aren't sufficiently well known but tend to instead just equate
524        // them both.
525        if source.is_ty_var() {
526            debug!("coerce_unsized: source is a TyVar, bailing out");
527            return Err(TypeError::Mismatch);
528        }
529        if target.is_ty_var() {
530            debug!("coerce_unsized: target is a TyVar, bailing out");
531            return Err(TypeError::Mismatch);
532        }
533
534        let traits =
535            (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
536        let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
537            debug!("missing Unsize or CoerceUnsized traits");
538            return Err(TypeError::Mismatch);
539        };
540
541        // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
542        // a DST unless we have to. This currently comes out in the wash since
543        // we can't unify [T] with U. But to properly support DST, we need to allow
544        // that, at which point we will need extra checks on the target here.
545
546        // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
547        let reborrow = match (source.kind(), target.kind()) {
548            (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
549                coerce_mutbls(mutbl_a, mutbl_b)?;
550
551                let coercion = RegionVariableOrigin::Coercion(self.cause.span);
552                let r_borrow = self.next_region_var(coercion);
553
554                // We don't allow two-phase borrows here, at least for initial
555                // implementation. If it happens that this coercion is a function argument,
556                // the reborrow in coerce_borrowed_ptr will pick it up.
557                let mutbl = AutoBorrowMutability::new(mutbl_b, AllowTwoPhase::No);
558
559                Some((
560                    Adjustment { kind: Adjust::Deref(None), target: ty_a },
561                    Adjustment {
562                        kind: Adjust::Borrow(AutoBorrow::Ref(mutbl)),
563                        target: Ty::new_ref(self.tcx, r_borrow, ty_a, mutbl_b),
564                    },
565                ))
566            }
567            (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(_, mt_b)) => {
568                coerce_mutbls(mt_a, mt_b)?;
569
570                Some((
571                    Adjustment { kind: Adjust::Deref(None), target: ty_a },
572                    Adjustment {
573                        kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
574                        target: Ty::new_ptr(self.tcx, ty_a, mt_b),
575                    },
576                ))
577            }
578            _ => None,
579        };
580        let coerce_source = reborrow.as_ref().map_or(source, |(_, r)| r.target);
581
582        // Setup either a subtyping or a LUB relationship between
583        // the `CoerceUnsized` target type and the expected type.
584        // We only have the latter, so we use an inference variable
585        // for the former and let type inference do the rest.
586        let coerce_target = self.next_ty_var(self.cause.span);
587
588        let mut coercion = self.unify_and(
589            coerce_target,
590            target,
591            reborrow.into_iter().flat_map(|(deref, autoref)| [deref, autoref]),
592            Adjust::Pointer(PointerCoercion::Unsize),
593        )?;
594
595        let mut selcx = traits::SelectionContext::new(self);
596
597        // Create an obligation for `Source: CoerceUnsized<Target>`.
598        let cause = self.cause(self.cause.span, ObligationCauseCode::Coercion { source, target });
599
600        // Use a FIFO queue for this custom fulfillment procedure.
601        //
602        // A Vec (or SmallVec) is not a natural choice for a queue. However,
603        // this code path is hot, and this queue usually has a max length of 1
604        // and almost never more than 3. By using a SmallVec we avoid an
605        // allocation, at the (very small) cost of (occasionally) having to
606        // shift subsequent elements down when removing the front element.
607        let mut queue: SmallVec<[PredicateObligation<'tcx>; 4]> = smallvec![Obligation::new(
608            self.tcx,
609            cause,
610            self.fcx.param_env,
611            ty::TraitRef::new(self.tcx, coerce_unsized_did, [coerce_source, coerce_target])
612        )];
613
614        // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
615        // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
616        // inference might unify those two inner type variables later.
617        let traits = [coerce_unsized_did, unsize_did];
618        while !queue.is_empty() {
619            let obligation = queue.remove(0);
620            let trait_pred = match obligation.predicate.kind().no_bound_vars() {
621                Some(ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)))
622                    if traits.contains(&trait_pred.def_id()) =>
623                {
624                    self.resolve_vars_if_possible(trait_pred)
625                }
626                // Eagerly process alias-relate obligations in new trait solver,
627                // since these can be emitted in the process of solving trait goals,
628                // but we need to constrain vars before processing goals mentioning
629                // them.
630                Some(ty::PredicateKind::AliasRelate(..)) => {
631                    let ocx = ObligationCtxt::new(self);
632                    ocx.register_obligation(obligation);
633                    if !ocx.select_where_possible().is_empty() {
634                        return Err(TypeError::Mismatch);
635                    }
636                    coercion.obligations.extend(ocx.into_pending_obligations());
637                    continue;
638                }
639                _ => {
640                    coercion.obligations.push(obligation);
641                    continue;
642                }
643            };
644            debug!("coerce_unsized resolve step: {:?}", trait_pred);
645            match selcx.select(&obligation.with(selcx.tcx(), trait_pred)) {
646                // Uncertain or unimplemented.
647                Ok(None) => {
648                    if trait_pred.def_id() == unsize_did {
649                        let self_ty = trait_pred.self_ty();
650                        let unsize_ty = trait_pred.trait_ref.args[1].expect_ty();
651                        debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
652                        match (self_ty.kind(), unsize_ty.kind()) {
653                            (&ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
654                                if self.type_var_is_sized(v) =>
655                            {
656                                debug!("coerce_unsized: have sized infer {:?}", v);
657                                coercion.obligations.push(obligation);
658                                // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
659                                // for unsizing.
660                            }
661                            _ => {
662                                // Some other case for `$0: Unsize<Something>`. Note that we
663                                // hit this case even if `Something` is a sized type, so just
664                                // don't do the coercion.
665                                debug!("coerce_unsized: ambiguous unsize");
666                                return Err(TypeError::Mismatch);
667                            }
668                        }
669                    } else {
670                        debug!("coerce_unsized: early return - ambiguous");
671                        return Err(TypeError::Mismatch);
672                    }
673                }
674                Err(SelectionError::Unimplemented) => {
675                    debug!("coerce_unsized: early return - can't prove obligation");
676                    return Err(TypeError::Mismatch);
677                }
678
679                Err(SelectionError::TraitDynIncompatible(_)) => {
680                    // Dyn compatibility errors in coercion will *always* be due to the
681                    // fact that the RHS of the coercion is a non-dyn compatible `dyn Trait`
682                    // writen in source somewhere (otherwise we will never have lowered
683                    // the dyn trait from HIR to middle).
684                    //
685                    // There's no reason to emit yet another dyn compatibility error,
686                    // especially since the span will differ slightly and thus not be
687                    // deduplicated at all!
688                    self.fcx.set_tainted_by_errors(
689                        self.fcx
690                            .dcx()
691                            .span_delayed_bug(self.cause.span, "dyn compatibility during coercion"),
692                    );
693                }
694                Err(err) => {
695                    let guar = self.err_ctxt().report_selection_error(
696                        obligation.clone(),
697                        &obligation,
698                        &err,
699                    );
700                    self.fcx.set_tainted_by_errors(guar);
701                    // Treat this like an obligation and follow through
702                    // with the unsizing - the lack of a coercion should
703                    // be silent, as it causes a type mismatch later.
704                }
705
706                Ok(Some(ImplSource::UserDefined(impl_source))) => {
707                    queue.extend(impl_source.nested);
708                    // Certain incoherent `CoerceUnsized` implementations may cause ICEs,
709                    // so check the impl's validity. Taint the body so that we don't try
710                    // to evaluate these invalid coercions in CTFE. We only need to do this
711                    // for local impls, since upstream impls should be valid.
712                    if impl_source.impl_def_id.is_local()
713                        && let Err(guar) =
714                            self.tcx.ensure_ok().coerce_unsized_info(impl_source.impl_def_id)
715                    {
716                        self.fcx.set_tainted_by_errors(guar);
717                    }
718                }
719                Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
720            }
721        }
722
723        Ok(coercion)
724    }
725
726    fn coerce_dyn_star(
727        &self,
728        a: Ty<'tcx>,
729        b: Ty<'tcx>,
730        predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
731        b_region: ty::Region<'tcx>,
732    ) -> CoerceResult<'tcx> {
733        if !self.tcx.features().dyn_star() {
734            return Err(TypeError::Mismatch);
735        }
736
737        // FIXME(dyn_star): We should probably allow things like casting from
738        // `dyn* Foo + Send` to `dyn* Foo`.
739        if let ty::Dynamic(a_data, _, ty::DynStar) = a.kind()
740            && let ty::Dynamic(b_data, _, ty::DynStar) = b.kind()
741            && a_data.principal_def_id() == b_data.principal_def_id()
742        {
743            return self.unify(a, b);
744        }
745
746        // Check the obligations of the cast -- for example, when casting
747        // `usize` to `dyn* Clone + 'static`:
748        let obligations = predicates
749            .iter()
750            .map(|predicate| {
751                // For each existential predicate (e.g., `?Self: Clone`) instantiate
752                // the type of the expression (e.g., `usize` in our example above)
753                // and then require that the resulting predicate (e.g., `usize: Clone`)
754                // holds (it does).
755                let predicate = predicate.with_self_ty(self.tcx, a);
756                Obligation::new(self.tcx, self.cause.clone(), self.param_env, predicate)
757            })
758            .chain([
759                // Enforce the region bound (e.g., `usize: 'static`, in our example).
760                Obligation::new(
761                    self.tcx,
762                    self.cause.clone(),
763                    self.param_env,
764                    ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(
765                        ty::OutlivesPredicate(a, b_region),
766                    ))),
767                ),
768                // Enforce that the type is `usize`/pointer-sized.
769                Obligation::new(
770                    self.tcx,
771                    self.cause.clone(),
772                    self.param_env,
773                    ty::TraitRef::new(
774                        self.tcx,
775                        self.tcx.require_lang_item(hir::LangItem::PointerLike, self.cause.span),
776                        [a],
777                    ),
778                ),
779            ])
780            .collect();
781
782        Ok(InferOk {
783            value: (
784                vec![Adjustment { kind: Adjust::Pointer(PointerCoercion::DynStar), target: b }],
785                b,
786            ),
787            obligations,
788        })
789    }
790
791    /// Applies reborrowing for `Pin`
792    ///
793    /// We currently only support reborrowing `Pin<&mut T>` as `Pin<&mut T>`. This is accomplished
794    /// by inserting a call to `Pin::as_mut` during MIR building.
795    ///
796    /// In the future we might want to support other reborrowing coercions, such as:
797    /// - `Pin<&mut T>` as `Pin<&T>`
798    /// - `Pin<&T>` as `Pin<&T>`
799    /// - `Pin<Box<T>>` as `Pin<&T>`
800    /// - `Pin<Box<T>>` as `Pin<&mut T>`
801    #[instrument(skip(self), level = "trace")]
802    fn coerce_pin_ref(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
803        // We need to make sure the two types are compatible for coercion.
804        // Then we will build a ReborrowPin adjustment and return that as an InferOk.
805
806        // Right now we can only reborrow if this is a `Pin<&mut T>`.
807        let extract_pin_mut = |ty: Ty<'tcx>| {
808            // Get the T out of Pin<T>
809            let (pin, ty) = match ty.kind() {
810                ty::Adt(pin, args) if self.tcx.is_lang_item(pin.did(), hir::LangItem::Pin) => {
811                    (*pin, args[0].expect_ty())
812                }
813                _ => {
814                    debug!("can't reborrow {:?} as pinned", ty);
815                    return Err(TypeError::Mismatch);
816                }
817            };
818            // Make sure the T is something we understand (just `&mut U` for now)
819            match ty.kind() {
820                ty::Ref(region, ty, mutbl) => Ok((pin, *region, *ty, *mutbl)),
821                _ => {
822                    debug!("can't reborrow pin of inner type {:?}", ty);
823                    Err(TypeError::Mismatch)
824                }
825            }
826        };
827
828        let (pin, a_region, a_ty, mut_a) = extract_pin_mut(a)?;
829        let (_, _, _b_ty, mut_b) = extract_pin_mut(b)?;
830
831        coerce_mutbls(mut_a, mut_b)?;
832
833        // update a with b's mutability since we'll be coercing mutability
834        let a = Ty::new_adt(
835            self.tcx,
836            pin,
837            self.tcx.mk_args(&[Ty::new_ref(self.tcx, a_region, a_ty, mut_b).into()]),
838        );
839
840        // To complete the reborrow, we need to make sure we can unify the inner types, and if so we
841        // add the adjustments.
842        self.unify_and(a, b, [], Adjust::ReborrowPin(mut_b))
843    }
844
845    fn coerce_from_safe_fn(
846        &self,
847        fn_ty_a: ty::PolyFnSig<'tcx>,
848        b: Ty<'tcx>,
849        adjustment: Option<Adjust>,
850    ) -> CoerceResult<'tcx> {
851        self.commit_if_ok(|snapshot| {
852            let outer_universe = self.infcx.universe();
853
854            let result = if let ty::FnPtr(_, hdr_b) = b.kind()
855                && fn_ty_a.safety().is_safe()
856                && hdr_b.safety.is_unsafe()
857            {
858                let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
859                self.unify_and(
860                    unsafe_a,
861                    b,
862                    adjustment
863                        .map(|kind| Adjustment { kind, target: Ty::new_fn_ptr(self.tcx, fn_ty_a) }),
864                    Adjust::Pointer(PointerCoercion::UnsafeFnPointer),
865                )
866            } else {
867                let a = Ty::new_fn_ptr(self.tcx, fn_ty_a);
868                match adjustment {
869                    Some(adjust) => self.unify_and(a, b, [], adjust),
870                    None => self.unify(a, b),
871                }
872            };
873
874            // FIXME(#73154): This is a hack. Currently LUB can generate
875            // unsolvable constraints. Additionally, it returns `a`
876            // unconditionally, even when the "LUB" is `b`. In the future, we
877            // want the coerced type to be the actual supertype of these two,
878            // but for now, we want to just error to ensure we don't lock
879            // ourselves into a specific behavior with NLL.
880            self.leak_check(outer_universe, Some(snapshot))?;
881
882            result
883        })
884    }
885
886    fn coerce_from_fn_pointer(
887        &self,
888        fn_ty_a: ty::PolyFnSig<'tcx>,
889        b: Ty<'tcx>,
890    ) -> CoerceResult<'tcx> {
891        //! Attempts to coerce from the type of a Rust function item
892        //! into a closure or a `proc`.
893        //!
894
895        let b = self.shallow_resolve(b);
896        debug!(?fn_ty_a, ?b, "coerce_from_fn_pointer");
897
898        self.coerce_from_safe_fn(fn_ty_a, b, None)
899    }
900
901    fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
902        //! Attempts to coerce from the type of a Rust function item
903        //! into a closure or a `proc`.
904
905        let b = self.shallow_resolve(b);
906        let InferOk { value: b, mut obligations } =
907            self.at(&self.cause, self.param_env).normalize(b);
908        debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
909
910        match b.kind() {
911            ty::FnPtr(_, b_hdr) => {
912                let mut a_sig = a.fn_sig(self.tcx);
913                if let ty::FnDef(def_id, _) = *a.kind() {
914                    // Intrinsics are not coercible to function pointers
915                    if self.tcx.intrinsic(def_id).is_some() {
916                        return Err(TypeError::IntrinsicCast);
917                    }
918
919                    let fn_attrs = self.tcx.codegen_fn_attrs(def_id);
920                    if matches!(fn_attrs.inline, InlineAttr::Force { .. }) {
921                        return Err(TypeError::ForceInlineCast);
922                    }
923
924                    if b_hdr.safety.is_safe()
925                        && self.tcx.codegen_fn_attrs(def_id).safe_target_features
926                    {
927                        // Allow the coercion if the current function has all the features that would be
928                        // needed to call the coercee safely.
929                        if let Some(safe_sig) = self.tcx.adjust_target_feature_sig(
930                            def_id,
931                            a_sig,
932                            self.fcx.body_id.into(),
933                        ) {
934                            a_sig = safe_sig;
935                        } else {
936                            return Err(TypeError::TargetFeatureCast(def_id));
937                        }
938                    }
939                }
940
941                let InferOk { value: a_sig, obligations: o1 } =
942                    self.at(&self.cause, self.param_env).normalize(a_sig);
943                obligations.extend(o1);
944
945                let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
946                    a_sig,
947                    b,
948                    Some(Adjust::Pointer(PointerCoercion::ReifyFnPointer)),
949                )?;
950
951                obligations.extend(o2);
952                Ok(InferOk { value, obligations })
953            }
954            _ => self.unify(a, b),
955        }
956    }
957
958    fn coerce_closure_to_fn(
959        &self,
960        a: Ty<'tcx>,
961        closure_def_id_a: DefId,
962        args_a: GenericArgsRef<'tcx>,
963        b: Ty<'tcx>,
964    ) -> CoerceResult<'tcx> {
965        //! Attempts to coerce from the type of a non-capturing closure
966        //! into a function pointer.
967        //!
968
969        let b = self.shallow_resolve(b);
970
971        match b.kind() {
972            // At this point we haven't done capture analysis, which means
973            // that the ClosureArgs just contains an inference variable instead
974            // of tuple of captured types.
975            //
976            // All we care here is if any variable is being captured and not the exact paths,
977            // so we check `upvars_mentioned` for root variables being captured.
978            ty::FnPtr(_, hdr)
979                if self
980                    .tcx
981                    .upvars_mentioned(closure_def_id_a.expect_local())
982                    .is_none_or(|u| u.is_empty()) =>
983            {
984                // We coerce the closure, which has fn type
985                //     `extern "rust-call" fn((arg0,arg1,...)) -> _`
986                // to
987                //     `fn(arg0,arg1,...) -> _`
988                // or
989                //     `unsafe fn(arg0,arg1,...) -> _`
990                let closure_sig = args_a.as_closure().sig();
991                let safety = hdr.safety;
992                let pointer_ty =
993                    Ty::new_fn_ptr(self.tcx, self.tcx.signature_unclosure(closure_sig, safety));
994                debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
995                self.unify_and(
996                    pointer_ty,
997                    b,
998                    [],
999                    Adjust::Pointer(PointerCoercion::ClosureFnPointer(safety)),
1000                )
1001            }
1002            _ => self.unify(a, b),
1003        }
1004    }
1005
1006    fn coerce_raw_ptr(
1007        &self,
1008        a: Ty<'tcx>,
1009        b: Ty<'tcx>,
1010        mutbl_b: hir::Mutability,
1011    ) -> CoerceResult<'tcx> {
1012        debug!("coerce_raw_ptr(a={:?}, b={:?})", a, b);
1013
1014        let (is_ref, mt_a) = match *a.kind() {
1015            ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
1016            ty::RawPtr(ty, mutbl) => (false, ty::TypeAndMut { ty, mutbl }),
1017            _ => return self.unify(a, b),
1018        };
1019        coerce_mutbls(mt_a.mutbl, mutbl_b)?;
1020
1021        // Check that the types which they point at are compatible.
1022        let a_raw = Ty::new_ptr(self.tcx, mt_a.ty, mutbl_b);
1023        // Although references and raw ptrs have the same
1024        // representation, we still register an Adjust::DerefRef so that
1025        // regionck knows that the region for `a` must be valid here.
1026        if is_ref {
1027            self.unify_and(
1028                a_raw,
1029                b,
1030                [Adjustment { kind: Adjust::Deref(None), target: mt_a.ty }],
1031                Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)),
1032            )
1033        } else if mt_a.mutbl != mutbl_b {
1034            self.unify_and(a_raw, b, [], Adjust::Pointer(PointerCoercion::MutToConstPointer))
1035        } else {
1036            self.unify(a_raw, b)
1037        }
1038    }
1039}
1040
1041impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
1042    /// Attempt to coerce an expression to a type, and return the
1043    /// adjusted type of the expression, if successful.
1044    /// Adjustments are only recorded if the coercion succeeded.
1045    /// The expressions *must not* have any preexisting adjustments.
1046    pub(crate) fn coerce(
1047        &self,
1048        expr: &'tcx hir::Expr<'tcx>,
1049        expr_ty: Ty<'tcx>,
1050        mut target: Ty<'tcx>,
1051        allow_two_phase: AllowTwoPhase,
1052        cause: Option<ObligationCause<'tcx>>,
1053    ) -> RelateResult<'tcx, Ty<'tcx>> {
1054        let source = self.try_structurally_resolve_type(expr.span, expr_ty);
1055        if self.next_trait_solver() {
1056            target = self.try_structurally_resolve_type(
1057                cause.as_ref().map_or(expr.span, |cause| cause.span),
1058                target,
1059            );
1060        }
1061        debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
1062
1063        let cause =
1064            cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
1065        let coerce = Coerce::new(
1066            self,
1067            cause,
1068            allow_two_phase,
1069            self.expr_guaranteed_to_constitute_read_for_never(expr),
1070        );
1071        let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
1072
1073        let (adjustments, _) = self.register_infer_ok_obligations(ok);
1074        self.apply_adjustments(expr, adjustments);
1075        Ok(if let Err(guar) = expr_ty.error_reported() {
1076            Ty::new_error(self.tcx, guar)
1077        } else {
1078            target
1079        })
1080    }
1081
1082    /// Probe whether `expr_ty` can be coerced to `target_ty`. This has no side-effects,
1083    /// and may return false positives if types are not yet fully constrained by inference.
1084    ///
1085    /// Returns false if the coercion is not possible, or if the coercion creates any
1086    /// sub-obligations that result in errors.
1087    ///
1088    /// This should only be used for diagnostics.
1089    pub(crate) fn may_coerce(&self, expr_ty: Ty<'tcx>, target_ty: Ty<'tcx>) -> bool {
1090        let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
1091        // We don't ever need two-phase here since we throw out the result of the coercion.
1092        // We also just always set `coerce_never` to true, since this is a heuristic.
1093        let coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No, true);
1094        self.probe(|_| {
1095            // Make sure to structurally resolve the types, since we use
1096            // the `TyKind`s heavily in coercion.
1097            let ocx = ObligationCtxt::new(self);
1098            let structurally_resolve = |ty| {
1099                let ty = self.shallow_resolve(ty);
1100                if self.next_trait_solver()
1101                    && let ty::Alias(..) = ty.kind()
1102                {
1103                    ocx.structurally_normalize_ty(&cause, self.param_env, ty)
1104                } else {
1105                    Ok(ty)
1106                }
1107            };
1108            let Ok(expr_ty) = structurally_resolve(expr_ty) else {
1109                return false;
1110            };
1111            let Ok(target_ty) = structurally_resolve(target_ty) else {
1112                return false;
1113            };
1114
1115            let Ok(ok) = coerce.coerce(expr_ty, target_ty) else {
1116                return false;
1117            };
1118            ocx.register_obligations(ok.obligations);
1119            ocx.select_where_possible().is_empty()
1120        })
1121    }
1122
1123    /// Given a type and a target type, this function will calculate and return
1124    /// how many dereference steps needed to coerce `expr_ty` to `target`. If
1125    /// it's not possible, return `None`.
1126    pub(crate) fn deref_steps_for_suggestion(
1127        &self,
1128        expr_ty: Ty<'tcx>,
1129        target: Ty<'tcx>,
1130    ) -> Option<usize> {
1131        let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
1132        // We don't ever need two-phase here since we throw out the result of the coercion.
1133        let coerce = Coerce::new(self, cause, AllowTwoPhase::No, true);
1134        coerce.autoderef(DUMMY_SP, expr_ty).find_map(|(ty, steps)| {
1135            self.probe(|_| coerce.unify_raw(ty, target)).ok().map(|_| steps)
1136        })
1137    }
1138
1139    /// Given a type, this function will calculate and return the type given
1140    /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
1141    ///
1142    /// This function is for diagnostics only, since it does not register
1143    /// trait or region sub-obligations. (presumably we could, but it's not
1144    /// particularly important for diagnostics...)
1145    pub(crate) fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
1146        self.autoderef(DUMMY_SP, expr_ty).silence_errors().nth(1).and_then(|(deref_ty, _)| {
1147            self.infcx
1148                .type_implements_trait(
1149                    self.tcx.lang_items().deref_mut_trait()?,
1150                    [expr_ty],
1151                    self.param_env,
1152                )
1153                .may_apply()
1154                .then_some(deref_ty)
1155        })
1156    }
1157
1158    /// Given some expressions, their known unified type and another expression,
1159    /// tries to unify the types, potentially inserting coercions on any of the
1160    /// provided expressions and returns their LUB (aka "common supertype").
1161    ///
1162    /// This is really an internal helper. From outside the coercion
1163    /// module, you should instantiate a `CoerceMany` instance.
1164    fn try_find_coercion_lub<E>(
1165        &self,
1166        cause: &ObligationCause<'tcx>,
1167        exprs: &[E],
1168        prev_ty: Ty<'tcx>,
1169        new: &hir::Expr<'_>,
1170        new_ty: Ty<'tcx>,
1171    ) -> RelateResult<'tcx, Ty<'tcx>>
1172    where
1173        E: AsCoercionSite,
1174    {
1175        let prev_ty = self.try_structurally_resolve_type(cause.span, prev_ty);
1176        let new_ty = self.try_structurally_resolve_type(new.span, new_ty);
1177        debug!(
1178            "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
1179            prev_ty,
1180            new_ty,
1181            exprs.len()
1182        );
1183
1184        // The following check fixes #88097, where the compiler erroneously
1185        // attempted to coerce a closure type to itself via a function pointer.
1186        if prev_ty == new_ty {
1187            return Ok(prev_ty);
1188        }
1189
1190        let is_force_inline = |ty: Ty<'tcx>| {
1191            if let ty::FnDef(did, _) = ty.kind() {
1192                matches!(self.tcx.codegen_fn_attrs(did).inline, InlineAttr::Force { .. })
1193            } else {
1194                false
1195            }
1196        };
1197        if is_force_inline(prev_ty) || is_force_inline(new_ty) {
1198            return Err(TypeError::ForceInlineCast);
1199        }
1200
1201        // Special-case that coercion alone cannot handle:
1202        // Function items or non-capturing closures of differing IDs or GenericArgs.
1203        let (a_sig, b_sig) = {
1204            let is_capturing_closure = |ty: Ty<'tcx>| {
1205                if let &ty::Closure(closure_def_id, _args) = ty.kind() {
1206                    self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some()
1207                } else {
1208                    false
1209                }
1210            };
1211            if is_capturing_closure(prev_ty) || is_capturing_closure(new_ty) {
1212                (None, None)
1213            } else {
1214                match (prev_ty.kind(), new_ty.kind()) {
1215                    (ty::FnDef(..), ty::FnDef(..)) => {
1216                        // Don't reify if the function types have a LUB, i.e., they
1217                        // are the same function and their parameters have a LUB.
1218                        match self.commit_if_ok(|_| {
1219                            // We need to eagerly handle nested obligations due to lazy norm.
1220                            if self.next_trait_solver() {
1221                                let ocx = ObligationCtxt::new(self);
1222                                let value = ocx.lub(cause, self.param_env, prev_ty, new_ty)?;
1223                                if ocx.select_where_possible().is_empty() {
1224                                    Ok(InferOk {
1225                                        value,
1226                                        obligations: ocx.into_pending_obligations(),
1227                                    })
1228                                } else {
1229                                    Err(TypeError::Mismatch)
1230                                }
1231                            } else {
1232                                self.at(cause, self.param_env).lub(prev_ty, new_ty)
1233                            }
1234                        }) {
1235                            // We have a LUB of prev_ty and new_ty, just return it.
1236                            Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
1237                            Err(_) => {
1238                                (Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx)))
1239                            }
1240                        }
1241                    }
1242                    (ty::Closure(_, args), ty::FnDef(..)) => {
1243                        let b_sig = new_ty.fn_sig(self.tcx);
1244                        let a_sig =
1245                            self.tcx.signature_unclosure(args.as_closure().sig(), b_sig.safety());
1246                        (Some(a_sig), Some(b_sig))
1247                    }
1248                    (ty::FnDef(..), ty::Closure(_, args)) => {
1249                        let a_sig = prev_ty.fn_sig(self.tcx);
1250                        let b_sig =
1251                            self.tcx.signature_unclosure(args.as_closure().sig(), a_sig.safety());
1252                        (Some(a_sig), Some(b_sig))
1253                    }
1254                    (ty::Closure(_, args_a), ty::Closure(_, args_b)) => (
1255                        Some(
1256                            self.tcx
1257                                .signature_unclosure(args_a.as_closure().sig(), hir::Safety::Safe),
1258                        ),
1259                        Some(
1260                            self.tcx
1261                                .signature_unclosure(args_b.as_closure().sig(), hir::Safety::Safe),
1262                        ),
1263                    ),
1264                    _ => (None, None),
1265                }
1266            }
1267        };
1268        if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) {
1269            // The signature must match.
1270            let (a_sig, b_sig) = self.normalize(new.span, (a_sig, b_sig));
1271            let sig = self
1272                .at(cause, self.param_env)
1273                .lub(a_sig, b_sig)
1274                .map(|ok| self.register_infer_ok_obligations(ok))?;
1275
1276            // Reify both sides and return the reified fn pointer type.
1277            let fn_ptr = Ty::new_fn_ptr(self.tcx, sig);
1278            let prev_adjustment = match prev_ty.kind() {
1279                ty::Closure(..) => {
1280                    Adjust::Pointer(PointerCoercion::ClosureFnPointer(a_sig.safety()))
1281                }
1282                ty::FnDef(..) => Adjust::Pointer(PointerCoercion::ReifyFnPointer),
1283                _ => span_bug!(cause.span, "should not try to coerce a {prev_ty} to a fn pointer"),
1284            };
1285            let next_adjustment = match new_ty.kind() {
1286                ty::Closure(..) => {
1287                    Adjust::Pointer(PointerCoercion::ClosureFnPointer(b_sig.safety()))
1288                }
1289                ty::FnDef(..) => Adjust::Pointer(PointerCoercion::ReifyFnPointer),
1290                _ => span_bug!(new.span, "should not try to coerce a {new_ty} to a fn pointer"),
1291            };
1292            for expr in exprs.iter().map(|e| e.as_coercion_site()) {
1293                self.apply_adjustments(
1294                    expr,
1295                    vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
1296                );
1297            }
1298            self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
1299            return Ok(fn_ptr);
1300        }
1301
1302        // Configure a Coerce instance to compute the LUB.
1303        // We don't allow two-phase borrows on any autorefs this creates since we
1304        // probably aren't processing function arguments here and even if we were,
1305        // they're going to get autorefed again anyway and we can apply 2-phase borrows
1306        // at that time.
1307        //
1308        // NOTE: we set `coerce_never` to `true` here because coercion LUBs only
1309        // operate on values and not places, so a never coercion is valid.
1310        let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No, true);
1311        coerce.use_lub = true;
1312
1313        // First try to coerce the new expression to the type of the previous ones,
1314        // but only if the new expression has no coercion already applied to it.
1315        let mut first_error = None;
1316        if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
1317            let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
1318            match result {
1319                Ok(ok) => {
1320                    let (adjustments, target) = self.register_infer_ok_obligations(ok);
1321                    self.apply_adjustments(new, adjustments);
1322                    debug!(
1323                        "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
1324                        new_ty, prev_ty, target
1325                    );
1326                    return Ok(target);
1327                }
1328                Err(e) => first_error = Some(e),
1329            }
1330        }
1331
1332        match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
1333            Err(_) => {
1334                // Avoid giving strange errors on failed attempts.
1335                if let Some(e) = first_error {
1336                    Err(e)
1337                } else {
1338                    Err(self
1339                        .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1340                        .unwrap_err())
1341                }
1342            }
1343            Ok(ok) => {
1344                let (adjustments, target) = self.register_infer_ok_obligations(ok);
1345                for expr in exprs {
1346                    let expr = expr.as_coercion_site();
1347                    self.apply_adjustments(expr, adjustments.clone());
1348                }
1349                debug!(
1350                    "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
1351                    prev_ty, new_ty, target
1352                );
1353                Ok(target)
1354            }
1355        }
1356    }
1357}
1358
1359/// Check whether `ty` can be coerced to `output_ty`.
1360/// Used from clippy.
1361pub fn can_coerce<'tcx>(
1362    tcx: TyCtxt<'tcx>,
1363    param_env: ty::ParamEnv<'tcx>,
1364    body_id: LocalDefId,
1365    ty: Ty<'tcx>,
1366    output_ty: Ty<'tcx>,
1367) -> bool {
1368    let root_ctxt = crate::typeck_root_ctxt::TypeckRootCtxt::new(tcx, body_id);
1369    let fn_ctxt = FnCtxt::new(&root_ctxt, param_env, body_id);
1370    fn_ctxt.may_coerce(ty, output_ty)
1371}
1372
1373/// CoerceMany encapsulates the pattern you should use when you have
1374/// many expressions that are all getting coerced to a common
1375/// type. This arises, for example, when you have a match (the result
1376/// of each arm is coerced to a common type). It also arises in less
1377/// obvious places, such as when you have many `break foo` expressions
1378/// that target the same loop, or the various `return` expressions in
1379/// a function.
1380///
1381/// The basic protocol is as follows:
1382///
1383/// - Instantiate the `CoerceMany` with an initial `expected_ty`.
1384///   This will also serve as the "starting LUB". The expectation is
1385///   that this type is something which all of the expressions *must*
1386///   be coercible to. Use a fresh type variable if needed.
1387/// - For each expression whose result is to be coerced, invoke `coerce()` with.
1388///   - In some cases we wish to coerce "non-expressions" whose types are implicitly
1389///     unit. This happens for example if you have a `break` with no expression,
1390///     or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
1391///   - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
1392///     from you so that you don't have to worry your pretty head about it.
1393///     But if an error is reported, the final type will be `err`.
1394///   - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
1395///     previously coerced expressions.
1396/// - When all done, invoke `complete()`. This will return the LUB of
1397///   all your expressions.
1398///   - WARNING: I don't believe this final type is guaranteed to be
1399///     related to your initial `expected_ty` in any particular way,
1400///     although it will typically be a subtype, so you should check it.
1401///   - Invoking `complete()` may cause us to go and adjust the "adjustments" on
1402///     previously coerced expressions.
1403///
1404/// Example:
1405///
1406/// ```ignore (illustrative)
1407/// let mut coerce = CoerceMany::new(expected_ty);
1408/// for expr in exprs {
1409///     let expr_ty = fcx.check_expr_with_expectation(expr, expected);
1410///     coerce.coerce(fcx, &cause, expr, expr_ty);
1411/// }
1412/// let final_ty = coerce.complete(fcx);
1413/// ```
1414pub(crate) struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
1415    expected_ty: Ty<'tcx>,
1416    final_ty: Option<Ty<'tcx>>,
1417    expressions: Expressions<'tcx, 'exprs, E>,
1418    pushed: usize,
1419}
1420
1421/// The type of a `CoerceMany` that is storing up the expressions into
1422/// a buffer. We use this in `check/mod.rs` for things like `break`.
1423pub(crate) type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;
1424
1425enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
1426    Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
1427    UpFront(&'exprs [E]),
1428}
1429
1430impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
1431    /// The usual case; collect the set of expressions dynamically.
1432    /// If the full set of coercion sites is known before hand,
1433    /// consider `with_coercion_sites()` instead to avoid allocation.
1434    pub(crate) fn new(expected_ty: Ty<'tcx>) -> Self {
1435        Self::make(expected_ty, Expressions::Dynamic(vec![]))
1436    }
1437
1438    /// As an optimization, you can create a `CoerceMany` with a
1439    /// preexisting slice of expressions. In this case, you are
1440    /// expected to pass each element in the slice to `coerce(...)` in
1441    /// order. This is used with arrays in particular to avoid
1442    /// needlessly cloning the slice.
1443    pub(crate) fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
1444        Self::make(expected_ty, Expressions::UpFront(coercion_sites))
1445    }
1446
1447    fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
1448        CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
1449    }
1450
1451    /// Returns the "expected type" with which this coercion was
1452    /// constructed. This represents the "downward propagated" type
1453    /// that was given to us at the start of typing whatever construct
1454    /// we are typing (e.g., the match expression).
1455    ///
1456    /// Typically, this is used as the expected type when
1457    /// type-checking each of the alternative expressions whose types
1458    /// we are trying to merge.
1459    pub(crate) fn expected_ty(&self) -> Ty<'tcx> {
1460        self.expected_ty
1461    }
1462
1463    /// Returns the current "merged type", representing our best-guess
1464    /// at the LUB of the expressions we've seen so far (if any). This
1465    /// isn't *final* until you call `self.complete()`, which will return
1466    /// the merged type.
1467    pub(crate) fn merged_ty(&self) -> Ty<'tcx> {
1468        self.final_ty.unwrap_or(self.expected_ty)
1469    }
1470
1471    /// Indicates that the value generated by `expression`, which is
1472    /// of type `expression_ty`, is one of the possibilities that we
1473    /// could coerce from. This will record `expression`, and later
1474    /// calls to `coerce` may come back and add adjustments and things
1475    /// if necessary.
1476    pub(crate) fn coerce<'a>(
1477        &mut self,
1478        fcx: &FnCtxt<'a, 'tcx>,
1479        cause: &ObligationCause<'tcx>,
1480        expression: &'tcx hir::Expr<'tcx>,
1481        expression_ty: Ty<'tcx>,
1482    ) {
1483        self.coerce_inner(fcx, cause, Some(expression), expression_ty, |_| {}, false)
1484    }
1485
1486    /// Indicates that one of the inputs is a "forced unit". This
1487    /// occurs in a case like `if foo { ... };`, where the missing else
1488    /// generates a "forced unit". Another example is a `loop { break;
1489    /// }`, where the `break` has no argument expression. We treat
1490    /// these cases slightly differently for error-reporting
1491    /// purposes. Note that these tend to correspond to cases where
1492    /// the `()` expression is implicit in the source, and hence we do
1493    /// not take an expression argument.
1494    ///
1495    /// The `augment_error` gives you a chance to extend the error
1496    /// message, in case any results (e.g., we use this to suggest
1497    /// removing a `;`).
1498    pub(crate) fn coerce_forced_unit<'a>(
1499        &mut self,
1500        fcx: &FnCtxt<'a, 'tcx>,
1501        cause: &ObligationCause<'tcx>,
1502        augment_error: impl FnOnce(&mut Diag<'_>),
1503        label_unit_as_expected: bool,
1504    ) {
1505        self.coerce_inner(
1506            fcx,
1507            cause,
1508            None,
1509            fcx.tcx.types.unit,
1510            augment_error,
1511            label_unit_as_expected,
1512        )
1513    }
1514
1515    /// The inner coercion "engine". If `expression` is `None`, this
1516    /// is a forced-unit case, and hence `expression_ty` must be
1517    /// `Nil`.
1518    #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
1519    pub(crate) fn coerce_inner<'a>(
1520        &mut self,
1521        fcx: &FnCtxt<'a, 'tcx>,
1522        cause: &ObligationCause<'tcx>,
1523        expression: Option<&'tcx hir::Expr<'tcx>>,
1524        mut expression_ty: Ty<'tcx>,
1525        augment_error: impl FnOnce(&mut Diag<'_>),
1526        label_expression_as_expected: bool,
1527    ) {
1528        // Incorporate whatever type inference information we have
1529        // until now; in principle we might also want to process
1530        // pending obligations, but doing so should only improve
1531        // compatibility (hopefully that is true) by helping us
1532        // uncover never types better.
1533        if expression_ty.is_ty_var() {
1534            expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1535        }
1536
1537        // If we see any error types, just propagate that error
1538        // upwards.
1539        if let Err(guar) = (expression_ty, self.merged_ty()).error_reported() {
1540            self.final_ty = Some(Ty::new_error(fcx.tcx, guar));
1541            return;
1542        }
1543
1544        let (expected, found) = if label_expression_as_expected {
1545            // In the case where this is a "forced unit", like
1546            // `break`, we want to call the `()` "expected"
1547            // since it is implied by the syntax.
1548            // (Note: not all force-units work this way.)"
1549            (expression_ty, self.merged_ty())
1550        } else {
1551            // Otherwise, the "expected" type for error
1552            // reporting is the current unification type,
1553            // which is basically the LUB of the expressions
1554            // we've seen so far (combined with the expected
1555            // type)
1556            (self.merged_ty(), expression_ty)
1557        };
1558
1559        // Handle the actual type unification etc.
1560        let result = if let Some(expression) = expression {
1561            if self.pushed == 0 {
1562                // Special-case the first expression we are coercing.
1563                // To be honest, I'm not entirely sure why we do this.
1564                // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1565                fcx.coerce(
1566                    expression,
1567                    expression_ty,
1568                    self.expected_ty,
1569                    AllowTwoPhase::No,
1570                    Some(cause.clone()),
1571                )
1572            } else {
1573                match self.expressions {
1574                    Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
1575                        cause,
1576                        exprs,
1577                        self.merged_ty(),
1578                        expression,
1579                        expression_ty,
1580                    ),
1581                    Expressions::UpFront(coercion_sites) => fcx.try_find_coercion_lub(
1582                        cause,
1583                        &coercion_sites[0..self.pushed],
1584                        self.merged_ty(),
1585                        expression,
1586                        expression_ty,
1587                    ),
1588                }
1589            }
1590        } else {
1591            // this is a hack for cases where we default to `()` because
1592            // the expression etc has been omitted from the source. An
1593            // example is an `if let` without an else:
1594            //
1595            //     if let Some(x) = ... { }
1596            //
1597            // we wind up with a second match arm that is like `_ =>
1598            // ()`. That is the case we are considering here. We take
1599            // a different path to get the right "expected, found"
1600            // message and so forth (and because we know that
1601            // `expression_ty` will be unit).
1602            //
1603            // Another example is `break` with no argument expression.
1604            assert!(expression_ty.is_unit(), "if let hack without unit type");
1605            fcx.at(cause, fcx.param_env)
1606                .eq(
1607                    // needed for tests/ui/type-alias-impl-trait/issue-65679-inst-opaque-ty-from-val-twice.rs
1608                    DefineOpaqueTypes::Yes,
1609                    expected,
1610                    found,
1611                )
1612                .map(|infer_ok| {
1613                    fcx.register_infer_ok_obligations(infer_ok);
1614                    expression_ty
1615                })
1616        };
1617
1618        debug!(?result);
1619        match result {
1620            Ok(v) => {
1621                self.final_ty = Some(v);
1622                if let Some(e) = expression {
1623                    match self.expressions {
1624                        Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1625                        Expressions::UpFront(coercion_sites) => {
1626                            // if the user gave us an array to validate, check that we got
1627                            // the next expression in the list, as expected
1628                            assert_eq!(
1629                                coercion_sites[self.pushed].as_coercion_site().hir_id,
1630                                e.hir_id
1631                            );
1632                        }
1633                    }
1634                    self.pushed += 1;
1635                }
1636            }
1637            Err(coercion_error) => {
1638                // Mark that we've failed to coerce the types here to suppress
1639                // any superfluous errors we might encounter while trying to
1640                // emit or provide suggestions on how to fix the initial error.
1641                fcx.set_tainted_by_errors(
1642                    fcx.dcx().span_delayed_bug(cause.span, "coercion error but no error emitted"),
1643                );
1644                let (expected, found) = fcx.resolve_vars_if_possible((expected, found));
1645
1646                let mut err;
1647                let mut unsized_return = false;
1648                match *cause.code() {
1649                    ObligationCauseCode::ReturnNoExpression => {
1650                        err = struct_span_code_err!(
1651                            fcx.dcx(),
1652                            cause.span,
1653                            E0069,
1654                            "`return;` in a function whose return type is not `()`"
1655                        );
1656                        if let Some(value) = fcx.err_ctxt().ty_kind_suggestion(fcx.param_env, found)
1657                        {
1658                            err.span_suggestion_verbose(
1659                                cause.span.shrink_to_hi(),
1660                                "give the `return` a value of the expected type",
1661                                format!(" {value}"),
1662                                Applicability::HasPlaceholders,
1663                            );
1664                        }
1665                        err.span_label(cause.span, "return type is not `()`");
1666                    }
1667                    ObligationCauseCode::BlockTailExpression(blk_id, ..) => {
1668                        err = self.report_return_mismatched_types(
1669                            cause,
1670                            expected,
1671                            found,
1672                            coercion_error,
1673                            fcx,
1674                            blk_id,
1675                            expression,
1676                        );
1677                        unsized_return = self.is_return_ty_definitely_unsized(fcx);
1678                    }
1679                    ObligationCauseCode::ReturnValue(return_expr_id) => {
1680                        err = self.report_return_mismatched_types(
1681                            cause,
1682                            expected,
1683                            found,
1684                            coercion_error,
1685                            fcx,
1686                            return_expr_id,
1687                            expression,
1688                        );
1689                        unsized_return = self.is_return_ty_definitely_unsized(fcx);
1690                    }
1691                    ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
1692                        arm_span,
1693                        arm_ty,
1694                        prior_arm_ty,
1695                        ref prior_non_diverging_arms,
1696                        tail_defines_return_position_impl_trait: Some(rpit_def_id),
1697                        ..
1698                    }) => {
1699                        err = fcx.err_ctxt().report_mismatched_types(
1700                            cause,
1701                            fcx.param_env,
1702                            expected,
1703                            found,
1704                            coercion_error,
1705                        );
1706                        // Check that we're actually in the second or later arm
1707                        if prior_non_diverging_arms.len() > 0 {
1708                            self.suggest_boxing_tail_for_return_position_impl_trait(
1709                                fcx,
1710                                &mut err,
1711                                rpit_def_id,
1712                                arm_ty,
1713                                prior_arm_ty,
1714                                prior_non_diverging_arms
1715                                    .iter()
1716                                    .chain(std::iter::once(&arm_span))
1717                                    .copied(),
1718                            );
1719                        }
1720                    }
1721                    ObligationCauseCode::IfExpression {
1722                        expr_id,
1723                        tail_defines_return_position_impl_trait: Some(rpit_def_id),
1724                    } => {
1725                        let hir::Node::Expr(hir::Expr {
1726                            kind: hir::ExprKind::If(_, then_expr, Some(else_expr)),
1727                            ..
1728                        }) = fcx.tcx.hir_node(expr_id)
1729                        else {
1730                            unreachable!();
1731                        };
1732                        err = fcx.err_ctxt().report_mismatched_types(
1733                            cause,
1734                            fcx.param_env,
1735                            expected,
1736                            found,
1737                            coercion_error,
1738                        );
1739                        let then_span = fcx.find_block_span_from_hir_id(then_expr.hir_id);
1740                        let else_span = fcx.find_block_span_from_hir_id(else_expr.hir_id);
1741                        // Don't suggest wrapping whole block in `Box::new`.
1742                        if then_span != then_expr.span && else_span != else_expr.span {
1743                            let then_ty = fcx.typeck_results.borrow().expr_ty(then_expr);
1744                            let else_ty = fcx.typeck_results.borrow().expr_ty(else_expr);
1745                            self.suggest_boxing_tail_for_return_position_impl_trait(
1746                                fcx,
1747                                &mut err,
1748                                rpit_def_id,
1749                                then_ty,
1750                                else_ty,
1751                                [then_span, else_span].into_iter(),
1752                            );
1753                        }
1754                    }
1755                    _ => {
1756                        err = fcx.err_ctxt().report_mismatched_types(
1757                            cause,
1758                            fcx.param_env,
1759                            expected,
1760                            found,
1761                            coercion_error,
1762                        );
1763                    }
1764                }
1765
1766                augment_error(&mut err);
1767
1768                if let Some(expr) = expression {
1769                    if let hir::ExprKind::Loop(
1770                        _,
1771                        _,
1772                        loop_src @ (hir::LoopSource::While | hir::LoopSource::ForLoop),
1773                        _,
1774                    ) = expr.kind
1775                    {
1776                        let loop_type = if loop_src == hir::LoopSource::While {
1777                            "`while` loops"
1778                        } else {
1779                            "`for` loops"
1780                        };
1781
1782                        err.note(format!("{loop_type} evaluate to unit type `()`"));
1783                    }
1784
1785                    fcx.emit_coerce_suggestions(
1786                        &mut err,
1787                        expr,
1788                        found,
1789                        expected,
1790                        None,
1791                        Some(coercion_error),
1792                    );
1793                }
1794
1795                let reported = err.emit_unless(unsized_return);
1796
1797                self.final_ty = Some(Ty::new_error(fcx.tcx, reported));
1798            }
1799        }
1800    }
1801
1802    fn suggest_boxing_tail_for_return_position_impl_trait(
1803        &self,
1804        fcx: &FnCtxt<'_, 'tcx>,
1805        err: &mut Diag<'_>,
1806        rpit_def_id: LocalDefId,
1807        a_ty: Ty<'tcx>,
1808        b_ty: Ty<'tcx>,
1809        arm_spans: impl Iterator<Item = Span>,
1810    ) {
1811        let compatible = |ty: Ty<'tcx>| {
1812            fcx.probe(|_| {
1813                let ocx = ObligationCtxt::new(fcx);
1814                ocx.register_obligations(
1815                    fcx.tcx.item_self_bounds(rpit_def_id).iter_identity().filter_map(|clause| {
1816                        let predicate = clause
1817                            .kind()
1818                            .map_bound(|clause| match clause {
1819                                ty::ClauseKind::Trait(trait_pred) => Some(ty::ClauseKind::Trait(
1820                                    trait_pred.with_self_ty(fcx.tcx, ty),
1821                                )),
1822                                ty::ClauseKind::Projection(proj_pred) => Some(
1823                                    ty::ClauseKind::Projection(proj_pred.with_self_ty(fcx.tcx, ty)),
1824                                ),
1825                                _ => None,
1826                            })
1827                            .transpose()?;
1828                        Some(Obligation::new(
1829                            fcx.tcx,
1830                            ObligationCause::dummy(),
1831                            fcx.param_env,
1832                            predicate,
1833                        ))
1834                    }),
1835                );
1836                ocx.select_where_possible().is_empty()
1837            })
1838        };
1839
1840        if !compatible(a_ty) || !compatible(b_ty) {
1841            return;
1842        }
1843
1844        let rpid_def_span = fcx.tcx.def_span(rpit_def_id);
1845        err.subdiagnostic(SuggestBoxingForReturnImplTrait::ChangeReturnType {
1846            start_sp: rpid_def_span.with_hi(rpid_def_span.lo() + BytePos(4)),
1847            end_sp: rpid_def_span.shrink_to_hi(),
1848        });
1849
1850        let (starts, ends) =
1851            arm_spans.map(|span| (span.shrink_to_lo(), span.shrink_to_hi())).unzip();
1852        err.subdiagnostic(SuggestBoxingForReturnImplTrait::BoxReturnExpr { starts, ends });
1853    }
1854
1855    fn report_return_mismatched_types<'infcx>(
1856        &self,
1857        cause: &ObligationCause<'tcx>,
1858        expected: Ty<'tcx>,
1859        found: Ty<'tcx>,
1860        ty_err: TypeError<'tcx>,
1861        fcx: &'infcx FnCtxt<'_, 'tcx>,
1862        block_or_return_id: hir::HirId,
1863        expression: Option<&'tcx hir::Expr<'tcx>>,
1864    ) -> Diag<'infcx> {
1865        let mut err =
1866            fcx.err_ctxt().report_mismatched_types(cause, fcx.param_env, expected, found, ty_err);
1867
1868        let due_to_block = matches!(fcx.tcx.hir_node(block_or_return_id), hir::Node::Block(..));
1869        let parent = fcx.tcx.parent_hir_node(block_or_return_id);
1870        if let Some(expr) = expression
1871            && let hir::Node::Expr(&hir::Expr {
1872                kind: hir::ExprKind::Closure(&hir::Closure { body, .. }),
1873                ..
1874            }) = parent
1875        {
1876            let needs_block =
1877                !matches!(fcx.tcx.hir_body(body).value.kind, hir::ExprKind::Block(..));
1878            fcx.suggest_missing_semicolon(&mut err, expr, expected, needs_block, true);
1879        }
1880        // Verify that this is a tail expression of a function, otherwise the
1881        // label pointing out the cause for the type coercion will be wrong
1882        // as prior return coercions would not be relevant (#57664).
1883        if let Some(expr) = expression
1884            && due_to_block
1885        {
1886            fcx.suggest_missing_semicolon(&mut err, expr, expected, false, false);
1887            let pointing_at_return_type = fcx.suggest_mismatched_types_on_tail(
1888                &mut err,
1889                expr,
1890                expected,
1891                found,
1892                block_or_return_id,
1893            );
1894            if let Some(cond_expr) = fcx.tcx.hir_get_if_cause(expr.hir_id)
1895                && expected.is_unit()
1896                && !pointing_at_return_type
1897                // If the block is from an external macro or try (`?`) desugaring, then
1898                // do not suggest adding a semicolon, because there's nowhere to put it.
1899                // See issues #81943 and #87051.
1900                && matches!(
1901                    cond_expr.span.desugaring_kind(),
1902                    None | Some(DesugaringKind::WhileLoop)
1903                )
1904                && !cond_expr.span.in_external_macro(fcx.tcx.sess.source_map())
1905                && !matches!(
1906                    cond_expr.kind,
1907                    hir::ExprKind::Match(.., hir::MatchSource::TryDesugar(_))
1908                )
1909            {
1910                err.span_label(cond_expr.span, "expected this to be `()`");
1911                if expr.can_have_side_effects() {
1912                    fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
1913                }
1914            }
1915        };
1916
1917        // If this is due to an explicit `return`, suggest adding a return type.
1918        if let Some((fn_id, fn_decl)) = fcx.get_fn_decl(block_or_return_id)
1919            && !due_to_block
1920        {
1921            fcx.suggest_missing_return_type(&mut err, fn_decl, expected, found, fn_id);
1922        }
1923
1924        // If this is due to a block, then maybe we forgot a `return`/`break`.
1925        if due_to_block
1926            && let Some(expr) = expression
1927            && let Some(parent_fn_decl) =
1928                fcx.tcx.hir_fn_decl_by_hir_id(fcx.tcx.local_def_id_to_hir_id(fcx.body_id))
1929        {
1930            fcx.suggest_missing_break_or_return_expr(
1931                &mut err,
1932                expr,
1933                parent_fn_decl,
1934                expected,
1935                found,
1936                block_or_return_id,
1937                fcx.body_id,
1938            );
1939        }
1940
1941        let ret_coercion_span = fcx.ret_coercion_span.get();
1942
1943        if let Some(sp) = ret_coercion_span
1944            // If the closure has an explicit return type annotation, or if
1945            // the closure's return type has been inferred from outside
1946            // requirements (such as an Fn* trait bound), then a type error
1947            // may occur at the first return expression we see in the closure
1948            // (if it conflicts with the declared return type). Skip adding a
1949            // note in this case, since it would be incorrect.
1950            && let Some(fn_sig) = fcx.body_fn_sig()
1951            && fn_sig.output().is_ty_var()
1952        {
1953            err.span_note(sp, format!("return type inferred to be `{expected}` here"));
1954        }
1955
1956        err
1957    }
1958
1959    /// Checks whether the return type is unsized via an obligation, which makes
1960    /// sure we consider `dyn Trait: Sized` where clauses, which are trivially
1961    /// false but technically valid for typeck.
1962    fn is_return_ty_definitely_unsized(&self, fcx: &FnCtxt<'_, 'tcx>) -> bool {
1963        if let Some(sig) = fcx.body_fn_sig() {
1964            !fcx.predicate_may_hold(&Obligation::new(
1965                fcx.tcx,
1966                ObligationCause::dummy(),
1967                fcx.param_env,
1968                ty::TraitRef::new(
1969                    fcx.tcx,
1970                    fcx.tcx.require_lang_item(hir::LangItem::Sized, DUMMY_SP),
1971                    [sig.output()],
1972                ),
1973            ))
1974        } else {
1975            false
1976        }
1977    }
1978
1979    pub(crate) fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
1980        if let Some(final_ty) = self.final_ty {
1981            final_ty
1982        } else {
1983            // If we only had inputs that were of type `!` (or no
1984            // inputs at all), then the final type is `!`.
1985            assert_eq!(self.pushed, 0);
1986            fcx.tcx.types.never
1987        }
1988    }
1989}
1990
1991/// Something that can be converted into an expression to which we can
1992/// apply a coercion.
1993pub(crate) trait AsCoercionSite {
1994    fn as_coercion_site(&self) -> &hir::Expr<'_>;
1995}
1996
1997impl AsCoercionSite for hir::Expr<'_> {
1998    fn as_coercion_site(&self) -> &hir::Expr<'_> {
1999        self
2000    }
2001}
2002
2003impl<'a, T> AsCoercionSite for &'a T
2004where
2005    T: AsCoercionSite,
2006{
2007    fn as_coercion_site(&self) -> &hir::Expr<'_> {
2008        (**self).as_coercion_site()
2009    }
2010}
2011
2012impl AsCoercionSite for ! {
2013    fn as_coercion_site(&self) -> &hir::Expr<'_> {
2014        *self
2015    }
2016}
2017
2018impl AsCoercionSite for hir::Arm<'_> {
2019    fn as_coercion_site(&self) -> &hir::Expr<'_> {
2020        self.body
2021    }
2022}