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