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::{Coercion, DefineOpaqueTypes, InferOk, InferResult};
48use rustc_infer::traits::{
49 IfExpressionCause, MatchExpressionArmCause, Obligation, PredicateObligation,
50 PredicateObligations, SelectionError,
51};
52use rustc_middle::span_bug;
53use rustc_middle::ty::adjustment::{
54 Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCoercion,
55};
56use rustc_middle::ty::error::TypeError;
57use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, TypeVisitableExt};
58use rustc_span::{BytePos, DUMMY_SP, DesugaringKind, Span};
59use rustc_trait_selection::infer::InferCtxtExt as _;
60use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
61use rustc_trait_selection::traits::{
62 self, NormalizeExt, ObligationCause, ObligationCauseCode, ObligationCtxt,
63};
64use smallvec::{SmallVec, smallvec};
65use tracing::{debug, instrument};
66
67use crate::FnCtxt;
68use crate::errors::SuggestBoxingForReturnImplTrait;
69
70struct Coerce<'a, 'tcx> {
71 fcx: &'a FnCtxt<'a, 'tcx>,
72 cause: ObligationCause<'tcx>,
73 use_lub: bool,
74 /// Determines whether or not allow_two_phase_borrow is set on any
75 /// autoref adjustments we create while coercing. We don't want to
76 /// allow deref coercions to create two-phase borrows, at least initially,
77 /// but we do need two-phase borrows for function argument reborrows.
78 /// See #47489 and #48598
79 /// See docs on the "AllowTwoPhase" type for a more detailed discussion
80 allow_two_phase: AllowTwoPhase,
81 /// Whether we allow `NeverToAny` coercions. This is unsound if we're
82 /// coercing a place expression without it counting as a read in the MIR.
83 /// This is a side-effect of HIR not really having a great distinction
84 /// between places and values.
85 coerce_never: bool,
86}
87
88impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
89 type Target = FnCtxt<'a, 'tcx>;
90 fn deref(&self) -> &Self::Target {
91 self.fcx
92 }
93}
94
95type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
96
97/// Coercing a mutable reference to an immutable works, while
98/// coercing `&T` to `&mut T` should be forbidden.
99fn coerce_mutbls<'tcx>(
100 from_mutbl: hir::Mutability,
101 to_mutbl: hir::Mutability,
102) -> RelateResult<'tcx, ()> {
103 if from_mutbl >= to_mutbl { Ok(()) } else { Err(TypeError::Mutability) }
104}
105
106/// This always returns `Ok(...)`.
107fn success<'tcx>(
108 adj: Vec<Adjustment<'tcx>>,
109 target: Ty<'tcx>,
110 obligations: PredicateObligations<'tcx>,
111) -> CoerceResult<'tcx> {
112 Ok(InferOk { value: (adj, target), obligations })
113}
114
115impl<'f, 'tcx> Coerce<'f, 'tcx> {
116 fn new(
117 fcx: &'f FnCtxt<'f, 'tcx>,
118 cause: ObligationCause<'tcx>,
119 allow_two_phase: AllowTwoPhase,
120 coerce_never: bool,
121 ) -> Self {
122 Coerce { fcx, cause, allow_two_phase, use_lub: false, coerce_never }
123 }
124
125 fn unify_raw(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
126 debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
127 self.commit_if_ok(|_| {
128 let at = self.at(&self.cause, self.fcx.param_env);
129
130 let res = if self.use_lub {
131 at.lub(b, a)
132 } else {
133 at.sup(DefineOpaqueTypes::Yes, b, a)
134 .map(|InferOk { value: (), obligations }| InferOk { value: b, obligations })
135 };
136
137 // In the new solver, lazy norm may allow us to shallowly equate
138 // more types, but we emit possibly impossible-to-satisfy obligations.
139 // Filter these cases out to make sure our coercion is more accurate.
140 match res {
141 Ok(InferOk { value, obligations }) if self.next_trait_solver() => {
142 let ocx = ObligationCtxt::new(self);
143 ocx.register_obligations(obligations);
144 if ocx.select_where_possible().is_empty() {
145 Ok(InferOk { value, obligations: ocx.into_pending_obligations() })
146 } else {
147 Err(TypeError::Mismatch)
148 }
149 }
150 res => res,
151 }
152 })
153 }
154
155 /// Unify two types (using sub or lub).
156 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
157 self.unify_raw(a, b)
158 .and_then(|InferOk { value: ty, obligations }| success(vec![], ty, obligations))
159 }
160
161 /// Unify two types (using sub or lub) and produce a specific coercion.
162 fn unify_and(
163 &self,
164 a: Ty<'tcx>,
165 b: Ty<'tcx>,
166 adjustments: impl IntoIterator<Item = Adjustment<'tcx>>,
167 final_adjustment: Adjust,
168 ) -> CoerceResult<'tcx> {
169 self.unify_raw(a, b).and_then(|InferOk { value: ty, obligations }| {
170 success(
171 adjustments
172 .into_iter()
173 .chain(std::iter::once(Adjustment { target: ty, kind: final_adjustment }))
174 .collect(),
175 ty,
176 obligations,
177 )
178 })
179 }
180
181 #[instrument(skip(self))]
182 fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
183 // First, remove any resolved type variables (at the top level, at least):
184 let a = self.shallow_resolve(a);
185 let b = self.shallow_resolve(b);
186 debug!("Coerce.tys({:?} => {:?})", a, b);
187
188 // Coercing from `!` to any type is allowed:
189 if a.is_never() {
190 if self.coerce_never {
191 return success(
192 vec![Adjustment { kind: Adjust::NeverToAny, target: b }],
193 b,
194 PredicateObligations::new(),
195 );
196 } else {
197 // Otherwise the only coercion we can do is unification.
198 return self.unify(a, b);
199 }
200 }
201
202 // Coercing *from* an unresolved inference variable means that
203 // we have no information about the source type. This will always
204 // ultimately fall back to some form of subtyping.
205 if a.is_ty_var() {
206 return self.coerce_from_inference_variable(a, b);
207 }
208
209 // Consider coercing the subtype to a DST
210 //
211 // NOTE: this is wrapped in a `commit_if_ok` because it creates
212 // a "spurious" type variable, and we don't want to have that
213 // type variable in memory if the coercion fails.
214 let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
215 match unsize {
216 Ok(_) => {
217 debug!("coerce: unsize successful");
218 return unsize;
219 }
220 Err(error) => {
221 debug!(?error, "coerce: unsize failed");
222 }
223 }
224
225 // Examine the supertype and consider type-specific coercions, such
226 // as auto-borrowing, coercing pointer mutability, a `dyn*` coercion,
227 // or pin-ergonomics.
228 match *b.kind() {
229 ty::RawPtr(_, b_mutbl) => {
230 return self.coerce_raw_ptr(a, b, b_mutbl);
231 }
232 ty::Ref(r_b, _, mutbl_b) => {
233 return self.coerce_borrowed_pointer(a, b, r_b, mutbl_b);
234 }
235 ty::Dynamic(predicates, region, ty::DynStar) if self.tcx.features().dyn_star() => {
236 return self.coerce_dyn_star(a, b, predicates, region);
237 }
238 ty::Adt(pin, _)
239 if self.tcx.features().pin_ergonomics()
240 && self.tcx.is_lang_item(pin.did(), hir::LangItem::Pin) =>
241 {
242 let pin_coerce = self.commit_if_ok(|_| self.coerce_pin_ref(a, b));
243 if pin_coerce.is_ok() {
244 return pin_coerce;
245 }
246 }
247 _ => {}
248 }
249
250 match *a.kind() {
251 ty::FnDef(..) => {
252 // Function items are coercible to any closure
253 // type; function pointers are not (that would
254 // require double indirection).
255 // Additionally, we permit coercion of function
256 // items to drop the unsafe qualifier.
257 self.coerce_from_fn_item(a, b)
258 }
259 ty::FnPtr(a_sig_tys, a_hdr) => {
260 // We permit coercion of fn pointers to drop the
261 // unsafe qualifier.
262 self.coerce_from_fn_pointer(a_sig_tys.with(a_hdr), b)
263 }
264 ty::Closure(closure_def_id_a, args_a) => {
265 // Non-capturing closures are coercible to
266 // function pointers or unsafe function pointers.
267 // It cannot convert closures that require unsafe.
268 self.coerce_closure_to_fn(a, closure_def_id_a, args_a, b)
269 }
270 _ => {
271 // Otherwise, just use unification rules.
272 self.unify(a, b)
273 }
274 }
275 }
276
277 /// Coercing *from* an inference variable. In this case, we have no information
278 /// about the source type, so we can't really do a true coercion and we always
279 /// fall back to subtyping (`unify_and`).
280 fn coerce_from_inference_variable(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
281 debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
282 assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
283 assert!(self.shallow_resolve(b) == b);
284
285 if b.is_ty_var() {
286 // Two unresolved type variables: create a `Coerce` predicate.
287 let target_ty = if self.use_lub { self.next_ty_var(self.cause.span) } else { b };
288
289 let mut obligations = PredicateObligations::with_capacity(2);
290 for &source_ty in &[a, b] {
291 if source_ty != target_ty {
292 obligations.push(Obligation::new(
293 self.tcx(),
294 self.cause.clone(),
295 self.param_env,
296 ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate {
297 a: source_ty,
298 b: target_ty,
299 })),
300 ));
301 }
302 }
303
304 debug!(
305 "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
306 target_ty, obligations
307 );
308 success(vec![], target_ty, obligations)
309 } else {
310 // One unresolved type variable: just apply subtyping, we may be able
311 // to do something useful.
312 self.unify(a, b)
313 }
314 }
315
316 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
317 /// To match `A` with `B`, autoderef will be performed,
318 /// calling `deref`/`deref_mut` where necessary.
319 fn coerce_borrowed_pointer(
320 &self,
321 a: Ty<'tcx>,
322 b: Ty<'tcx>,
323 r_b: ty::Region<'tcx>,
324 mutbl_b: hir::Mutability,
325 ) -> CoerceResult<'tcx> {
326 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
327
328 // If we have a parameter of type `&M T_a` and the value
329 // provided is `expr`, we will be adding an implicit borrow,
330 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
331 // to type check, we will construct the type that `&M*expr` would
332 // yield.
333
334 let (r_a, mt_a) = match *a.kind() {
335 ty::Ref(r_a, ty, mutbl) => {
336 let mt_a = ty::TypeAndMut { ty, mutbl };
337 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
338 (r_a, mt_a)
339 }
340 _ => return self.unify(a, b),
341 };
342
343 let span = self.cause.span;
344
345 let mut first_error = None;
346 let mut r_borrow_var = None;
347 let mut autoderef = self.autoderef(span, a);
348 let mut found = None;
349
350 for (referent_ty, autoderefs) in autoderef.by_ref() {
351 if autoderefs == 0 {
352 // Don't let this pass, otherwise it would cause
353 // &T to autoref to &&T.
354 continue;
355 }
356
357 // At this point, we have deref'd `a` to `referent_ty`. So
358 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
359 // In the autoderef loop for `&'a mut Vec<T>`, we would get
360 // three callbacks:
361 //
362 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
363 // - `Vec<T>` -- 1 deref
364 // - `[T]` -- 2 deref
365 //
366 // At each point after the first callback, we want to
367 // check to see whether this would match out target type
368 // (`&'b mut [T]`) if we autoref'd it. We can't just
369 // compare the referent types, though, because we still
370 // have to consider the mutability. E.g., in the case
371 // we've been considering, we have an `&mut` reference, so
372 // the `T` in `[T]` needs to be unified with equality.
373 //
374 // Therefore, we construct reference types reflecting what
375 // the types will be after we do the final auto-ref and
376 // compare those. Note that this means we use the target
377 // mutability [1], since it may be that we are coercing
378 // from `&mut T` to `&U`.
379 //
380 // One fine point concerns the region that we use. We
381 // choose the region such that the region of the final
382 // type that results from `unify` will be the region we
383 // want for the autoref:
384 //
385 // - if in sub mode, that means we want to use `'b` (the
386 // region from the target reference) for both
387 // pointers [2]. This is because sub mode (somewhat
388 // arbitrarily) returns the subtype region. In the case
389 // where we are coercing to a target type, we know we
390 // want to use that target type region (`'b`) because --
391 // for the program to type-check -- it must be the
392 // smaller of the two.
393 // - One fine point. It may be surprising that we can
394 // use `'b` without relating `'a` and `'b`. The reason
395 // that this is ok is that what we produce is
396 // effectively a `&'b *x` expression (if you could
397 // annotate the region of a borrow), and regionck has
398 // code that adds edges from the region of a borrow
399 // (`'b`, here) into the regions in the borrowed
400 // expression (`*x`, here). (Search for "link".)
401 // - if in lub mode, things can get fairly complicated. The
402 // easiest thing is just to make a fresh
403 // region variable [4], which effectively means we defer
404 // the decision to region inference (and regionck, which will add
405 // some more edges to this variable). However, this can wind up
406 // creating a crippling number of variables in some cases --
407 // e.g., #32278 -- so we optimize one particular case [3].
408 // Let me try to explain with some examples:
409 // - The "running example" above represents the simple case,
410 // where we have one `&` reference at the outer level and
411 // ownership all the rest of the way down. In this case,
412 // we want `LUB('a, 'b)` as the resulting region.
413 // - However, if there are nested borrows, that region is
414 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
415 // `&'b T`. In this case, `'a` is actually irrelevant.
416 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
417 // we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
418 // (The errors actually show up in borrowck, typically, because
419 // this extra edge causes the region `'a` to be inferred to something
420 // too big, which then results in borrowck errors.)
421 // - We could track the innermost shared reference, but there is already
422 // code in regionck that has the job of creating links between
423 // the region of a borrow and the regions in the thing being
424 // borrowed (here, `'a` and `'x`), and it knows how to handle
425 // all the various cases. So instead we just make a region variable
426 // and let regionck figure it out.
427 let r = if !self.use_lub {
428 r_b // [2] above
429 } else if autoderefs == 1 {
430 r_a // [3] above
431 } else {
432 if r_borrow_var.is_none() {
433 // create var lazily, at most once
434 let coercion = Coercion(span);
435 let r = self.next_region_var(coercion);
436 r_borrow_var = Some(r); // [4] above
437 }
438 r_borrow_var.unwrap()
439 };
440 let derefd_ty_a = Ty::new_ref(
441 self.tcx,
442 r,
443 referent_ty,
444 mutbl_b, // [1] above
445 );
446 match self.unify_raw(derefd_ty_a, b) {
447 Ok(ok) => {
448 found = Some(ok);
449 break;
450 }
451 Err(err) => {
452 if first_error.is_none() {
453 first_error = Some(err);
454 }
455 }
456 }
457 }
458
459 // Extract type or return an error. We return the first error
460 // we got, which should be from relating the "base" type
461 // (e.g., in example above, the failure from relating `Vec<T>`
462 // to the target type), since that should be the least
463 // confusing.
464 let Some(InferOk { value: ty, mut obligations }) = found else {
465 if let Some(first_error) = first_error {
466 debug!("coerce_borrowed_pointer: failed with err = {:?}", first_error);
467 return Err(first_error);
468 } else {
469 // This may happen in the new trait solver since autoderef requires
470 // the pointee to be structurally normalizable, or else it'll just bail.
471 // So when we have a type like `&<not well formed>`, then we get no
472 // autoderef steps (even though there should be at least one). That means
473 // we get no type mismatches, since the loop above just exits early.
474 return Err(TypeError::Mismatch);
475 }
476 };
477
478 if ty == a && mt_a.mutbl.is_not() && autoderef.step_count() == 1 {
479 // As a special case, if we would produce `&'a *x`, that's
480 // a total no-op. We end up with the type `&'a T` just as
481 // we started with. In that case, just skip it
482 // altogether. This is just an optimization.
483 //
484 // Note that for `&mut`, we DO want to reborrow --
485 // otherwise, this would be a move, which might be an
486 // error. For example `foo(self.x)` where `self` and
487 // `self.x` both have `&mut `type would be a move of
488 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
489 // which is a borrow.
490 assert!(mutbl_b.is_not()); // can only coerce &T -> &U
491 return success(vec![], ty, obligations);
492 }
493
494 let InferOk { value: mut adjustments, obligations: o } =
495 self.adjust_steps_as_infer_ok(&autoderef);
496 obligations.extend(o);
497 obligations.extend(autoderef.into_obligations());
498
499 // Now apply the autoref. We have to extract the region out of
500 // the final ref type we got.
501 let ty::Ref(..) = ty.kind() else {
502 span_bug!(span, "expected a ref type, got {:?}", ty);
503 };
504 let mutbl = AutoBorrowMutability::new(mutbl_b, self.allow_two_phase);
505 adjustments.push(Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(mutbl)), target: ty });
506
507 debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
508
509 success(adjustments, ty, obligations)
510 }
511
512 /// Performs [unsized coercion] by emulating a fulfillment loop on a
513 /// `CoerceUnsized` goal until all `CoerceUnsized` and `Unsize` goals
514 /// are successfully selected.
515 ///
516 /// [unsized coercion](https://doc.rust-lang.org/reference/type-coercions.html#unsized-coercions)
517 #[instrument(skip(self), level = "debug")]
518 fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
519 source = self.shallow_resolve(source);
520 target = self.shallow_resolve(target);
521 debug!(?source, ?target);
522
523 // We don't apply any coercions incase either the source or target
524 // aren't sufficiently well known but tend to instead just equate
525 // them both.
526 if source.is_ty_var() {
527 debug!("coerce_unsized: source is a TyVar, bailing out");
528 return Err(TypeError::Mismatch);
529 }
530 if target.is_ty_var() {
531 debug!("coerce_unsized: target is a TyVar, bailing out");
532 return Err(TypeError::Mismatch);
533 }
534
535 let traits =
536 (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
537 let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
538 debug!("missing Unsize or CoerceUnsized traits");
539 return Err(TypeError::Mismatch);
540 };
541
542 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
543 // a DST unless we have to. This currently comes out in the wash since
544 // we can't unify [T] with U. But to properly support DST, we need to allow
545 // that, at which point we will need extra checks on the target here.
546
547 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
548 let reborrow = match (source.kind(), target.kind()) {
549 (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
550 coerce_mutbls(mutbl_a, mutbl_b)?;
551
552 let coercion = Coercion(self.cause.span);
553 let r_borrow = self.next_region_var(coercion);
554
555 // We don't allow two-phase borrows here, at least for initial
556 // implementation. If it happens that this coercion is a function argument,
557 // the reborrow in coerce_borrowed_ptr will pick it up.
558 let mutbl = AutoBorrowMutability::new(mutbl_b, AllowTwoPhase::No);
559
560 Some((
561 Adjustment { kind: Adjust::Deref(None), target: ty_a },
562 Adjustment {
563 kind: Adjust::Borrow(AutoBorrow::Ref(mutbl)),
564 target: Ty::new_ref(self.tcx, r_borrow, ty_a, mutbl_b),
565 },
566 ))
567 }
568 (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(_, mt_b)) => {
569 coerce_mutbls(mt_a, mt_b)?;
570
571 Some((
572 Adjustment { kind: Adjust::Deref(None), target: ty_a },
573 Adjustment {
574 kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
575 target: Ty::new_ptr(self.tcx, ty_a, mt_b),
576 },
577 ))
578 }
579 _ => None,
580 };
581 let coerce_source = reborrow.as_ref().map_or(source, |(_, r)| r.target);
582
583 // Setup either a subtyping or a LUB relationship between
584 // the `CoerceUnsized` target type and the expected type.
585 // We only have the latter, so we use an inference variable
586 // for the former and let type inference do the rest.
587 let coerce_target = self.next_ty_var(self.cause.span);
588
589 let mut coercion = self.unify_and(
590 coerce_target,
591 target,
592 reborrow.into_iter().flat_map(|(deref, autoref)| [deref, autoref]),
593 Adjust::Pointer(PointerCoercion::Unsize),
594 )?;
595
596 let mut selcx = traits::SelectionContext::new(self);
597
598 // Create an obligation for `Source: CoerceUnsized<Target>`.
599 let cause = self.cause(self.cause.span, ObligationCauseCode::Coercion { source, target });
600
601 // Use a FIFO queue for this custom fulfillment procedure.
602 //
603 // A Vec (or SmallVec) is not a natural choice for a queue. However,
604 // this code path is hot, and this queue usually has a max length of 1
605 // and almost never more than 3. By using a SmallVec we avoid an
606 // allocation, at the (very small) cost of (occasionally) having to
607 // shift subsequent elements down when removing the front element.
608 let mut queue: SmallVec<[PredicateObligation<'tcx>; 4]> = smallvec![Obligation::new(
609 self.tcx,
610 cause,
611 self.fcx.param_env,
612 ty::TraitRef::new(self.tcx, coerce_unsized_did, [coerce_source, coerce_target])
613 )];
614
615 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
616 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
617 // inference might unify those two inner type variables later.
618 let traits = [coerce_unsized_did, unsize_did];
619 while !queue.is_empty() {
620 let obligation = queue.remove(0);
621 let trait_pred = match obligation.predicate.kind().no_bound_vars() {
622 Some(ty::PredicateKind::Clause(ty::ClauseKind::Trait(trait_pred)))
623 if traits.contains(&trait_pred.def_id()) =>
624 {
625 self.resolve_vars_if_possible(trait_pred)
626 }
627 // Eagerly process alias-relate obligations in new trait solver,
628 // since these can be emitted in the process of solving trait goals,
629 // but we need to constrain vars before processing goals mentioning
630 // them.
631 Some(ty::PredicateKind::AliasRelate(..)) => {
632 let ocx = ObligationCtxt::new(self);
633 ocx.register_obligation(obligation);
634 if !ocx.select_where_possible().is_empty() {
635 return Err(TypeError::Mismatch);
636 }
637 coercion.obligations.extend(ocx.into_pending_obligations());
638 continue;
639 }
640 _ => {
641 coercion.obligations.push(obligation);
642 continue;
643 }
644 };
645 debug!("coerce_unsized resolve step: {:?}", trait_pred);
646 match selcx.select(&obligation.with(selcx.tcx(), trait_pred)) {
647 // Uncertain or unimplemented.
648 Ok(None) => {
649 if trait_pred.def_id() == unsize_did {
650 let self_ty = trait_pred.self_ty();
651 let unsize_ty = trait_pred.trait_ref.args[1].expect_ty();
652 debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
653 match (self_ty.kind(), unsize_ty.kind()) {
654 (&ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
655 if self.type_var_is_sized(v) =>
656 {
657 debug!("coerce_unsized: have sized infer {:?}", v);
658 coercion.obligations.push(obligation);
659 // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
660 // for unsizing.
661 }
662 _ => {
663 // Some other case for `$0: Unsize<Something>`. Note that we
664 // hit this case even if `Something` is a sized type, so just
665 // don't do the coercion.
666 debug!("coerce_unsized: ambiguous unsize");
667 return Err(TypeError::Mismatch);
668 }
669 }
670 } else {
671 debug!("coerce_unsized: early return - ambiguous");
672 return Err(TypeError::Mismatch);
673 }
674 }
675 Err(traits::Unimplemented) => {
676 debug!("coerce_unsized: early return - can't prove obligation");
677 return Err(TypeError::Mismatch);
678 }
679
680 Err(SelectionError::TraitDynIncompatible(_)) => {
681 // Dyn compatibility errors in coercion will *always* be due to the
682 // fact that the RHS of the coercion is a non-dyn compatible `dyn Trait`
683 // writen in source somewhere (otherwise we will never have lowered
684 // the dyn trait from HIR to middle).
685 //
686 // There's no reason to emit yet another dyn compatibility error,
687 // especially since the span will differ slightly and thus not be
688 // deduplicated at all!
689 self.fcx.set_tainted_by_errors(
690 self.fcx
691 .dcx()
692 .span_delayed_bug(self.cause.span, "dyn compatibility during coercion"),
693 );
694 }
695 Err(err) => {
696 let guar = self.err_ctxt().report_selection_error(
697 obligation.clone(),
698 &obligation,
699 &err,
700 );
701 self.fcx.set_tainted_by_errors(guar);
702 // Treat this like an obligation and follow through
703 // with the unsizing - the lack of a coercion should
704 // be silent, as it causes a type mismatch later.
705 }
706
707 Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
708 }
709 }
710
711 Ok(coercion)
712 }
713
714 fn coerce_dyn_star(
715 &self,
716 a: Ty<'tcx>,
717 b: Ty<'tcx>,
718 predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
719 b_region: ty::Region<'tcx>,
720 ) -> CoerceResult<'tcx> {
721 if !self.tcx.features().dyn_star() {
722 return Err(TypeError::Mismatch);
723 }
724
725 // FIXME(dyn_star): We should probably allow things like casting from
726 // `dyn* Foo + Send` to `dyn* Foo`.
727 if let ty::Dynamic(a_data, _, ty::DynStar) = a.kind()
728 && let ty::Dynamic(b_data, _, ty::DynStar) = b.kind()
729 && a_data.principal_def_id() == b_data.principal_def_id()
730 {
731 return self.unify(a, b);
732 }
733
734 // Check the obligations of the cast -- for example, when casting
735 // `usize` to `dyn* Clone + 'static`:
736 let obligations = predicates
737 .iter()
738 .map(|predicate| {
739 // For each existential predicate (e.g., `?Self: Clone`) instantiate
740 // the type of the expression (e.g., `usize` in our example above)
741 // and then require that the resulting predicate (e.g., `usize: Clone`)
742 // holds (it does).
743 let predicate = predicate.with_self_ty(self.tcx, a);
744 Obligation::new(self.tcx, self.cause.clone(), self.param_env, predicate)
745 })
746 .chain([
747 // Enforce the region bound (e.g., `usize: 'static`, in our example).
748 Obligation::new(
749 self.tcx,
750 self.cause.clone(),
751 self.param_env,
752 ty::Binder::dummy(ty::PredicateKind::Clause(ty::ClauseKind::TypeOutlives(
753 ty::OutlivesPredicate(a, b_region),
754 ))),
755 ),
756 // Enforce that the type is `usize`/pointer-sized.
757 Obligation::new(
758 self.tcx,
759 self.cause.clone(),
760 self.param_env,
761 ty::TraitRef::new(
762 self.tcx,
763 self.tcx.require_lang_item(hir::LangItem::PointerLike, self.cause.span),
764 [a],
765 ),
766 ),
767 ])
768 .collect();
769
770 Ok(InferOk {
771 value: (
772 vec![Adjustment { kind: Adjust::Pointer(PointerCoercion::DynStar), target: b }],
773 b,
774 ),
775 obligations,
776 })
777 }
778
779 /// Applies reborrowing for `Pin`
780 ///
781 /// We currently only support reborrowing `Pin<&mut T>` as `Pin<&mut T>`. This is accomplished
782 /// by inserting a call to `Pin::as_mut` during MIR building.
783 ///
784 /// In the future we might want to support other reborrowing coercions, such as:
785 /// - `Pin<&mut T>` as `Pin<&T>`
786 /// - `Pin<&T>` as `Pin<&T>`
787 /// - `Pin<Box<T>>` as `Pin<&T>`
788 /// - `Pin<Box<T>>` as `Pin<&mut T>`
789 #[instrument(skip(self), level = "trace")]
790 fn coerce_pin_ref(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
791 // We need to make sure the two types are compatible for coercion.
792 // Then we will build a ReborrowPin adjustment and return that as an InferOk.
793
794 // Right now we can only reborrow if this is a `Pin<&mut T>`.
795 let extract_pin_mut = |ty: Ty<'tcx>| {
796 // Get the T out of Pin<T>
797 let (pin, ty) = match ty.kind() {
798 ty::Adt(pin, args) if self.tcx.is_lang_item(pin.did(), hir::LangItem::Pin) => {
799 (*pin, args[0].expect_ty())
800 }
801 _ => {
802 debug!("can't reborrow {:?} as pinned", ty);
803 return Err(TypeError::Mismatch);
804 }
805 };
806 // Make sure the T is something we understand (just `&mut U` for now)
807 match ty.kind() {
808 ty::Ref(region, ty, mutbl) => Ok((pin, *region, *ty, *mutbl)),
809 _ => {
810 debug!("can't reborrow pin of inner type {:?}", ty);
811 Err(TypeError::Mismatch)
812 }
813 }
814 };
815
816 let (pin, a_region, a_ty, mut_a) = extract_pin_mut(a)?;
817 let (_, _, _b_ty, mut_b) = extract_pin_mut(b)?;
818
819 coerce_mutbls(mut_a, mut_b)?;
820
821 // update a with b's mutability since we'll be coercing mutability
822 let a = Ty::new_adt(
823 self.tcx,
824 pin,
825 self.tcx.mk_args(&[Ty::new_ref(self.tcx, a_region, a_ty, mut_b).into()]),
826 );
827
828 // To complete the reborrow, we need to make sure we can unify the inner types, and if so we
829 // add the adjustments.
830 self.unify_and(a, b, [], Adjust::ReborrowPin(mut_b))
831 }
832
833 fn coerce_from_safe_fn(
834 &self,
835 fn_ty_a: ty::PolyFnSig<'tcx>,
836 b: Ty<'tcx>,
837 adjustment: Option<Adjust>,
838 ) -> CoerceResult<'tcx> {
839 self.commit_if_ok(|snapshot| {
840 let outer_universe = self.infcx.universe();
841
842 let result = if let ty::FnPtr(_, hdr_b) = b.kind()
843 && fn_ty_a.safety().is_safe()
844 && hdr_b.safety.is_unsafe()
845 {
846 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
847 self.unify_and(
848 unsafe_a,
849 b,
850 adjustment
851 .map(|kind| Adjustment { kind, target: Ty::new_fn_ptr(self.tcx, fn_ty_a) }),
852 Adjust::Pointer(PointerCoercion::UnsafeFnPointer),
853 )
854 } else {
855 let a = Ty::new_fn_ptr(self.tcx, fn_ty_a);
856 match adjustment {
857 Some(adjust) => self.unify_and(a, b, [], adjust),
858 None => self.unify(a, b),
859 }
860 };
861
862 // FIXME(#73154): This is a hack. Currently LUB can generate
863 // unsolvable constraints. Additionally, it returns `a`
864 // unconditionally, even when the "LUB" is `b`. In the future, we
865 // want the coerced type to be the actual supertype of these two,
866 // but for now, we want to just error to ensure we don't lock
867 // ourselves into a specific behavior with NLL.
868 self.leak_check(outer_universe, Some(snapshot))?;
869
870 result
871 })
872 }
873
874 fn coerce_from_fn_pointer(
875 &self,
876 fn_ty_a: ty::PolyFnSig<'tcx>,
877 b: Ty<'tcx>,
878 ) -> CoerceResult<'tcx> {
879 //! Attempts to coerce from the type of a Rust function item
880 //! into a closure or a `proc`.
881 //!
882
883 let b = self.shallow_resolve(b);
884 debug!(?fn_ty_a, ?b, "coerce_from_fn_pointer");
885
886 self.coerce_from_safe_fn(fn_ty_a, b, None)
887 }
888
889 fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
890 //! Attempts to coerce from the type of a Rust function item
891 //! into a closure or a `proc`.
892
893 let b = self.shallow_resolve(b);
894 let InferOk { value: b, mut obligations } =
895 self.at(&self.cause, self.param_env).normalize(b);
896 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
897
898 match b.kind() {
899 ty::FnPtr(_, b_hdr) => {
900 let mut a_sig = a.fn_sig(self.tcx);
901 if let ty::FnDef(def_id, _) = *a.kind() {
902 // Intrinsics are not coercible to function pointers
903 if self.tcx.intrinsic(def_id).is_some() {
904 return Err(TypeError::IntrinsicCast);
905 }
906
907 let fn_attrs = self.tcx.codegen_fn_attrs(def_id);
908 if matches!(fn_attrs.inline, InlineAttr::Force { .. }) {
909 return Err(TypeError::ForceInlineCast);
910 }
911
912 if b_hdr.safety.is_safe()
913 && self.tcx.codegen_fn_attrs(def_id).safe_target_features
914 {
915 // Allow the coercion if the current function has all the features that would be
916 // needed to call the coercee safely.
917 if let Some(safe_sig) = self.tcx.adjust_target_feature_sig(
918 def_id,
919 a_sig,
920 self.fcx.body_id.into(),
921 ) {
922 a_sig = safe_sig;
923 } else {
924 return Err(TypeError::TargetFeatureCast(def_id));
925 }
926 }
927 }
928
929 let InferOk { value: a_sig, obligations: o1 } =
930 self.at(&self.cause, self.param_env).normalize(a_sig);
931 obligations.extend(o1);
932
933 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
934 a_sig,
935 b,
936 Some(Adjust::Pointer(PointerCoercion::ReifyFnPointer)),
937 )?;
938
939 obligations.extend(o2);
940 Ok(InferOk { value, obligations })
941 }
942 _ => self.unify(a, b),
943 }
944 }
945
946 fn coerce_closure_to_fn(
947 &self,
948 a: Ty<'tcx>,
949 closure_def_id_a: DefId,
950 args_a: GenericArgsRef<'tcx>,
951 b: Ty<'tcx>,
952 ) -> CoerceResult<'tcx> {
953 //! Attempts to coerce from the type of a non-capturing closure
954 //! into a function pointer.
955 //!
956
957 let b = self.shallow_resolve(b);
958
959 match b.kind() {
960 // At this point we haven't done capture analysis, which means
961 // that the ClosureArgs just contains an inference variable instead
962 // of tuple of captured types.
963 //
964 // All we care here is if any variable is being captured and not the exact paths,
965 // so we check `upvars_mentioned` for root variables being captured.
966 ty::FnPtr(_, hdr)
967 if self
968 .tcx
969 .upvars_mentioned(closure_def_id_a.expect_local())
970 .is_none_or(|u| u.is_empty()) =>
971 {
972 // We coerce the closure, which has fn type
973 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
974 // to
975 // `fn(arg0,arg1,...) -> _`
976 // or
977 // `unsafe fn(arg0,arg1,...) -> _`
978 let closure_sig = args_a.as_closure().sig();
979 let safety = hdr.safety;
980 let pointer_ty =
981 Ty::new_fn_ptr(self.tcx, self.tcx.signature_unclosure(closure_sig, safety));
982 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
983 self.unify_and(
984 pointer_ty,
985 b,
986 [],
987 Adjust::Pointer(PointerCoercion::ClosureFnPointer(safety)),
988 )
989 }
990 _ => self.unify(a, b),
991 }
992 }
993
994 fn coerce_raw_ptr(
995 &self,
996 a: Ty<'tcx>,
997 b: Ty<'tcx>,
998 mutbl_b: hir::Mutability,
999 ) -> CoerceResult<'tcx> {
1000 debug!("coerce_raw_ptr(a={:?}, b={:?})", a, b);
1001
1002 let (is_ref, mt_a) = match *a.kind() {
1003 ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
1004 ty::RawPtr(ty, mutbl) => (false, ty::TypeAndMut { ty, mutbl }),
1005 _ => return self.unify(a, b),
1006 };
1007 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
1008
1009 // Check that the types which they point at are compatible.
1010 let a_raw = Ty::new_ptr(self.tcx, mt_a.ty, mutbl_b);
1011 // Although references and raw ptrs have the same
1012 // representation, we still register an Adjust::DerefRef so that
1013 // regionck knows that the region for `a` must be valid here.
1014 if is_ref {
1015 self.unify_and(
1016 a_raw,
1017 b,
1018 [Adjustment { kind: Adjust::Deref(None), target: mt_a.ty }],
1019 Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)),
1020 )
1021 } else if mt_a.mutbl != mutbl_b {
1022 self.unify_and(a_raw, b, [], Adjust::Pointer(PointerCoercion::MutToConstPointer))
1023 } else {
1024 self.unify(a_raw, b)
1025 }
1026 }
1027}
1028
1029impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
1030 /// Attempt to coerce an expression to a type, and return the
1031 /// adjusted type of the expression, if successful.
1032 /// Adjustments are only recorded if the coercion succeeded.
1033 /// The expressions *must not* have any preexisting adjustments.
1034 pub(crate) fn coerce(
1035 &self,
1036 expr: &'tcx hir::Expr<'tcx>,
1037 expr_ty: Ty<'tcx>,
1038 mut target: Ty<'tcx>,
1039 allow_two_phase: AllowTwoPhase,
1040 cause: Option<ObligationCause<'tcx>>,
1041 ) -> RelateResult<'tcx, Ty<'tcx>> {
1042 let source = self.try_structurally_resolve_type(expr.span, expr_ty);
1043 if self.next_trait_solver() {
1044 target = self.try_structurally_resolve_type(
1045 cause.as_ref().map_or(expr.span, |cause| cause.span),
1046 target,
1047 );
1048 }
1049 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
1050
1051 let cause =
1052 cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
1053 let coerce = Coerce::new(
1054 self,
1055 cause,
1056 allow_two_phase,
1057 self.expr_guaranteed_to_constitute_read_for_never(expr),
1058 );
1059 let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
1060
1061 let (adjustments, _) = self.register_infer_ok_obligations(ok);
1062 self.apply_adjustments(expr, adjustments);
1063 Ok(if let Err(guar) = expr_ty.error_reported() {
1064 Ty::new_error(self.tcx, guar)
1065 } else {
1066 target
1067 })
1068 }
1069
1070 /// Probe whether `expr_ty` can be coerced to `target_ty`. This has no side-effects,
1071 /// and may return false positives if types are not yet fully constrained by inference.
1072 ///
1073 /// Returns false if the coercion is not possible, or if the coercion creates any
1074 /// sub-obligations that result in errors.
1075 ///
1076 /// This should only be used for diagnostics.
1077 pub(crate) fn may_coerce(&self, expr_ty: Ty<'tcx>, target_ty: Ty<'tcx>) -> bool {
1078 let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
1079 // We don't ever need two-phase here since we throw out the result of the coercion.
1080 // We also just always set `coerce_never` to true, since this is a heuristic.
1081 let coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No, true);
1082 self.probe(|_| {
1083 // Make sure to structurally resolve the types, since we use
1084 // the `TyKind`s heavily in coercion.
1085 let ocx = ObligationCtxt::new(self);
1086 let structurally_resolve = |ty| {
1087 let ty = self.shallow_resolve(ty);
1088 if self.next_trait_solver()
1089 && let ty::Alias(..) = ty.kind()
1090 {
1091 ocx.structurally_normalize_ty(&cause, self.param_env, ty)
1092 } else {
1093 Ok(ty)
1094 }
1095 };
1096 let Ok(expr_ty) = structurally_resolve(expr_ty) else {
1097 return false;
1098 };
1099 let Ok(target_ty) = structurally_resolve(target_ty) else {
1100 return false;
1101 };
1102
1103 let Ok(ok) = coerce.coerce(expr_ty, target_ty) else {
1104 return false;
1105 };
1106 ocx.register_obligations(ok.obligations);
1107 ocx.select_where_possible().is_empty()
1108 })
1109 }
1110
1111 /// Given a type and a target type, this function will calculate and return
1112 /// how many dereference steps needed to coerce `expr_ty` to `target`. If
1113 /// it's not possible, return `None`.
1114 pub(crate) fn deref_steps_for_suggestion(
1115 &self,
1116 expr_ty: Ty<'tcx>,
1117 target: Ty<'tcx>,
1118 ) -> Option<usize> {
1119 let cause = self.cause(DUMMY_SP, ObligationCauseCode::ExprAssignable);
1120 // We don't ever need two-phase here since we throw out the result of the coercion.
1121 let coerce = Coerce::new(self, cause, AllowTwoPhase::No, true);
1122 coerce.autoderef(DUMMY_SP, expr_ty).find_map(|(ty, steps)| {
1123 self.probe(|_| coerce.unify_raw(ty, target)).ok().map(|_| steps)
1124 })
1125 }
1126
1127 /// Given a type, this function will calculate and return the type given
1128 /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
1129 ///
1130 /// This function is for diagnostics only, since it does not register
1131 /// trait or region sub-obligations. (presumably we could, but it's not
1132 /// particularly important for diagnostics...)
1133 pub(crate) fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
1134 self.autoderef(DUMMY_SP, expr_ty).silence_errors().nth(1).and_then(|(deref_ty, _)| {
1135 self.infcx
1136 .type_implements_trait(
1137 self.tcx.lang_items().deref_mut_trait()?,
1138 [expr_ty],
1139 self.param_env,
1140 )
1141 .may_apply()
1142 .then_some(deref_ty)
1143 })
1144 }
1145
1146 /// Given some expressions, their known unified type and another expression,
1147 /// tries to unify the types, potentially inserting coercions on any of the
1148 /// provided expressions and returns their LUB (aka "common supertype").
1149 ///
1150 /// This is really an internal helper. From outside the coercion
1151 /// module, you should instantiate a `CoerceMany` instance.
1152 fn try_find_coercion_lub<E>(
1153 &self,
1154 cause: &ObligationCause<'tcx>,
1155 exprs: &[E],
1156 prev_ty: Ty<'tcx>,
1157 new: &hir::Expr<'_>,
1158 new_ty: Ty<'tcx>,
1159 ) -> RelateResult<'tcx, Ty<'tcx>>
1160 where
1161 E: AsCoercionSite,
1162 {
1163 let prev_ty = self.try_structurally_resolve_type(cause.span, prev_ty);
1164 let new_ty = self.try_structurally_resolve_type(new.span, new_ty);
1165 debug!(
1166 "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
1167 prev_ty,
1168 new_ty,
1169 exprs.len()
1170 );
1171
1172 // The following check fixes #88097, where the compiler erroneously
1173 // attempted to coerce a closure type to itself via a function pointer.
1174 if prev_ty == new_ty {
1175 return Ok(prev_ty);
1176 }
1177
1178 let is_force_inline = |ty: Ty<'tcx>| {
1179 if let ty::FnDef(did, _) = ty.kind() {
1180 matches!(self.tcx.codegen_fn_attrs(did).inline, InlineAttr::Force { .. })
1181 } else {
1182 false
1183 }
1184 };
1185 if is_force_inline(prev_ty) || is_force_inline(new_ty) {
1186 return Err(TypeError::ForceInlineCast);
1187 }
1188
1189 // Special-case that coercion alone cannot handle:
1190 // Function items or non-capturing closures of differing IDs or GenericArgs.
1191 let (a_sig, b_sig) = {
1192 let is_capturing_closure = |ty: Ty<'tcx>| {
1193 if let &ty::Closure(closure_def_id, _args) = ty.kind() {
1194 self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some()
1195 } else {
1196 false
1197 }
1198 };
1199 if is_capturing_closure(prev_ty) || is_capturing_closure(new_ty) {
1200 (None, None)
1201 } else {
1202 match (prev_ty.kind(), new_ty.kind()) {
1203 (ty::FnDef(..), ty::FnDef(..)) => {
1204 // Don't reify if the function types have a LUB, i.e., they
1205 // are the same function and their parameters have a LUB.
1206 match self.commit_if_ok(|_| {
1207 // We need to eagerly handle nested obligations due to lazy norm.
1208 if self.next_trait_solver() {
1209 let ocx = ObligationCtxt::new(self);
1210 let value = ocx.lub(cause, self.param_env, prev_ty, new_ty)?;
1211 if ocx.select_where_possible().is_empty() {
1212 Ok(InferOk {
1213 value,
1214 obligations: ocx.into_pending_obligations(),
1215 })
1216 } else {
1217 Err(TypeError::Mismatch)
1218 }
1219 } else {
1220 self.at(cause, self.param_env).lub(prev_ty, new_ty)
1221 }
1222 }) {
1223 // We have a LUB of prev_ty and new_ty, just return it.
1224 Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
1225 Err(_) => {
1226 (Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx)))
1227 }
1228 }
1229 }
1230 (ty::Closure(_, args), ty::FnDef(..)) => {
1231 let b_sig = new_ty.fn_sig(self.tcx);
1232 let a_sig =
1233 self.tcx.signature_unclosure(args.as_closure().sig(), b_sig.safety());
1234 (Some(a_sig), Some(b_sig))
1235 }
1236 (ty::FnDef(..), ty::Closure(_, args)) => {
1237 let a_sig = prev_ty.fn_sig(self.tcx);
1238 let b_sig =
1239 self.tcx.signature_unclosure(args.as_closure().sig(), a_sig.safety());
1240 (Some(a_sig), Some(b_sig))
1241 }
1242 (ty::Closure(_, args_a), ty::Closure(_, args_b)) => (
1243 Some(
1244 self.tcx
1245 .signature_unclosure(args_a.as_closure().sig(), hir::Safety::Safe),
1246 ),
1247 Some(
1248 self.tcx
1249 .signature_unclosure(args_b.as_closure().sig(), hir::Safety::Safe),
1250 ),
1251 ),
1252 _ => (None, None),
1253 }
1254 }
1255 };
1256 if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) {
1257 // The signature must match.
1258 let (a_sig, b_sig) = self.normalize(new.span, (a_sig, b_sig));
1259 let sig = self
1260 .at(cause, self.param_env)
1261 .lub(a_sig, b_sig)
1262 .map(|ok| self.register_infer_ok_obligations(ok))?;
1263
1264 // Reify both sides and return the reified fn pointer type.
1265 let fn_ptr = Ty::new_fn_ptr(self.tcx, sig);
1266 let prev_adjustment = match prev_ty.kind() {
1267 ty::Closure(..) => {
1268 Adjust::Pointer(PointerCoercion::ClosureFnPointer(a_sig.safety()))
1269 }
1270 ty::FnDef(..) => Adjust::Pointer(PointerCoercion::ReifyFnPointer),
1271 _ => span_bug!(cause.span, "should not try to coerce a {prev_ty} to a fn pointer"),
1272 };
1273 let next_adjustment = match new_ty.kind() {
1274 ty::Closure(..) => {
1275 Adjust::Pointer(PointerCoercion::ClosureFnPointer(b_sig.safety()))
1276 }
1277 ty::FnDef(..) => Adjust::Pointer(PointerCoercion::ReifyFnPointer),
1278 _ => span_bug!(new.span, "should not try to coerce a {new_ty} to a fn pointer"),
1279 };
1280 for expr in exprs.iter().map(|e| e.as_coercion_site()) {
1281 self.apply_adjustments(
1282 expr,
1283 vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
1284 );
1285 }
1286 self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
1287 return Ok(fn_ptr);
1288 }
1289
1290 // Configure a Coerce instance to compute the LUB.
1291 // We don't allow two-phase borrows on any autorefs this creates since we
1292 // probably aren't processing function arguments here and even if we were,
1293 // they're going to get autorefed again anyway and we can apply 2-phase borrows
1294 // at that time.
1295 //
1296 // NOTE: we set `coerce_never` to `true` here because coercion LUBs only
1297 // operate on values and not places, so a never coercion is valid.
1298 let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No, true);
1299 coerce.use_lub = true;
1300
1301 // First try to coerce the new expression to the type of the previous ones,
1302 // but only if the new expression has no coercion already applied to it.
1303 let mut first_error = None;
1304 if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
1305 let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
1306 match result {
1307 Ok(ok) => {
1308 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1309 self.apply_adjustments(new, adjustments);
1310 debug!(
1311 "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
1312 new_ty, prev_ty, target
1313 );
1314 return Ok(target);
1315 }
1316 Err(e) => first_error = Some(e),
1317 }
1318 }
1319
1320 match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
1321 Err(_) => {
1322 // Avoid giving strange errors on failed attempts.
1323 if let Some(e) = first_error {
1324 Err(e)
1325 } else {
1326 Err(self
1327 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1328 .unwrap_err())
1329 }
1330 }
1331 Ok(ok) => {
1332 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1333 for expr in exprs {
1334 let expr = expr.as_coercion_site();
1335 self.apply_adjustments(expr, adjustments.clone());
1336 }
1337 debug!(
1338 "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
1339 prev_ty, new_ty, target
1340 );
1341 Ok(target)
1342 }
1343 }
1344 }
1345}
1346
1347/// Check whether `ty` can be coerced to `output_ty`.
1348/// Used from clippy.
1349pub fn can_coerce<'tcx>(
1350 tcx: TyCtxt<'tcx>,
1351 param_env: ty::ParamEnv<'tcx>,
1352 body_id: LocalDefId,
1353 ty: Ty<'tcx>,
1354 output_ty: Ty<'tcx>,
1355) -> bool {
1356 let root_ctxt = crate::typeck_root_ctxt::TypeckRootCtxt::new(tcx, body_id);
1357 let fn_ctxt = FnCtxt::new(&root_ctxt, param_env, body_id);
1358 fn_ctxt.may_coerce(ty, output_ty)
1359}
1360
1361/// CoerceMany encapsulates the pattern you should use when you have
1362/// many expressions that are all getting coerced to a common
1363/// type. This arises, for example, when you have a match (the result
1364/// of each arm is coerced to a common type). It also arises in less
1365/// obvious places, such as when you have many `break foo` expressions
1366/// that target the same loop, or the various `return` expressions in
1367/// a function.
1368///
1369/// The basic protocol is as follows:
1370///
1371/// - Instantiate the `CoerceMany` with an initial `expected_ty`.
1372/// This will also serve as the "starting LUB". The expectation is
1373/// that this type is something which all of the expressions *must*
1374/// be coercible to. Use a fresh type variable if needed.
1375/// - For each expression whose result is to be coerced, invoke `coerce()` with.
1376/// - In some cases we wish to coerce "non-expressions" whose types are implicitly
1377/// unit. This happens for example if you have a `break` with no expression,
1378/// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
1379/// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
1380/// from you so that you don't have to worry your pretty head about it.
1381/// But if an error is reported, the final type will be `err`.
1382/// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
1383/// previously coerced expressions.
1384/// - When all done, invoke `complete()`. This will return the LUB of
1385/// all your expressions.
1386/// - WARNING: I don't believe this final type is guaranteed to be
1387/// related to your initial `expected_ty` in any particular way,
1388/// although it will typically be a subtype, so you should check it.
1389/// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
1390/// previously coerced expressions.
1391///
1392/// Example:
1393///
1394/// ```ignore (illustrative)
1395/// let mut coerce = CoerceMany::new(expected_ty);
1396/// for expr in exprs {
1397/// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
1398/// coerce.coerce(fcx, &cause, expr, expr_ty);
1399/// }
1400/// let final_ty = coerce.complete(fcx);
1401/// ```
1402pub(crate) struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
1403 expected_ty: Ty<'tcx>,
1404 final_ty: Option<Ty<'tcx>>,
1405 expressions: Expressions<'tcx, 'exprs, E>,
1406 pushed: usize,
1407}
1408
1409/// The type of a `CoerceMany` that is storing up the expressions into
1410/// a buffer. We use this in `check/mod.rs` for things like `break`.
1411pub(crate) type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;
1412
1413enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
1414 Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
1415 UpFront(&'exprs [E]),
1416}
1417
1418impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
1419 /// The usual case; collect the set of expressions dynamically.
1420 /// If the full set of coercion sites is known before hand,
1421 /// consider `with_coercion_sites()` instead to avoid allocation.
1422 pub(crate) fn new(expected_ty: Ty<'tcx>) -> Self {
1423 Self::make(expected_ty, Expressions::Dynamic(vec![]))
1424 }
1425
1426 /// As an optimization, you can create a `CoerceMany` with a
1427 /// preexisting slice of expressions. In this case, you are
1428 /// expected to pass each element in the slice to `coerce(...)` in
1429 /// order. This is used with arrays in particular to avoid
1430 /// needlessly cloning the slice.
1431 pub(crate) fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
1432 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
1433 }
1434
1435 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
1436 CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
1437 }
1438
1439 /// Returns the "expected type" with which this coercion was
1440 /// constructed. This represents the "downward propagated" type
1441 /// that was given to us at the start of typing whatever construct
1442 /// we are typing (e.g., the match expression).
1443 ///
1444 /// Typically, this is used as the expected type when
1445 /// type-checking each of the alternative expressions whose types
1446 /// we are trying to merge.
1447 pub(crate) fn expected_ty(&self) -> Ty<'tcx> {
1448 self.expected_ty
1449 }
1450
1451 /// Returns the current "merged type", representing our best-guess
1452 /// at the LUB of the expressions we've seen so far (if any). This
1453 /// isn't *final* until you call `self.complete()`, which will return
1454 /// the merged type.
1455 pub(crate) fn merged_ty(&self) -> Ty<'tcx> {
1456 self.final_ty.unwrap_or(self.expected_ty)
1457 }
1458
1459 /// Indicates that the value generated by `expression`, which is
1460 /// of type `expression_ty`, is one of the possibilities that we
1461 /// could coerce from. This will record `expression`, and later
1462 /// calls to `coerce` may come back and add adjustments and things
1463 /// if necessary.
1464 pub(crate) fn coerce<'a>(
1465 &mut self,
1466 fcx: &FnCtxt<'a, 'tcx>,
1467 cause: &ObligationCause<'tcx>,
1468 expression: &'tcx hir::Expr<'tcx>,
1469 expression_ty: Ty<'tcx>,
1470 ) {
1471 self.coerce_inner(fcx, cause, Some(expression), expression_ty, |_| {}, false)
1472 }
1473
1474 /// Indicates that one of the inputs is a "forced unit". This
1475 /// occurs in a case like `if foo { ... };`, where the missing else
1476 /// generates a "forced unit". Another example is a `loop { break;
1477 /// }`, where the `break` has no argument expression. We treat
1478 /// these cases slightly differently for error-reporting
1479 /// purposes. Note that these tend to correspond to cases where
1480 /// the `()` expression is implicit in the source, and hence we do
1481 /// not take an expression argument.
1482 ///
1483 /// The `augment_error` gives you a chance to extend the error
1484 /// message, in case any results (e.g., we use this to suggest
1485 /// removing a `;`).
1486 pub(crate) fn coerce_forced_unit<'a>(
1487 &mut self,
1488 fcx: &FnCtxt<'a, 'tcx>,
1489 cause: &ObligationCause<'tcx>,
1490 augment_error: impl FnOnce(&mut Diag<'_>),
1491 label_unit_as_expected: bool,
1492 ) {
1493 self.coerce_inner(
1494 fcx,
1495 cause,
1496 None,
1497 fcx.tcx.types.unit,
1498 augment_error,
1499 label_unit_as_expected,
1500 )
1501 }
1502
1503 /// The inner coercion "engine". If `expression` is `None`, this
1504 /// is a forced-unit case, and hence `expression_ty` must be
1505 /// `Nil`.
1506 #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
1507 pub(crate) fn coerce_inner<'a>(
1508 &mut self,
1509 fcx: &FnCtxt<'a, 'tcx>,
1510 cause: &ObligationCause<'tcx>,
1511 expression: Option<&'tcx hir::Expr<'tcx>>,
1512 mut expression_ty: Ty<'tcx>,
1513 augment_error: impl FnOnce(&mut Diag<'_>),
1514 label_expression_as_expected: bool,
1515 ) {
1516 // Incorporate whatever type inference information we have
1517 // until now; in principle we might also want to process
1518 // pending obligations, but doing so should only improve
1519 // compatibility (hopefully that is true) by helping us
1520 // uncover never types better.
1521 if expression_ty.is_ty_var() {
1522 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1523 }
1524
1525 // If we see any error types, just propagate that error
1526 // upwards.
1527 if let Err(guar) = (expression_ty, self.merged_ty()).error_reported() {
1528 self.final_ty = Some(Ty::new_error(fcx.tcx, guar));
1529 return;
1530 }
1531
1532 let (expected, found) = if label_expression_as_expected {
1533 // In the case where this is a "forced unit", like
1534 // `break`, we want to call the `()` "expected"
1535 // since it is implied by the syntax.
1536 // (Note: not all force-units work this way.)"
1537 (expression_ty, self.merged_ty())
1538 } else {
1539 // Otherwise, the "expected" type for error
1540 // reporting is the current unification type,
1541 // which is basically the LUB of the expressions
1542 // we've seen so far (combined with the expected
1543 // type)
1544 (self.merged_ty(), expression_ty)
1545 };
1546
1547 // Handle the actual type unification etc.
1548 let result = if let Some(expression) = expression {
1549 if self.pushed == 0 {
1550 // Special-case the first expression we are coercing.
1551 // To be honest, I'm not entirely sure why we do this.
1552 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1553 fcx.coerce(
1554 expression,
1555 expression_ty,
1556 self.expected_ty,
1557 AllowTwoPhase::No,
1558 Some(cause.clone()),
1559 )
1560 } else {
1561 match self.expressions {
1562 Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
1563 cause,
1564 exprs,
1565 self.merged_ty(),
1566 expression,
1567 expression_ty,
1568 ),
1569 Expressions::UpFront(coercion_sites) => fcx.try_find_coercion_lub(
1570 cause,
1571 &coercion_sites[0..self.pushed],
1572 self.merged_ty(),
1573 expression,
1574 expression_ty,
1575 ),
1576 }
1577 }
1578 } else {
1579 // this is a hack for cases where we default to `()` because
1580 // the expression etc has been omitted from the source. An
1581 // example is an `if let` without an else:
1582 //
1583 // if let Some(x) = ... { }
1584 //
1585 // we wind up with a second match arm that is like `_ =>
1586 // ()`. That is the case we are considering here. We take
1587 // a different path to get the right "expected, found"
1588 // message and so forth (and because we know that
1589 // `expression_ty` will be unit).
1590 //
1591 // Another example is `break` with no argument expression.
1592 assert!(expression_ty.is_unit(), "if let hack without unit type");
1593 fcx.at(cause, fcx.param_env)
1594 .eq(
1595 // needed for tests/ui/type-alias-impl-trait/issue-65679-inst-opaque-ty-from-val-twice.rs
1596 DefineOpaqueTypes::Yes,
1597 expected,
1598 found,
1599 )
1600 .map(|infer_ok| {
1601 fcx.register_infer_ok_obligations(infer_ok);
1602 expression_ty
1603 })
1604 };
1605
1606 debug!(?result);
1607 match result {
1608 Ok(v) => {
1609 self.final_ty = Some(v);
1610 if let Some(e) = expression {
1611 match self.expressions {
1612 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1613 Expressions::UpFront(coercion_sites) => {
1614 // if the user gave us an array to validate, check that we got
1615 // the next expression in the list, as expected
1616 assert_eq!(
1617 coercion_sites[self.pushed].as_coercion_site().hir_id,
1618 e.hir_id
1619 );
1620 }
1621 }
1622 self.pushed += 1;
1623 }
1624 }
1625 Err(coercion_error) => {
1626 // Mark that we've failed to coerce the types here to suppress
1627 // any superfluous errors we might encounter while trying to
1628 // emit or provide suggestions on how to fix the initial error.
1629 fcx.set_tainted_by_errors(
1630 fcx.dcx().span_delayed_bug(cause.span, "coercion error but no error emitted"),
1631 );
1632 let (expected, found) = fcx.resolve_vars_if_possible((expected, found));
1633
1634 let mut err;
1635 let mut unsized_return = false;
1636 match *cause.code() {
1637 ObligationCauseCode::ReturnNoExpression => {
1638 err = struct_span_code_err!(
1639 fcx.dcx(),
1640 cause.span,
1641 E0069,
1642 "`return;` in a function whose return type is not `()`"
1643 );
1644 if let Some(value) = fcx.err_ctxt().ty_kind_suggestion(fcx.param_env, found)
1645 {
1646 err.span_suggestion_verbose(
1647 cause.span.shrink_to_hi(),
1648 "give the `return` a value of the expected type",
1649 format!(" {value}"),
1650 Applicability::HasPlaceholders,
1651 );
1652 }
1653 err.span_label(cause.span, "return type is not `()`");
1654 }
1655 ObligationCauseCode::BlockTailExpression(blk_id, ..) => {
1656 err = self.report_return_mismatched_types(
1657 cause,
1658 expected,
1659 found,
1660 coercion_error,
1661 fcx,
1662 blk_id,
1663 expression,
1664 );
1665 if !fcx.tcx.features().unsized_locals() {
1666 unsized_return = self.is_return_ty_definitely_unsized(fcx);
1667 }
1668 }
1669 ObligationCauseCode::ReturnValue(return_expr_id) => {
1670 err = self.report_return_mismatched_types(
1671 cause,
1672 expected,
1673 found,
1674 coercion_error,
1675 fcx,
1676 return_expr_id,
1677 expression,
1678 );
1679 if !fcx.tcx.features().unsized_locals() {
1680 unsized_return = self.is_return_ty_definitely_unsized(fcx);
1681 }
1682 }
1683 ObligationCauseCode::MatchExpressionArm(box MatchExpressionArmCause {
1684 arm_span,
1685 arm_ty,
1686 prior_arm_ty,
1687 ref prior_non_diverging_arms,
1688 tail_defines_return_position_impl_trait: Some(rpit_def_id),
1689 ..
1690 }) => {
1691 err = fcx.err_ctxt().report_mismatched_types(
1692 cause,
1693 fcx.param_env,
1694 expected,
1695 found,
1696 coercion_error,
1697 );
1698 // Check that we're actually in the second or later arm
1699 if prior_non_diverging_arms.len() > 0 {
1700 self.suggest_boxing_tail_for_return_position_impl_trait(
1701 fcx,
1702 &mut err,
1703 rpit_def_id,
1704 arm_ty,
1705 prior_arm_ty,
1706 prior_non_diverging_arms
1707 .iter()
1708 .chain(std::iter::once(&arm_span))
1709 .copied(),
1710 );
1711 }
1712 }
1713 ObligationCauseCode::IfExpression(box IfExpressionCause {
1714 then_id,
1715 else_id,
1716 then_ty,
1717 else_ty,
1718 tail_defines_return_position_impl_trait: Some(rpit_def_id),
1719 ..
1720 }) => {
1721 err = fcx.err_ctxt().report_mismatched_types(
1722 cause,
1723 fcx.param_env,
1724 expected,
1725 found,
1726 coercion_error,
1727 );
1728 let then_span = fcx.find_block_span_from_hir_id(then_id);
1729 let else_span = fcx.find_block_span_from_hir_id(else_id);
1730 // don't suggest wrapping either blocks in `if .. {} else {}`
1731 let is_empty_arm = |id| {
1732 let hir::Node::Block(blk) = fcx.tcx.hir_node(id) else {
1733 return false;
1734 };
1735 if blk.expr.is_some() || !blk.stmts.is_empty() {
1736 return false;
1737 }
1738 let Some((_, hir::Node::Expr(expr))) =
1739 fcx.tcx.hir_parent_iter(id).nth(1)
1740 else {
1741 return false;
1742 };
1743 matches!(expr.kind, hir::ExprKind::If(..))
1744 };
1745 if !is_empty_arm(then_id) && !is_empty_arm(else_id) {
1746 self.suggest_boxing_tail_for_return_position_impl_trait(
1747 fcx,
1748 &mut err,
1749 rpit_def_id,
1750 then_ty,
1751 else_ty,
1752 [then_span, else_span].into_iter(),
1753 );
1754 }
1755 }
1756 _ => {
1757 err = fcx.err_ctxt().report_mismatched_types(
1758 cause,
1759 fcx.param_env,
1760 expected,
1761 found,
1762 coercion_error,
1763 );
1764 }
1765 }
1766
1767 augment_error(&mut err);
1768
1769 if let Some(expr) = expression {
1770 if let hir::ExprKind::Loop(
1771 _,
1772 _,
1773 loop_src @ (hir::LoopSource::While | hir::LoopSource::ForLoop),
1774 _,
1775 ) = expr.kind
1776 {
1777 let loop_type = if loop_src == hir::LoopSource::While {
1778 "`while` loops"
1779 } else {
1780 "`for` loops"
1781 };
1782
1783 err.note(format!("{loop_type} evaluate to unit type `()`"));
1784 }
1785
1786 fcx.emit_coerce_suggestions(
1787 &mut err,
1788 expr,
1789 found,
1790 expected,
1791 None,
1792 Some(coercion_error),
1793 );
1794 }
1795
1796 let reported = err.emit_unless(unsized_return);
1797
1798 self.final_ty = Some(Ty::new_error(fcx.tcx, reported));
1799 }
1800 }
1801 }
1802
1803 fn suggest_boxing_tail_for_return_position_impl_trait(
1804 &self,
1805 fcx: &FnCtxt<'_, 'tcx>,
1806 err: &mut Diag<'_>,
1807 rpit_def_id: LocalDefId,
1808 a_ty: Ty<'tcx>,
1809 b_ty: Ty<'tcx>,
1810 arm_spans: impl Iterator<Item = Span>,
1811 ) {
1812 let compatible = |ty: Ty<'tcx>| {
1813 fcx.probe(|_| {
1814 let ocx = ObligationCtxt::new(fcx);
1815 ocx.register_obligations(
1816 fcx.tcx.item_self_bounds(rpit_def_id).iter_identity().filter_map(|clause| {
1817 let predicate = clause
1818 .kind()
1819 .map_bound(|clause| match clause {
1820 ty::ClauseKind::Trait(trait_pred) => Some(ty::ClauseKind::Trait(
1821 trait_pred.with_self_ty(fcx.tcx, ty),
1822 )),
1823 ty::ClauseKind::Projection(proj_pred) => Some(
1824 ty::ClauseKind::Projection(proj_pred.with_self_ty(fcx.tcx, ty)),
1825 ),
1826 _ => None,
1827 })
1828 .transpose()?;
1829 Some(Obligation::new(
1830 fcx.tcx,
1831 ObligationCause::dummy(),
1832 fcx.param_env,
1833 predicate,
1834 ))
1835 }),
1836 );
1837 ocx.select_where_possible().is_empty()
1838 })
1839 };
1840
1841 if !compatible(a_ty) || !compatible(b_ty) {
1842 return;
1843 }
1844
1845 let rpid_def_span = fcx.tcx.def_span(rpit_def_id);
1846 err.subdiagnostic(SuggestBoxingForReturnImplTrait::ChangeReturnType {
1847 start_sp: rpid_def_span.with_hi(rpid_def_span.lo() + BytePos(4)),
1848 end_sp: rpid_def_span.shrink_to_hi(),
1849 });
1850
1851 let (starts, ends) =
1852 arm_spans.map(|span| (span.shrink_to_lo(), span.shrink_to_hi())).unzip();
1853 err.subdiagnostic(SuggestBoxingForReturnImplTrait::BoxReturnExpr { starts, ends });
1854 }
1855
1856 fn report_return_mismatched_types<'infcx>(
1857 &self,
1858 cause: &ObligationCause<'tcx>,
1859 expected: Ty<'tcx>,
1860 found: Ty<'tcx>,
1861 ty_err: TypeError<'tcx>,
1862 fcx: &'infcx FnCtxt<'_, 'tcx>,
1863 block_or_return_id: hir::HirId,
1864 expression: Option<&'tcx hir::Expr<'tcx>>,
1865 ) -> Diag<'infcx> {
1866 let mut err =
1867 fcx.err_ctxt().report_mismatched_types(cause, fcx.param_env, expected, found, ty_err);
1868
1869 let due_to_block = matches!(fcx.tcx.hir_node(block_or_return_id), hir::Node::Block(..));
1870 let parent = fcx.tcx.parent_hir_node(block_or_return_id);
1871 if let Some(expr) = expression
1872 && let hir::Node::Expr(&hir::Expr {
1873 kind: hir::ExprKind::Closure(&hir::Closure { body, .. }),
1874 ..
1875 }) = parent
1876 {
1877 let needs_block =
1878 !matches!(fcx.tcx.hir_body(body).value.kind, hir::ExprKind::Block(..));
1879 fcx.suggest_missing_semicolon(&mut err, expr, expected, needs_block, true);
1880 }
1881 // Verify that this is a tail expression of a function, otherwise the
1882 // label pointing out the cause for the type coercion will be wrong
1883 // as prior return coercions would not be relevant (#57664).
1884 if let Some(expr) = expression
1885 && due_to_block
1886 {
1887 fcx.suggest_missing_semicolon(&mut err, expr, expected, false, false);
1888 let pointing_at_return_type = fcx.suggest_mismatched_types_on_tail(
1889 &mut err,
1890 expr,
1891 expected,
1892 found,
1893 block_or_return_id,
1894 );
1895 if let Some(cond_expr) = fcx.tcx.hir_get_if_cause(expr.hir_id)
1896 && expected.is_unit()
1897 && !pointing_at_return_type
1898 // If the block is from an external macro or try (`?`) desugaring, then
1899 // do not suggest adding a semicolon, because there's nowhere to put it.
1900 // See issues #81943 and #87051.
1901 && matches!(
1902 cond_expr.span.desugaring_kind(),
1903 None | Some(DesugaringKind::WhileLoop)
1904 )
1905 && !cond_expr.span.in_external_macro(fcx.tcx.sess.source_map())
1906 && !matches!(
1907 cond_expr.kind,
1908 hir::ExprKind::Match(.., hir::MatchSource::TryDesugar(_))
1909 )
1910 {
1911 err.span_label(cond_expr.span, "expected this to be `()`");
1912 if expr.can_have_side_effects() {
1913 fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
1914 }
1915 }
1916 };
1917
1918 // If this is due to an explicit `return`, suggest adding a return type.
1919 if let Some((fn_id, fn_decl)) = fcx.get_fn_decl(block_or_return_id)
1920 && !due_to_block
1921 {
1922 fcx.suggest_missing_return_type(&mut err, fn_decl, expected, found, fn_id);
1923 }
1924
1925 // If this is due to a block, then maybe we forgot a `return`/`break`.
1926 if due_to_block
1927 && let Some(expr) = expression
1928 && let Some(parent_fn_decl) =
1929 fcx.tcx.hir_fn_decl_by_hir_id(fcx.tcx.local_def_id_to_hir_id(fcx.body_id))
1930 {
1931 fcx.suggest_missing_break_or_return_expr(
1932 &mut err,
1933 expr,
1934 parent_fn_decl,
1935 expected,
1936 found,
1937 block_or_return_id,
1938 fcx.body_id,
1939 );
1940 }
1941
1942 let ret_coercion_span = fcx.ret_coercion_span.get();
1943
1944 if let Some(sp) = ret_coercion_span
1945 // If the closure has an explicit return type annotation, or if
1946 // the closure's return type has been inferred from outside
1947 // requirements (such as an Fn* trait bound), then a type error
1948 // may occur at the first return expression we see in the closure
1949 // (if it conflicts with the declared return type). Skip adding a
1950 // note in this case, since it would be incorrect.
1951 && let Some(fn_sig) = fcx.body_fn_sig()
1952 && fn_sig.output().is_ty_var()
1953 {
1954 err.span_note(sp, format!("return type inferred to be `{expected}` here"));
1955 }
1956
1957 err
1958 }
1959
1960 /// Checks whether the return type is unsized via an obligation, which makes
1961 /// sure we consider `dyn Trait: Sized` where clauses, which are trivially
1962 /// false but technically valid for typeck.
1963 fn is_return_ty_definitely_unsized(&self, fcx: &FnCtxt<'_, 'tcx>) -> bool {
1964 if let Some(sig) = fcx.body_fn_sig() {
1965 !fcx.predicate_may_hold(&Obligation::new(
1966 fcx.tcx,
1967 ObligationCause::dummy(),
1968 fcx.param_env,
1969 ty::TraitRef::new(
1970 fcx.tcx,
1971 fcx.tcx.require_lang_item(hir::LangItem::Sized, DUMMY_SP),
1972 [sig.output()],
1973 ),
1974 ))
1975 } else {
1976 false
1977 }
1978 }
1979
1980 pub(crate) fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
1981 if let Some(final_ty) = self.final_ty {
1982 final_ty
1983 } else {
1984 // If we only had inputs that were of type `!` (or no
1985 // inputs at all), then the final type is `!`.
1986 assert_eq!(self.pushed, 0);
1987 fcx.tcx.types.never
1988 }
1989 }
1990}
1991
1992/// Something that can be converted into an expression to which we can
1993/// apply a coercion.
1994pub(crate) trait AsCoercionSite {
1995 fn as_coercion_site(&self) -> &hir::Expr<'_>;
1996}
1997
1998impl AsCoercionSite for hir::Expr<'_> {
1999 fn as_coercion_site(&self) -> &hir::Expr<'_> {
2000 self
2001 }
2002}
2003
2004impl<'a, T> AsCoercionSite for &'a T
2005where
2006 T: AsCoercionSite,
2007{
2008 fn as_coercion_site(&self) -> &hir::Expr<'_> {
2009 (**self).as_coercion_site()
2010 }
2011}
2012
2013impl AsCoercionSite for ! {
2014 fn as_coercion_site(&self) -> &hir::Expr<'_> {
2015 *self
2016 }
2017}
2018
2019impl AsCoercionSite for hir::Arm<'_> {
2020 fn as_coercion_site(&self) -> &hir::Expr<'_> {
2021 self.body
2022 }
2023}