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).
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 `src/test/ui/coerce/*.rs` tests for
17 //! examples of where this is useful.
21 //! When inferring the generic arguments of functions, the argument
22 //! order is relevant, which can lead to the following edge case:
24 //! ```ignore (illustrative)
25 //! fn foo<T>(a: T, b: T) {
29 //! foo(&7i32, &mut 7i32);
30 //! // This compiles, as we first infer `T` to be `&i32`,
31 //! // and then coerce `&mut 7i32` to `&7i32`.
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`.
38 use crate::astconv::AstConv;
39 use crate::check::FnCtxt;
41 struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed,
44 use rustc_hir::def_id::DefId;
45 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
46 use rustc_infer::infer::{Coercion, InferOk, InferResult};
47 use rustc_infer::traits::{Obligation, TraitEngine, TraitEngineExt};
48 use rustc_middle::lint::in_external_macro;
49 use rustc_middle::ty::adjustment::{
50 Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast,
52 use rustc_middle::ty::error::TypeError;
53 use rustc_middle::ty::relate::RelateResult;
54 use rustc_middle::ty::subst::SubstsRef;
55 use rustc_middle::ty::visit::TypeVisitable;
56 use rustc_middle::ty::{self, ToPredicate, Ty, TypeAndMut};
57 use rustc_session::parse::feature_err;
58 use rustc_span::symbol::sym;
59 use rustc_span::{self, BytePos, DesugaringKind, Span};
60 use rustc_target::spec::abi::Abi;
61 use rustc_trait_selection::infer::InferCtxtExt as _;
62 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
63 use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
65 use smallvec::{smallvec, SmallVec};
68 struct Coerce<'a, 'tcx> {
69 fcx: &'a FnCtxt<'a, 'tcx>,
70 cause: ObligationCause<'tcx>,
72 /// Determines whether or not allow_two_phase_borrow is set on any
73 /// autoref adjustments we create while coercing. We don't want to
74 /// allow deref coercions to create two-phase borrows, at least initially,
75 /// but we do need two-phase borrows for function argument reborrows.
76 /// See #47489 and #48598
77 /// See docs on the "AllowTwoPhase" type for a more detailed discussion
78 allow_two_phase: AllowTwoPhase,
81 impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
82 type Target = FnCtxt<'a, 'tcx>;
83 fn deref(&self) -> &Self::Target {
88 type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
90 /// Coercing a mutable reference to an immutable works, while
91 /// coercing `&T` to `&mut T` should be forbidden.
92 fn coerce_mutbls<'tcx>(
93 from_mutbl: hir::Mutability,
94 to_mutbl: hir::Mutability,
95 ) -> RelateResult<'tcx, ()> {
96 match (from_mutbl, to_mutbl) {
97 (hir::Mutability::Mut, hir::Mutability::Mut | hir::Mutability::Not)
98 | (hir::Mutability::Not, hir::Mutability::Not) => Ok(()),
99 (hir::Mutability::Not, hir::Mutability::Mut) => Err(TypeError::Mutability),
103 /// Do not require any adjustments, i.e. coerce `x -> x`.
104 fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> {
108 fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
109 move |target| vec![Adjustment { kind, target }]
112 /// This always returns `Ok(...)`.
114 adj: Vec<Adjustment<'tcx>>,
116 obligations: traits::PredicateObligations<'tcx>,
117 ) -> CoerceResult<'tcx> {
118 Ok(InferOk { value: (adj, target), obligations })
121 impl<'f, 'tcx> Coerce<'f, 'tcx> {
123 fcx: &'f FnCtxt<'f, 'tcx>,
124 cause: ObligationCause<'tcx>,
125 allow_two_phase: AllowTwoPhase,
127 Coerce { fcx, cause, allow_two_phase, use_lub: false }
130 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
131 debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
132 self.commit_if_ok(|_| {
134 self.at(&self.cause, self.fcx.param_env).lub(b, a)
136 self.at(&self.cause, self.fcx.param_env)
138 .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
143 /// Unify two types (using sub or lub) and produce a specific coercion.
144 fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
146 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
149 .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
152 #[instrument(skip(self))]
153 fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
154 // First, remove any resolved type variables (at the top level, at least):
155 let a = self.shallow_resolve(a);
156 let b = self.shallow_resolve(b);
157 debug!("Coerce.tys({:?} => {:?})", a, b);
159 // Just ignore error types.
160 if a.references_error() || b.references_error() {
161 return success(vec![], self.fcx.tcx.ty_error(), vec![]);
164 // Coercing from `!` to any type is allowed:
166 return success(simple(Adjust::NeverToAny)(b), b, vec![]);
169 // Coercing *from* an unresolved inference variable means that
170 // we have no information about the source type. This will always
171 // ultimately fall back to some form of subtyping.
173 return self.coerce_from_inference_variable(a, b, identity);
176 // Consider coercing the subtype to a DST
178 // NOTE: this is wrapped in a `commit_if_ok` because it creates
179 // a "spurious" type variable, and we don't want to have that
180 // type variable in memory if the coercion fails.
181 let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
184 debug!("coerce: unsize successful");
187 Err(TypeError::ObjectUnsafeCoercion(did)) => {
188 debug!("coerce: unsize not object safe");
189 return Err(TypeError::ObjectUnsafeCoercion(did));
192 debug!(?error, "coerce: unsize failed");
196 // Examine the supertype and consider auto-borrowing.
198 ty::RawPtr(mt_b) => {
199 return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
201 ty::Ref(r_b, _, mutbl_b) => {
202 return self.coerce_borrowed_pointer(a, b, r_b, mutbl_b);
209 // Function items are coercible to any closure
210 // type; function pointers are not (that would
211 // require double indirection).
212 // Additionally, we permit coercion of function
213 // items to drop the unsafe qualifier.
214 self.coerce_from_fn_item(a, b)
217 // We permit coercion of fn pointers to drop the
219 self.coerce_from_fn_pointer(a, a_f, b)
221 ty::Closure(closure_def_id_a, substs_a) => {
222 // Non-capturing closures are coercible to
223 // function pointers or unsafe function pointers.
224 // It cannot convert closures that require unsafe.
225 self.coerce_closure_to_fn(a, closure_def_id_a, substs_a, b)
228 // Otherwise, just use unification rules.
229 self.unify_and(a, b, identity)
234 /// Coercing *from* an inference variable. In this case, we have no information
235 /// about the source type, so we can't really do a true coercion and we always
236 /// fall back to subtyping (`unify_and`).
237 fn coerce_from_inference_variable(
241 make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
242 ) -> CoerceResult<'tcx> {
243 debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
244 assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
245 assert!(self.shallow_resolve(b) == b);
248 // Two unresolved type variables: create a `Coerce` predicate.
249 let target_ty = if self.use_lub {
250 self.next_ty_var(TypeVariableOrigin {
251 kind: TypeVariableOriginKind::LatticeVariable,
252 span: self.cause.span,
258 let mut obligations = Vec::with_capacity(2);
259 for &source_ty in &[a, b] {
260 if source_ty != target_ty {
261 obligations.push(Obligation::new(
264 ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate {
268 .to_predicate(self.tcx()),
274 "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
275 target_ty, obligations
277 let adjustments = make_adjustments(target_ty);
278 InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
280 // One unresolved type variable: just apply subtyping, we may be able
281 // to do something useful.
282 self.unify_and(a, b, make_adjustments)
286 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
287 /// To match `A` with `B`, autoderef will be performed,
288 /// calling `deref`/`deref_mut` where necessary.
289 fn coerce_borrowed_pointer(
293 r_b: ty::Region<'tcx>,
294 mutbl_b: hir::Mutability,
295 ) -> CoerceResult<'tcx> {
296 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
298 // If we have a parameter of type `&M T_a` and the value
299 // provided is `expr`, we will be adding an implicit borrow,
300 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
301 // to type check, we will construct the type that `&M*expr` would
304 let (r_a, mt_a) = match *a.kind() {
305 ty::Ref(r_a, ty, mutbl) => {
306 let mt_a = ty::TypeAndMut { ty, mutbl };
307 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
310 _ => return self.unify_and(a, b, identity),
313 let span = self.cause.span;
315 let mut first_error = None;
316 let mut r_borrow_var = None;
317 let mut autoderef = self.autoderef(span, a);
318 let mut found = None;
320 for (referent_ty, autoderefs) in autoderef.by_ref() {
322 // Don't let this pass, otherwise it would cause
323 // &T to autoref to &&T.
327 // At this point, we have deref'd `a` to `referent_ty`. So
328 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
329 // In the autoderef loop for `&'a mut Vec<T>`, we would get
332 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
333 // - `Vec<T>` -- 1 deref
334 // - `[T]` -- 2 deref
336 // At each point after the first callback, we want to
337 // check to see whether this would match out target type
338 // (`&'b mut [T]`) if we autoref'd it. We can't just
339 // compare the referent types, though, because we still
340 // have to consider the mutability. E.g., in the case
341 // we've been considering, we have an `&mut` reference, so
342 // the `T` in `[T]` needs to be unified with equality.
344 // Therefore, we construct reference types reflecting what
345 // the types will be after we do the final auto-ref and
346 // compare those. Note that this means we use the target
347 // mutability [1], since it may be that we are coercing
348 // from `&mut T` to `&U`.
350 // One fine point concerns the region that we use. We
351 // choose the region such that the region of the final
352 // type that results from `unify` will be the region we
353 // want for the autoref:
355 // - if in sub mode, that means we want to use `'b` (the
356 // region from the target reference) for both
357 // pointers [2]. This is because sub mode (somewhat
358 // arbitrarily) returns the subtype region. In the case
359 // where we are coercing to a target type, we know we
360 // want to use that target type region (`'b`) because --
361 // for the program to type-check -- it must be the
362 // smaller of the two.
363 // - One fine point. It may be surprising that we can
364 // use `'b` without relating `'a` and `'b`. The reason
365 // that this is ok is that what we produce is
366 // effectively a `&'b *x` expression (if you could
367 // annotate the region of a borrow), and regionck has
368 // code that adds edges from the region of a borrow
369 // (`'b`, here) into the regions in the borrowed
370 // expression (`*x`, here). (Search for "link".)
371 // - if in lub mode, things can get fairly complicated. The
372 // easiest thing is just to make a fresh
373 // region variable [4], which effectively means we defer
374 // the decision to region inference (and regionck, which will add
375 // some more edges to this variable). However, this can wind up
376 // creating a crippling number of variables in some cases --
377 // e.g., #32278 -- so we optimize one particular case [3].
378 // Let me try to explain with some examples:
379 // - The "running example" above represents the simple case,
380 // where we have one `&` reference at the outer level and
381 // ownership all the rest of the way down. In this case,
382 // we want `LUB('a, 'b)` as the resulting region.
383 // - However, if there are nested borrows, that region is
384 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
385 // `&'b T`. In this case, `'a` is actually irrelevant.
386 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
387 // we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
388 // (The errors actually show up in borrowck, typically, because
389 // this extra edge causes the region `'a` to be inferred to something
390 // too big, which then results in borrowck errors.)
391 // - We could track the innermost shared reference, but there is already
392 // code in regionck that has the job of creating links between
393 // the region of a borrow and the regions in the thing being
394 // borrowed (here, `'a` and `'x`), and it knows how to handle
395 // all the various cases. So instead we just make a region variable
396 // and let regionck figure it out.
397 let r = if !self.use_lub {
399 } else if autoderefs == 1 {
402 if r_borrow_var.is_none() {
403 // create var lazily, at most once
404 let coercion = Coercion(span);
405 let r = self.next_region_var(coercion);
406 r_borrow_var = Some(r); // [4] above
408 r_borrow_var.unwrap()
410 let derefd_ty_a = self.tcx.mk_ref(
414 mutbl: mutbl_b, // [1] above
417 match self.unify(derefd_ty_a, b) {
423 if first_error.is_none() {
424 first_error = Some(err);
430 // Extract type or return an error. We return the first error
431 // we got, which should be from relating the "base" type
432 // (e.g., in example above, the failure from relating `Vec<T>`
433 // to the target type), since that should be the least
435 let Some(InferOk { value: ty, mut obligations }) = found else {
436 let err = first_error.expect("coerce_borrowed_pointer had no error");
437 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
441 if ty == a && mt_a.mutbl == hir::Mutability::Not && autoderef.step_count() == 1 {
442 // As a special case, if we would produce `&'a *x`, that's
443 // a total no-op. We end up with the type `&'a T` just as
444 // we started with. In that case, just skip it
445 // altogether. This is just an optimization.
447 // Note that for `&mut`, we DO want to reborrow --
448 // otherwise, this would be a move, which might be an
449 // error. For example `foo(self.x)` where `self` and
450 // `self.x` both have `&mut `type would be a move of
451 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
452 // which is a borrow.
453 assert_eq!(mutbl_b, hir::Mutability::Not); // can only coerce &T -> &U
454 return success(vec![], ty, obligations);
457 let InferOk { value: mut adjustments, obligations: o } =
458 self.adjust_steps_as_infer_ok(&autoderef);
459 obligations.extend(o);
460 obligations.extend(autoderef.into_obligations());
462 // Now apply the autoref. We have to extract the region out of
463 // the final ref type we got.
464 let ty::Ref(r_borrow, _, _) = ty.kind() else {
465 span_bug!(span, "expected a ref type, got {:?}", ty);
467 let mutbl = match mutbl_b {
468 hir::Mutability::Not => AutoBorrowMutability::Not,
469 hir::Mutability::Mut => {
470 AutoBorrowMutability::Mut { allow_two_phase_borrow: self.allow_two_phase }
473 adjustments.push(Adjustment {
474 kind: Adjust::Borrow(AutoBorrow::Ref(*r_borrow, mutbl)),
478 debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
480 success(adjustments, ty, obligations)
483 // &[T; n] or &mut [T; n] -> &[T]
484 // or &mut [T; n] -> &mut [T]
485 // or &Concrete -> &Trait, etc.
486 #[instrument(skip(self), level = "debug")]
487 fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
488 source = self.shallow_resolve(source);
489 target = self.shallow_resolve(target);
490 debug!(?source, ?target);
492 // These 'if' statements require some explanation.
493 // The `CoerceUnsized` trait is special - it is only
494 // possible to write `impl CoerceUnsized<B> for A` where
495 // A and B have 'matching' fields. This rules out the following
496 // two types of blanket impls:
498 // `impl<T> CoerceUnsized<T> for SomeType`
499 // `impl<T> CoerceUnsized<SomeType> for T`
501 // Both of these trigger a special `CoerceUnsized`-related error (E0376)
503 // We can take advantage of this fact to avoid performing unnecessary work.
504 // If either `source` or `target` is a type variable, then any applicable impl
505 // would need to be generic over the self-type (`impl<T> CoerceUnsized<SomeType> for T`)
506 // or generic over the `CoerceUnsized` type parameter (`impl<T> CoerceUnsized<T> for
509 // However, these are exactly the kinds of impls which are forbidden by
510 // the compiler! Therefore, we can be sure that coercion will always fail
511 // when either the source or target type is a type variable. This allows us
512 // to skip performing any trait selection, and immediately bail out.
513 if source.is_ty_var() {
514 debug!("coerce_unsized: source is a TyVar, bailing out");
515 return Err(TypeError::Mismatch);
517 if target.is_ty_var() {
518 debug!("coerce_unsized: target is a TyVar, bailing out");
519 return Err(TypeError::Mismatch);
523 (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
524 let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
525 debug!("missing Unsize or CoerceUnsized traits");
526 return Err(TypeError::Mismatch);
529 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
530 // a DST unless we have to. This currently comes out in the wash since
531 // we can't unify [T] with U. But to properly support DST, we need to allow
532 // that, at which point we will need extra checks on the target here.
534 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
535 let reborrow = match (source.kind(), target.kind()) {
536 (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
537 coerce_mutbls(mutbl_a, mutbl_b)?;
539 let coercion = Coercion(self.cause.span);
540 let r_borrow = self.next_region_var(coercion);
541 let mutbl = match mutbl_b {
542 hir::Mutability::Not => AutoBorrowMutability::Not,
543 hir::Mutability::Mut => AutoBorrowMutability::Mut {
544 // We don't allow two-phase borrows here, at least for initial
545 // implementation. If it happens that this coercion is a function argument,
546 // the reborrow in coerce_borrowed_ptr will pick it up.
547 allow_two_phase_borrow: AllowTwoPhase::No,
551 Adjustment { kind: Adjust::Deref(None), target: ty_a },
553 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
556 .mk_ref(r_borrow, ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a }),
560 (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
561 coerce_mutbls(mt_a, mt_b)?;
564 Adjustment { kind: Adjust::Deref(None), target: ty_a },
566 kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
567 target: self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mt_b, ty: ty_a }),
573 let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
575 // Setup either a subtyping or a LUB relationship between
576 // the `CoerceUnsized` target type and the expected type.
577 // We only have the latter, so we use an inference variable
578 // for the former and let type inference do the rest.
579 let origin = TypeVariableOrigin {
580 kind: TypeVariableOriginKind::MiscVariable,
581 span: self.cause.span,
583 let coerce_target = self.next_ty_var(origin);
584 let mut coercion = self.unify_and(coerce_target, target, |target| {
585 let unsize = Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target };
587 None => vec![unsize],
588 Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
592 let mut selcx = traits::SelectionContext::new(self);
594 // Create an obligation for `Source: CoerceUnsized<Target>`.
595 let cause = ObligationCause::new(
598 ObligationCauseCode::Coercion { source, target },
601 // Use a FIFO queue for this custom fulfillment procedure.
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<[_; 4]> = smallvec![traits::predicate_for_trait_def(
615 &[coerce_target.into()]
618 let mut has_unsized_tuple_coercion = false;
619 let mut has_trait_upcasting_coercion = None;
621 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
622 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
623 // inference might unify those two inner type variables later.
624 let traits = [coerce_unsized_did, unsize_did];
625 while !queue.is_empty() {
626 let obligation = queue.remove(0);
627 debug!("coerce_unsized resolve step: {:?}", obligation);
628 let bound_predicate = obligation.predicate.kind();
629 let trait_pred = match bound_predicate.skip_binder() {
630 ty::PredicateKind::Trait(trait_pred) if traits.contains(&trait_pred.def_id()) => {
631 if unsize_did == trait_pred.def_id() {
632 let self_ty = trait_pred.self_ty();
633 let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty();
634 if let (ty::Dynamic(ref data_a, ..), ty::Dynamic(ref data_b, ..)) =
635 (self_ty.kind(), unsize_ty.kind())
636 && data_a.principal_def_id() != data_b.principal_def_id()
638 debug!("coerce_unsized: found trait upcasting coercion");
639 has_trait_upcasting_coercion = Some((self_ty, unsize_ty));
641 if let ty::Tuple(..) = unsize_ty.kind() {
642 debug!("coerce_unsized: found unsized tuple coercion");
643 has_unsized_tuple_coercion = true;
646 bound_predicate.rebind(trait_pred)
649 coercion.obligations.push(obligation);
653 match selcx.select(&obligation.with(trait_pred)) {
654 // Uncertain or unimplemented.
656 if trait_pred.def_id() == unsize_did {
657 let trait_pred = self.resolve_vars_if_possible(trait_pred);
658 let self_ty = trait_pred.skip_binder().self_ty();
659 let unsize_ty = trait_pred.skip_binder().trait_ref.substs[1].expect_ty();
660 debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
661 match (&self_ty.kind(), &unsize_ty.kind()) {
662 (ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
663 if self.type_var_is_sized(*v) =>
665 debug!("coerce_unsized: have sized infer {:?}", v);
666 coercion.obligations.push(obligation);
667 // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
671 // Some other case for `$0: Unsize<Something>`. Note that we
672 // hit this case even if `Something` is a sized type, so just
673 // don't do the coercion.
674 debug!("coerce_unsized: ambiguous unsize");
675 return Err(TypeError::Mismatch);
679 debug!("coerce_unsized: early return - ambiguous");
680 return Err(TypeError::Mismatch);
683 Err(traits::Unimplemented) => {
684 debug!("coerce_unsized: early return - can't prove obligation");
685 return Err(TypeError::Mismatch);
688 // Object safety violations or miscellaneous.
690 self.report_selection_error(obligation.clone(), &obligation, &err, false);
691 // Treat this like an obligation and follow through
692 // with the unsizing - the lack of a coercion should
693 // be silent, as it causes a type mismatch later.
696 Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
700 if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
702 &self.tcx.sess.parse_sess,
703 sym::unsized_tuple_coercion,
705 "unsized tuple coercion is not stable enough for use and is subject to change",
710 if let Some((sub, sup)) = has_trait_upcasting_coercion
711 && !self.tcx().features().trait_upcasting
713 // Renders better when we erase regions, since they're not really the point here.
714 let (sub, sup) = self.tcx.erase_regions((sub, sup));
715 let mut err = feature_err(
716 &self.tcx.sess.parse_sess,
717 sym::trait_upcasting,
719 &format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
721 err.note(&format!("required when coercing `{source}` into `{target}`"));
728 fn coerce_from_safe_fn<F, G>(
731 fn_ty_a: ty::PolyFnSig<'tcx>,
735 ) -> CoerceResult<'tcx>
737 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
738 G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
740 self.commit_if_ok(|snapshot| {
741 let result = if let ty::FnPtr(fn_ty_b) = b.kind()
742 && let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) =
743 (fn_ty_a.unsafety(), fn_ty_b.unsafety())
745 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
746 self.unify_and(unsafe_a, b, to_unsafe)
748 self.unify_and(a, b, normal)
751 // FIXME(#73154): This is a hack. Currently LUB can generate
752 // unsolvable constraints. Additionally, it returns `a`
753 // unconditionally, even when the "LUB" is `b`. In the future, we
754 // want the coerced type to be the actual supertype of these two,
755 // but for now, we want to just error to ensure we don't lock
756 // ourselves into a specific behavior with NLL.
757 self.leak_check(false, snapshot)?;
763 fn coerce_from_fn_pointer(
766 fn_ty_a: ty::PolyFnSig<'tcx>,
768 ) -> CoerceResult<'tcx> {
769 //! Attempts to coerce from the type of a Rust function item
770 //! into a closure or a `proc`.
773 let b = self.shallow_resolve(b);
774 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
776 self.coerce_from_safe_fn(
780 simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)),
785 fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
786 //! Attempts to coerce from the type of a Rust function item
787 //! into a closure or a `proc`.
789 let b = self.shallow_resolve(b);
790 let InferOk { value: b, mut obligations } =
791 self.normalize_associated_types_in_as_infer_ok(self.cause.span, b);
792 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
795 ty::FnPtr(b_sig) => {
796 let a_sig = a.fn_sig(self.tcx);
797 if let ty::FnDef(def_id, _) = *a.kind() {
798 // Intrinsics are not coercible to function pointers
799 if self.tcx.is_intrinsic(def_id) {
800 return Err(TypeError::IntrinsicCast);
803 // Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).
805 if b_sig.unsafety() == hir::Unsafety::Normal
806 && !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
808 return Err(TypeError::TargetFeatureCast(def_id));
812 let InferOk { value: a_sig, obligations: o1 } =
813 self.normalize_associated_types_in_as_infer_ok(self.cause.span, a_sig);
814 obligations.extend(o1);
816 let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
817 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
824 kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
825 target: a_fn_pointer,
828 kind: Adjust::Pointer(PointerCast::UnsafeFnPointer),
833 simple(Adjust::Pointer(PointerCast::ReifyFnPointer)),
836 obligations.extend(o2);
837 Ok(InferOk { value, obligations })
839 _ => self.unify_and(a, b, identity),
843 fn coerce_closure_to_fn(
846 closure_def_id_a: DefId,
847 substs_a: SubstsRef<'tcx>,
849 ) -> CoerceResult<'tcx> {
850 //! Attempts to coerce from the type of a non-capturing closure
851 //! into a function pointer.
854 let b = self.shallow_resolve(b);
857 // At this point we haven't done capture analysis, which means
858 // that the ClosureSubsts just contains an inference variable instead
859 // of tuple of captured types.
861 // All we care here is if any variable is being captured and not the exact paths,
862 // so we check `upvars_mentioned` for root variables being captured.
866 .upvars_mentioned(closure_def_id_a.expect_local())
867 .map_or(true, |u| u.is_empty()) =>
869 // We coerce the closure, which has fn type
870 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
872 // `fn(arg0,arg1,...) -> _`
874 // `unsafe fn(arg0,arg1,...) -> _`
875 let closure_sig = substs_a.as_closure().sig();
876 let unsafety = fn_ty.unsafety();
878 self.tcx.mk_fn_ptr(self.tcx.signature_unclosure(closure_sig, unsafety));
879 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
883 simple(Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))),
886 _ => self.unify_and(a, b, identity),
890 fn coerce_unsafe_ptr(
894 mutbl_b: hir::Mutability,
895 ) -> CoerceResult<'tcx> {
896 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
898 let (is_ref, mt_a) = match *a.kind() {
899 ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
900 ty::RawPtr(mt) => (false, mt),
901 _ => return self.unify_and(a, b, identity),
903 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
905 // Check that the types which they point at are compatible.
906 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty });
907 // Although references and unsafe ptrs have the same
908 // representation, we still register an Adjust::DerefRef so that
909 // regionck knows that the region for `a` must be valid here.
911 self.unify_and(a_unsafe, b, |target| {
913 Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
914 Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
917 } else if mt_a.mutbl != mutbl_b {
918 self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer)))
920 self.unify_and(a_unsafe, b, identity)
925 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
926 /// Attempt to coerce an expression to a type, and return the
927 /// adjusted type of the expression, if successful.
928 /// Adjustments are only recorded if the coercion succeeded.
929 /// The expressions *must not* have any pre-existing adjustments.
932 expr: &hir::Expr<'_>,
935 allow_two_phase: AllowTwoPhase,
936 cause: Option<ObligationCause<'tcx>>,
937 ) -> RelateResult<'tcx, Ty<'tcx>> {
938 let source = self.resolve_vars_with_obligations(expr_ty);
939 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
942 cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
943 let coerce = Coerce::new(self, cause, allow_two_phase);
944 let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
946 let (adjustments, _) = self.register_infer_ok_obligations(ok);
947 self.apply_adjustments(expr, adjustments);
948 Ok(if expr_ty.references_error() { self.tcx.ty_error() } else { target })
951 /// Same as `try_coerce()`, but without side-effects.
953 /// Returns false if the coercion creates any obligations that result in
955 pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
956 let source = self.resolve_vars_with_obligations(expr_ty);
957 debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);
959 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
960 // We don't ever need two-phase here since we throw out the result of the coercion
961 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
963 let Ok(ok) = coerce.coerce(source, target) else {
966 let mut fcx = traits::FulfillmentContext::new_in_snapshot();
967 fcx.register_predicate_obligations(self, ok.obligations);
968 fcx.select_where_possible(&self).is_empty()
972 /// Given a type and a target type, this function will calculate and return
973 /// how many dereference steps needed to achieve `expr_ty <: target`. If
974 /// it's not possible, return `None`.
975 pub fn deref_steps(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> Option<usize> {
976 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
977 // We don't ever need two-phase here since we throw out the result of the coercion
978 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
980 .autoderef(rustc_span::DUMMY_SP, expr_ty)
981 .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
984 /// Given a type, this function will calculate and return the type given
985 /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
987 /// This function is for diagnostics only, since it does not register
988 /// trait or region sub-obligations. (presumably we could, but it's not
989 /// particularly important for diagnostics...)
990 pub fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
991 self.autoderef(rustc_span::DUMMY_SP, expr_ty).nth(1).and_then(|(deref_ty, _)| {
993 .type_implements_trait(
994 self.tcx.lang_items().deref_mut_trait()?,
1004 /// Given some expressions, their known unified type and another expression,
1005 /// tries to unify the types, potentially inserting coercions on any of the
1006 /// provided expressions and returns their LUB (aka "common supertype").
1008 /// This is really an internal helper. From outside the coercion
1009 /// module, you should instantiate a `CoerceMany` instance.
1010 fn try_find_coercion_lub<E>(
1012 cause: &ObligationCause<'tcx>,
1015 new: &hir::Expr<'_>,
1017 ) -> RelateResult<'tcx, Ty<'tcx>>
1021 let prev_ty = self.resolve_vars_with_obligations(prev_ty);
1022 let new_ty = self.resolve_vars_with_obligations(new_ty);
1024 "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
1030 // The following check fixes #88097, where the compiler erroneously
1031 // attempted to coerce a closure type to itself via a function pointer.
1032 if prev_ty == new_ty {
1036 // Special-case that coercion alone cannot handle:
1037 // Function items or non-capturing closures of differing IDs or InternalSubsts.
1038 let (a_sig, b_sig) = {
1039 #[allow(rustc::usage_of_ty_tykind)]
1040 let is_capturing_closure = |ty: &ty::TyKind<'tcx>| {
1041 if let &ty::Closure(closure_def_id, _substs) = ty {
1042 self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some()
1047 if is_capturing_closure(prev_ty.kind()) || is_capturing_closure(new_ty.kind()) {
1050 match (prev_ty.kind(), new_ty.kind()) {
1051 (ty::FnDef(..), ty::FnDef(..)) => {
1052 // Don't reify if the function types have a LUB, i.e., they
1053 // are the same function and their parameters have a LUB.
1055 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1057 // We have a LUB of prev_ty and new_ty, just return it.
1058 Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
1060 (Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx)))
1064 (ty::Closure(_, substs), ty::FnDef(..)) => {
1065 let b_sig = new_ty.fn_sig(self.tcx);
1068 .signature_unclosure(substs.as_closure().sig(), b_sig.unsafety());
1069 (Some(a_sig), Some(b_sig))
1071 (ty::FnDef(..), ty::Closure(_, substs)) => {
1072 let a_sig = prev_ty.fn_sig(self.tcx);
1075 .signature_unclosure(substs.as_closure().sig(), a_sig.unsafety());
1076 (Some(a_sig), Some(b_sig))
1078 (ty::Closure(_, substs_a), ty::Closure(_, substs_b)) => (
1079 Some(self.tcx.signature_unclosure(
1080 substs_a.as_closure().sig(),
1081 hir::Unsafety::Normal,
1083 Some(self.tcx.signature_unclosure(
1084 substs_b.as_closure().sig(),
1085 hir::Unsafety::Normal,
1092 if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) {
1093 // Intrinsics are not coercible to function pointers.
1094 if a_sig.abi() == Abi::RustIntrinsic
1095 || a_sig.abi() == Abi::PlatformIntrinsic
1096 || b_sig.abi() == Abi::RustIntrinsic
1097 || b_sig.abi() == Abi::PlatformIntrinsic
1099 return Err(TypeError::IntrinsicCast);
1101 // The signature must match.
1102 let a_sig = self.normalize_associated_types_in(new.span, a_sig);
1103 let b_sig = self.normalize_associated_types_in(new.span, b_sig);
1105 .at(cause, self.param_env)
1106 .trace(prev_ty, new_ty)
1108 .map(|ok| self.register_infer_ok_obligations(ok))?;
1110 // Reify both sides and return the reified fn pointer type.
1111 let fn_ptr = self.tcx.mk_fn_ptr(sig);
1112 let prev_adjustment = match prev_ty.kind() {
1113 ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(a_sig.unsafety())),
1114 ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
1115 _ => unreachable!(),
1117 let next_adjustment = match new_ty.kind() {
1118 ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(b_sig.unsafety())),
1119 ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
1120 _ => unreachable!(),
1122 for expr in exprs.iter().map(|e| e.as_coercion_site()) {
1123 self.apply_adjustments(
1125 vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
1128 self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
1132 // Configure a Coerce instance to compute the LUB.
1133 // We don't allow two-phase borrows on any autorefs this creates since we
1134 // probably aren't processing function arguments here and even if we were,
1135 // they're going to get autorefed again anyway and we can apply 2-phase borrows
1137 let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
1138 coerce.use_lub = true;
1140 // First try to coerce the new expression to the type of the previous ones,
1141 // but only if the new expression has no coercion already applied to it.
1142 let mut first_error = None;
1143 if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
1144 let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
1147 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1148 self.apply_adjustments(new, adjustments);
1150 "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
1151 new_ty, prev_ty, target
1155 Err(e) => first_error = Some(e),
1159 // Then try to coerce the previous expressions to the type of the new one.
1160 // This requires ensuring there are no coercions applied to *any* of the
1161 // previous expressions, other than noop reborrows (ignoring lifetimes).
1163 let expr = expr.as_coercion_site();
1164 let noop = match self.typeck_results.borrow().expr_adjustments(expr) {
1166 Adjustment { kind: Adjust::Deref(_), .. },
1167 Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. },
1169 match *self.node_ty(expr.hir_id).kind() {
1170 ty::Ref(_, _, mt_orig) => {
1171 let mutbl_adj: hir::Mutability = mutbl_adj.into();
1172 // Reborrow that we can safely ignore, because
1173 // the next adjustment can only be a Deref
1174 // which will be merged into it.
1175 mutbl_adj == mt_orig
1180 &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
1186 "coercion::try_find_coercion_lub: older expression {:?} had adjustments, requiring LUB",
1191 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1192 .map(|ok| self.register_infer_ok_obligations(ok));
1196 match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
1198 // Avoid giving strange errors on failed attempts.
1199 if let Some(e) = first_error {
1202 self.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1203 .map(|ok| self.register_infer_ok_obligations(ok))
1207 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1209 let expr = expr.as_coercion_site();
1210 self.apply_adjustments(expr, adjustments.clone());
1213 "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
1214 prev_ty, new_ty, target
1222 /// CoerceMany encapsulates the pattern you should use when you have
1223 /// many expressions that are all getting coerced to a common
1224 /// type. This arises, for example, when you have a match (the result
1225 /// of each arm is coerced to a common type). It also arises in less
1226 /// obvious places, such as when you have many `break foo` expressions
1227 /// that target the same loop, or the various `return` expressions in
1230 /// The basic protocol is as follows:
1232 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
1233 /// This will also serve as the "starting LUB". The expectation is
1234 /// that this type is something which all of the expressions *must*
1235 /// be coercible to. Use a fresh type variable if needed.
1236 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
1237 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
1238 /// unit. This happens for example if you have a `break` with no expression,
1239 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
1240 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
1241 /// from you so that you don't have to worry your pretty head about it.
1242 /// But if an error is reported, the final type will be `err`.
1243 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
1244 /// previously coerced expressions.
1245 /// - When all done, invoke `complete()`. This will return the LUB of
1246 /// all your expressions.
1247 /// - WARNING: I don't believe this final type is guaranteed to be
1248 /// related to your initial `expected_ty` in any particular way,
1249 /// although it will typically be a subtype, so you should check it.
1250 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
1251 /// previously coerced expressions.
1255 /// ```ignore (illustrative)
1256 /// let mut coerce = CoerceMany::new(expected_ty);
1257 /// for expr in exprs {
1258 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
1259 /// coerce.coerce(fcx, &cause, expr, expr_ty);
1261 /// let final_ty = coerce.complete(fcx);
1263 pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
1264 expected_ty: Ty<'tcx>,
1265 final_ty: Option<Ty<'tcx>>,
1266 expressions: Expressions<'tcx, 'exprs, E>,
1270 /// The type of a `CoerceMany` that is storing up the expressions into
1271 /// a buffer. We use this in `check/mod.rs` for things like `break`.
1272 pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;
1274 enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
1275 Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
1276 UpFront(&'exprs [E]),
1279 impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
1280 /// The usual case; collect the set of expressions dynamically.
1281 /// If the full set of coercion sites is known before hand,
1282 /// consider `with_coercion_sites()` instead to avoid allocation.
1283 pub fn new(expected_ty: Ty<'tcx>) -> Self {
1284 Self::make(expected_ty, Expressions::Dynamic(vec![]))
1287 /// As an optimization, you can create a `CoerceMany` with a
1288 /// pre-existing slice of expressions. In this case, you are
1289 /// expected to pass each element in the slice to `coerce(...)` in
1290 /// order. This is used with arrays in particular to avoid
1291 /// needlessly cloning the slice.
1292 pub fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
1293 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
1296 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
1297 CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
1300 /// Returns the "expected type" with which this coercion was
1301 /// constructed. This represents the "downward propagated" type
1302 /// that was given to us at the start of typing whatever construct
1303 /// we are typing (e.g., the match expression).
1305 /// Typically, this is used as the expected type when
1306 /// type-checking each of the alternative expressions whose types
1307 /// we are trying to merge.
1308 pub fn expected_ty(&self) -> Ty<'tcx> {
1312 /// Returns the current "merged type", representing our best-guess
1313 /// at the LUB of the expressions we've seen so far (if any). This
1314 /// isn't *final* until you call `self.complete()`, which will return
1315 /// the merged type.
1316 pub fn merged_ty(&self) -> Ty<'tcx> {
1317 self.final_ty.unwrap_or(self.expected_ty)
1320 /// Indicates that the value generated by `expression`, which is
1321 /// of type `expression_ty`, is one of the possibilities that we
1322 /// could coerce from. This will record `expression`, and later
1323 /// calls to `coerce` may come back and add adjustments and things
1327 fcx: &FnCtxt<'a, 'tcx>,
1328 cause: &ObligationCause<'tcx>,
1329 expression: &'tcx hir::Expr<'tcx>,
1330 expression_ty: Ty<'tcx>,
1332 self.coerce_inner(fcx, cause, Some(expression), expression_ty, None, false)
1335 /// Indicates that one of the inputs is a "forced unit". This
1336 /// occurs in a case like `if foo { ... };`, where the missing else
1337 /// generates a "forced unit". Another example is a `loop { break;
1338 /// }`, where the `break` has no argument expression. We treat
1339 /// these cases slightly differently for error-reporting
1340 /// purposes. Note that these tend to correspond to cases where
1341 /// the `()` expression is implicit in the source, and hence we do
1342 /// not take an expression argument.
1344 /// The `augment_error` gives you a chance to extend the error
1345 /// message, in case any results (e.g., we use this to suggest
1346 /// removing a `;`).
1347 pub fn coerce_forced_unit<'a>(
1349 fcx: &FnCtxt<'a, 'tcx>,
1350 cause: &ObligationCause<'tcx>,
1351 augment_error: &mut dyn FnMut(&mut Diagnostic),
1352 label_unit_as_expected: bool,
1359 Some(augment_error),
1360 label_unit_as_expected,
1364 /// The inner coercion "engine". If `expression` is `None`, this
1365 /// is a forced-unit case, and hence `expression_ty` must be
1367 #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
1368 pub(crate) fn coerce_inner<'a>(
1370 fcx: &FnCtxt<'a, 'tcx>,
1371 cause: &ObligationCause<'tcx>,
1372 expression: Option<&'tcx hir::Expr<'tcx>>,
1373 mut expression_ty: Ty<'tcx>,
1374 augment_error: Option<&mut dyn FnMut(&mut Diagnostic)>,
1375 label_expression_as_expected: bool,
1377 // Incorporate whatever type inference information we have
1378 // until now; in principle we might also want to process
1379 // pending obligations, but doing so should only improve
1380 // compatibility (hopefully that is true) by helping us
1381 // uncover never types better.
1382 if expression_ty.is_ty_var() {
1383 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1386 // If we see any error types, just propagate that error
1388 if expression_ty.references_error() || self.merged_ty().references_error() {
1389 self.final_ty = Some(fcx.tcx.ty_error());
1393 // Handle the actual type unification etc.
1394 let result = if let Some(expression) = expression {
1395 if self.pushed == 0 {
1396 // Special-case the first expression we are coercing.
1397 // To be honest, I'm not entirely sure why we do this.
1398 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1404 Some(cause.clone()),
1407 match self.expressions {
1408 Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
1415 Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub(
1417 &coercion_sites[0..self.pushed],
1425 // this is a hack for cases where we default to `()` because
1426 // the expression etc has been omitted from the source. An
1427 // example is an `if let` without an else:
1429 // if let Some(x) = ... { }
1431 // we wind up with a second match arm that is like `_ =>
1432 // ()`. That is the case we are considering here. We take
1433 // a different path to get the right "expected, found"
1434 // message and so forth (and because we know that
1435 // `expression_ty` will be unit).
1437 // Another example is `break` with no argument expression.
1438 assert!(expression_ty.is_unit(), "if let hack without unit type");
1439 fcx.at(cause, fcx.param_env)
1440 .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
1442 fcx.register_infer_ok_obligations(infer_ok);
1450 self.final_ty = Some(v);
1451 if let Some(e) = expression {
1452 match self.expressions {
1453 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1454 Expressions::UpFront(coercion_sites) => {
1455 // if the user gave us an array to validate, check that we got
1456 // the next expression in the list, as expected
1458 coercion_sites[self.pushed].as_coercion_site().hir_id,
1466 Err(coercion_error) => {
1467 let (expected, found) = if label_expression_as_expected {
1468 // In the case where this is a "forced unit", like
1469 // `break`, we want to call the `()` "expected"
1470 // since it is implied by the syntax.
1471 // (Note: not all force-units work this way.)"
1472 (expression_ty, self.final_ty.unwrap_or(self.expected_ty))
1474 // Otherwise, the "expected" type for error
1475 // reporting is the current unification type,
1476 // which is basically the LUB of the expressions
1477 // we've seen so far (combined with the expected
1479 (self.final_ty.unwrap_or(self.expected_ty), expression_ty)
1483 let mut unsized_return = false;
1484 match *cause.code() {
1485 ObligationCauseCode::ReturnNoExpression => {
1486 err = struct_span_err!(
1490 "`return;` in a function whose return type is not `()`"
1492 err.span_label(cause.span, "return type is not `()`");
1494 ObligationCauseCode::BlockTailExpression(blk_id) => {
1495 let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
1496 err = self.report_return_mismatched_types(
1500 coercion_error.clone(),
1506 if !fcx.tcx.features().unsized_locals {
1507 unsized_return = self.is_return_ty_unsized(fcx, blk_id);
1510 ObligationCauseCode::ReturnValue(id) => {
1511 err = self.report_return_mismatched_types(
1515 coercion_error.clone(),
1521 if !fcx.tcx.features().unsized_locals {
1522 let id = fcx.tcx.hir().get_parent_node(id);
1523 unsized_return = self.is_return_ty_unsized(fcx, id);
1527 err = fcx.report_mismatched_types(
1531 coercion_error.clone(),
1536 if let Some(augment_error) = augment_error {
1537 augment_error(&mut err);
1540 let is_insufficiently_polymorphic =
1541 matches!(coercion_error, TypeError::RegionsInsufficientlyPolymorphic(..));
1543 if !is_insufficiently_polymorphic && let Some(expr) = expression {
1544 fcx.emit_coerce_suggestions(
1550 Some(coercion_error),
1554 err.emit_unless(unsized_return);
1556 self.final_ty = Some(fcx.tcx.ty_error());
1561 fn report_return_mismatched_types<'a>(
1563 cause: &ObligationCause<'tcx>,
1566 ty_err: TypeError<'tcx>,
1567 fcx: &FnCtxt<'a, 'tcx>,
1569 expression: Option<&'tcx hir::Expr<'tcx>>,
1570 blk_id: Option<hir::HirId>,
1571 ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
1572 let mut err = fcx.report_mismatched_types(cause, expected, found, ty_err);
1574 let mut pointing_at_return_type = false;
1575 let mut fn_output = None;
1577 let parent_id = fcx.tcx.hir().get_parent_node(id);
1578 let parent = fcx.tcx.hir().get(parent_id);
1579 if let Some(expr) = expression
1580 && let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(&hir::Closure { body, .. }), .. }) = parent
1581 && !matches!(fcx.tcx.hir().body(body).value.kind, hir::ExprKind::Block(..))
1583 fcx.suggest_missing_semicolon(&mut err, expr, expected, true);
1585 // Verify that this is a tail expression of a function, otherwise the
1586 // label pointing out the cause for the type coercion will be wrong
1587 // as prior return coercions would not be relevant (#57664).
1588 let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) {
1589 pointing_at_return_type =
1590 fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id);
1591 if let (Some(cond_expr), true, false) = (
1592 fcx.tcx.hir().get_if_cause(expr.hir_id),
1594 pointing_at_return_type,
1596 // If the block is from an external macro or try (`?`) desugaring, then
1597 // do not suggest adding a semicolon, because there's nowhere to put it.
1598 // See issues #81943 and #87051.
1600 cond_expr.span.desugaring_kind(),
1601 None | Some(DesugaringKind::WhileLoop)
1602 ) && !in_external_macro(fcx.tcx.sess, cond_expr.span)
1605 hir::ExprKind::Match(.., hir::MatchSource::TryDesugar)
1608 err.span_label(cond_expr.span, "expected this to be `()`");
1609 if expr.can_have_side_effects() {
1610 fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
1613 fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
1615 fcx.get_fn_decl(parent_id)
1618 if let Some((fn_decl, can_suggest)) = fn_decl {
1619 if blk_id.is_none() {
1620 pointing_at_return_type |= fcx.suggest_missing_return_type(
1626 fcx.tcx.hir().local_def_id_to_hir_id(fcx.tcx.hir().get_parent_item(id)),
1629 if !pointing_at_return_type {
1630 fn_output = Some(&fn_decl.output); // `impl Trait` return type
1634 let parent_id = fcx.tcx.hir().get_parent_item(id);
1635 let parent_item = fcx.tcx.hir().get_by_def_id(parent_id);
1637 if let (Some(expr), Some(_), Some((fn_decl, _, _))) =
1638 (expression, blk_id, fcx.get_node_fn_decl(parent_item))
1640 fcx.suggest_missing_break_or_return_expr(
1647 fcx.tcx.hir().local_def_id_to_hir_id(parent_id),
1651 let ret_coercion_span = fcx.ret_coercion_span.get();
1653 if let Some(sp) = ret_coercion_span
1654 // If the closure has an explicit return type annotation, or if
1655 // the closure's return type has been inferred from outside
1656 // requirements (such as an Fn* trait bound), then a type error
1657 // may occur at the first return expression we see in the closure
1658 // (if it conflicts with the declared return type). Skip adding a
1659 // note in this case, since it would be incorrect.
1660 && !fcx.return_type_pre_known
1665 "return type inferred to be `{}` here",
1666 fcx.resolve_vars_if_possible(expected)
1671 if let (Some(sp), Some(fn_output)) = (ret_coercion_span, fn_output) {
1672 self.add_impl_trait_explanation(&mut err, cause, fcx, expected, sp, fn_output);
1678 fn add_impl_trait_explanation<'a>(
1680 err: &mut Diagnostic,
1681 cause: &ObligationCause<'tcx>,
1682 fcx: &FnCtxt<'a, 'tcx>,
1685 fn_output: &hir::FnRetTy<'_>,
1687 let return_sp = fn_output.span();
1688 err.span_label(return_sp, "expected because this return type...");
1691 format!("...is found to be `{}` here", fcx.resolve_vars_with_obligations(expected)),
1693 let impl_trait_msg = "for information on `impl Trait`, see \
1694 <https://doc.rust-lang.org/book/ch10-02-traits.html\
1695 #returning-types-that-implement-traits>";
1696 let trait_obj_msg = "for information on trait objects, see \
1697 <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
1698 #using-trait-objects-that-allow-for-values-of-different-types>";
1699 err.note("to return `impl Trait`, all returned values must be of the same type");
1700 err.note(impl_trait_msg);
1705 .span_to_snippet(return_sp)
1706 .unwrap_or_else(|_| "dyn Trait".to_string());
1707 let mut snippet_iter = snippet.split_whitespace();
1708 let has_impl = snippet_iter.next().map_or(false, |s| s == "impl");
1709 // Only suggest `Box<dyn Trait>` if `Trait` in `impl Trait` is object safe.
1710 let mut is_object_safe = false;
1711 if let hir::FnRetTy::Return(ty) = fn_output
1712 // Get the return type.
1713 && let hir::TyKind::OpaqueDef(..) = ty.kind
1715 let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty);
1716 // Get the `impl Trait`'s `DefId`.
1717 if let ty::Opaque(def_id, _) = ty.kind()
1718 // Get the `impl Trait`'s `Item` so that we can get its trait bounds and
1719 // get the `Trait`'s `DefId`.
1720 && let hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, .. }) =
1721 fcx.tcx.hir().expect_item(def_id.expect_local()).kind
1723 // Are of this `impl Trait`'s traits object safe?
1724 is_object_safe = bounds.iter().all(|bound| {
1727 .and_then(|t| t.trait_def_id())
1728 .map_or(false, |def_id| {
1729 fcx.tcx.object_safety_violations(def_id).is_empty()
1736 err.multipart_suggestion(
1737 "you could change the return type to be a boxed trait object",
1739 (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()),
1740 (return_sp.shrink_to_hi(), ">".to_string()),
1742 Applicability::MachineApplicable,
1744 let sugg = [sp, cause.span]
1748 (sp.shrink_to_lo(), "Box::new(".to_string()),
1749 (sp.shrink_to_hi(), ")".to_string()),
1753 .collect::<Vec<_>>();
1754 err.multipart_suggestion(
1755 "if you change the return type to expect trait objects, box the returned \
1758 Applicability::MaybeIncorrect,
1762 "if the trait `{}` were object safe, you could return a boxed trait object",
1766 err.note(trait_obj_msg);
1768 err.help("you could instead create a new `enum` with a variant for each returned type");
1771 fn is_return_ty_unsized<'a>(&self, fcx: &FnCtxt<'a, 'tcx>, blk_id: hir::HirId) -> bool {
1772 if let Some((fn_decl, _)) = fcx.get_fn_decl(blk_id)
1773 && let hir::FnRetTy::Return(ty) = fn_decl.output
1774 && let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty)
1775 && let ty::Dynamic(..) = ty.kind()
1782 pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
1783 if let Some(final_ty) = self.final_ty {
1786 // If we only had inputs that were of type `!` (or no
1787 // inputs at all), then the final type is `!`.
1788 assert_eq!(self.pushed, 0);
1794 /// Something that can be converted into an expression to which we can
1795 /// apply a coercion.
1796 pub trait AsCoercionSite {
1797 fn as_coercion_site(&self) -> &hir::Expr<'_>;
1800 impl AsCoercionSite for hir::Expr<'_> {
1801 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1806 impl<'a, T> AsCoercionSite for &'a T
1810 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1811 (**self).as_coercion_site()
1815 impl AsCoercionSite for ! {
1816 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1821 impl AsCoercionSite for hir::Arm<'_> {
1822 fn as_coercion_site(&self) -> &hir::Expr<'_> {