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, MultiSpan,
44 use rustc_hir::def_id::DefId;
45 use rustc_hir::intravisit::{self, Visitor};
47 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
48 use rustc_infer::infer::{Coercion, InferOk, InferResult};
49 use rustc_infer::traits::{Obligation, TraitEngine, TraitEngineExt};
50 use rustc_middle::lint::in_external_macro;
51 use rustc_middle::ty::adjustment::{
52 Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast,
54 use rustc_middle::ty::error::TypeError;
55 use rustc_middle::ty::relate::RelateResult;
56 use rustc_middle::ty::subst::SubstsRef;
57 use rustc_middle::ty::visit::TypeVisitable;
58 use rustc_middle::ty::{self, ToPredicate, Ty, TypeAndMut};
59 use rustc_session::parse::feature_err;
60 use rustc_span::symbol::sym;
61 use rustc_span::{self, BytePos, DesugaringKind, Span};
62 use rustc_target::spec::abi::Abi;
63 use rustc_trait_selection::infer::InferCtxtExt as _;
64 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
65 use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
67 use smallvec::{smallvec, SmallVec};
70 struct Coerce<'a, 'tcx> {
71 fcx: &'a FnCtxt<'a, 'tcx>,
72 cause: ObligationCause<'tcx>,
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,
83 impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
84 type Target = FnCtxt<'a, 'tcx>;
85 fn deref(&self) -> &Self::Target {
90 type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
92 struct CollectRetsVisitor<'tcx> {
93 ret_exprs: Vec<&'tcx hir::Expr<'tcx>>,
96 impl<'tcx> Visitor<'tcx> for CollectRetsVisitor<'tcx> {
97 fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
98 if let hir::ExprKind::Ret(_) = expr.kind {
99 self.ret_exprs.push(expr);
101 intravisit::walk_expr(self, expr);
105 /// Coercing a mutable reference to an immutable works, while
106 /// coercing `&T` to `&mut T` should be forbidden.
107 fn coerce_mutbls<'tcx>(
108 from_mutbl: hir::Mutability,
109 to_mutbl: hir::Mutability,
110 ) -> RelateResult<'tcx, ()> {
111 match (from_mutbl, to_mutbl) {
112 (hir::Mutability::Mut, hir::Mutability::Mut | hir::Mutability::Not)
113 | (hir::Mutability::Not, hir::Mutability::Not) => Ok(()),
114 (hir::Mutability::Not, hir::Mutability::Mut) => Err(TypeError::Mutability),
118 /// Do not require any adjustments, i.e. coerce `x -> x`.
119 fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> {
123 fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
124 move |target| vec![Adjustment { kind, target }]
127 /// This always returns `Ok(...)`.
129 adj: Vec<Adjustment<'tcx>>,
131 obligations: traits::PredicateObligations<'tcx>,
132 ) -> CoerceResult<'tcx> {
133 Ok(InferOk { value: (adj, target), obligations })
136 impl<'f, 'tcx> Coerce<'f, 'tcx> {
138 fcx: &'f FnCtxt<'f, 'tcx>,
139 cause: ObligationCause<'tcx>,
140 allow_two_phase: AllowTwoPhase,
142 Coerce { fcx, cause, allow_two_phase, use_lub: false }
145 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
146 debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
147 self.commit_if_ok(|_| {
149 self.at(&self.cause, self.fcx.param_env).lub(b, a)
151 self.at(&self.cause, self.fcx.param_env)
153 .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
158 /// Unify two types (using sub or lub) and produce a specific coercion.
159 fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
161 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
164 .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
167 #[instrument(skip(self))]
168 fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
169 // First, remove any resolved type variables (at the top level, at least):
170 let a = self.shallow_resolve(a);
171 let b = self.shallow_resolve(b);
172 debug!("Coerce.tys({:?} => {:?})", a, b);
174 // Just ignore error types.
175 if a.references_error() || b.references_error() {
176 return success(vec![], self.fcx.tcx.ty_error(), vec![]);
179 // Coercing from `!` to any type is allowed:
181 return success(simple(Adjust::NeverToAny)(b), b, vec![]);
184 // Coercing *from* an unresolved inference variable means that
185 // we have no information about the source type. This will always
186 // ultimately fall back to some form of subtyping.
188 return self.coerce_from_inference_variable(a, b, identity);
191 // Consider coercing the subtype to a DST
193 // NOTE: this is wrapped in a `commit_if_ok` because it creates
194 // a "spurious" type variable, and we don't want to have that
195 // type variable in memory if the coercion fails.
196 let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
199 debug!("coerce: unsize successful");
202 Err(TypeError::ObjectUnsafeCoercion(did)) => {
203 debug!("coerce: unsize not object safe");
204 return Err(TypeError::ObjectUnsafeCoercion(did));
207 debug!(?error, "coerce: unsize failed");
211 // Examine the supertype and consider auto-borrowing.
213 ty::RawPtr(mt_b) => {
214 return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
216 ty::Ref(r_b, _, mutbl_b) => {
217 return self.coerce_borrowed_pointer(a, b, r_b, mutbl_b);
224 // Function items are coercible to any closure
225 // type; function pointers are not (that would
226 // require double indirection).
227 // Additionally, we permit coercion of function
228 // items to drop the unsafe qualifier.
229 self.coerce_from_fn_item(a, b)
232 // We permit coercion of fn pointers to drop the
234 self.coerce_from_fn_pointer(a, a_f, b)
236 ty::Closure(closure_def_id_a, substs_a) => {
237 // Non-capturing closures are coercible to
238 // function pointers or unsafe function pointers.
239 // It cannot convert closures that require unsafe.
240 self.coerce_closure_to_fn(a, closure_def_id_a, substs_a, b)
243 // Otherwise, just use unification rules.
244 self.unify_and(a, b, identity)
249 /// Coercing *from* an inference variable. In this case, we have no information
250 /// about the source type, so we can't really do a true coercion and we always
251 /// fall back to subtyping (`unify_and`).
252 fn coerce_from_inference_variable(
256 make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
257 ) -> CoerceResult<'tcx> {
258 debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
259 assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
260 assert!(self.shallow_resolve(b) == b);
263 // Two unresolved type variables: create a `Coerce` predicate.
264 let target_ty = if self.use_lub {
265 self.next_ty_var(TypeVariableOrigin {
266 kind: TypeVariableOriginKind::LatticeVariable,
267 span: self.cause.span,
273 let mut obligations = Vec::with_capacity(2);
274 for &source_ty in &[a, b] {
275 if source_ty != target_ty {
276 obligations.push(Obligation::new(
279 ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate {
283 .to_predicate(self.tcx()),
289 "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
290 target_ty, obligations
292 let adjustments = make_adjustments(target_ty);
293 InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
295 // One unresolved type variable: just apply subtyping, we may be able
296 // to do something useful.
297 self.unify_and(a, b, make_adjustments)
301 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
302 /// To match `A` with `B`, autoderef will be performed,
303 /// calling `deref`/`deref_mut` where necessary.
304 fn coerce_borrowed_pointer(
308 r_b: ty::Region<'tcx>,
309 mutbl_b: hir::Mutability,
310 ) -> CoerceResult<'tcx> {
311 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
313 // If we have a parameter of type `&M T_a` and the value
314 // provided is `expr`, we will be adding an implicit borrow,
315 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
316 // to type check, we will construct the type that `&M*expr` would
319 let (r_a, mt_a) = match *a.kind() {
320 ty::Ref(r_a, ty, mutbl) => {
321 let mt_a = ty::TypeAndMut { ty, mutbl };
322 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
325 _ => return self.unify_and(a, b, identity),
328 let span = self.cause.span;
330 let mut first_error = None;
331 let mut r_borrow_var = None;
332 let mut autoderef = self.autoderef(span, a);
333 let mut found = None;
335 for (referent_ty, autoderefs) in autoderef.by_ref() {
337 // Don't let this pass, otherwise it would cause
338 // &T to autoref to &&T.
342 // At this point, we have deref'd `a` to `referent_ty`. So
343 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
344 // In the autoderef loop for `&'a mut Vec<T>`, we would get
347 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
348 // - `Vec<T>` -- 1 deref
349 // - `[T]` -- 2 deref
351 // At each point after the first callback, we want to
352 // check to see whether this would match out target type
353 // (`&'b mut [T]`) if we autoref'd it. We can't just
354 // compare the referent types, though, because we still
355 // have to consider the mutability. E.g., in the case
356 // we've been considering, we have an `&mut` reference, so
357 // the `T` in `[T]` needs to be unified with equality.
359 // Therefore, we construct reference types reflecting what
360 // the types will be after we do the final auto-ref and
361 // compare those. Note that this means we use the target
362 // mutability [1], since it may be that we are coercing
363 // from `&mut T` to `&U`.
365 // One fine point concerns the region that we use. We
366 // choose the region such that the region of the final
367 // type that results from `unify` will be the region we
368 // want for the autoref:
370 // - if in sub mode, that means we want to use `'b` (the
371 // region from the target reference) for both
372 // pointers [2]. This is because sub mode (somewhat
373 // arbitrarily) returns the subtype region. In the case
374 // where we are coercing to a target type, we know we
375 // want to use that target type region (`'b`) because --
376 // for the program to type-check -- it must be the
377 // smaller of the two.
378 // - One fine point. It may be surprising that we can
379 // use `'b` without relating `'a` and `'b`. The reason
380 // that this is ok is that what we produce is
381 // effectively a `&'b *x` expression (if you could
382 // annotate the region of a borrow), and regionck has
383 // code that adds edges from the region of a borrow
384 // (`'b`, here) into the regions in the borrowed
385 // expression (`*x`, here). (Search for "link".)
386 // - if in lub mode, things can get fairly complicated. The
387 // easiest thing is just to make a fresh
388 // region variable [4], which effectively means we defer
389 // the decision to region inference (and regionck, which will add
390 // some more edges to this variable). However, this can wind up
391 // creating a crippling number of variables in some cases --
392 // e.g., #32278 -- so we optimize one particular case [3].
393 // Let me try to explain with some examples:
394 // - The "running example" above represents the simple case,
395 // where we have one `&` reference at the outer level and
396 // ownership all the rest of the way down. In this case,
397 // we want `LUB('a, 'b)` as the resulting region.
398 // - However, if there are nested borrows, that region is
399 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
400 // `&'b T`. In this case, `'a` is actually irrelevant.
401 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
402 // we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
403 // (The errors actually show up in borrowck, typically, because
404 // this extra edge causes the region `'a` to be inferred to something
405 // too big, which then results in borrowck errors.)
406 // - We could track the innermost shared reference, but there is already
407 // code in regionck that has the job of creating links between
408 // the region of a borrow and the regions in the thing being
409 // borrowed (here, `'a` and `'x`), and it knows how to handle
410 // all the various cases. So instead we just make a region variable
411 // and let regionck figure it out.
412 let r = if !self.use_lub {
414 } else if autoderefs == 1 {
417 if r_borrow_var.is_none() {
418 // create var lazily, at most once
419 let coercion = Coercion(span);
420 let r = self.next_region_var(coercion);
421 r_borrow_var = Some(r); // [4] above
423 r_borrow_var.unwrap()
425 let derefd_ty_a = self.tcx.mk_ref(
429 mutbl: mutbl_b, // [1] above
432 match self.unify(derefd_ty_a, b) {
438 if first_error.is_none() {
439 first_error = Some(err);
445 // Extract type or return an error. We return the first error
446 // we got, which should be from relating the "base" type
447 // (e.g., in example above, the failure from relating `Vec<T>`
448 // to the target type), since that should be the least
450 let Some(InferOk { value: ty, mut obligations }) = found else {
451 let err = first_error.expect("coerce_borrowed_pointer had no error");
452 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
456 if ty == a && mt_a.mutbl == hir::Mutability::Not && autoderef.step_count() == 1 {
457 // As a special case, if we would produce `&'a *x`, that's
458 // a total no-op. We end up with the type `&'a T` just as
459 // we started with. In that case, just skip it
460 // altogether. This is just an optimization.
462 // Note that for `&mut`, we DO want to reborrow --
463 // otherwise, this would be a move, which might be an
464 // error. For example `foo(self.x)` where `self` and
465 // `self.x` both have `&mut `type would be a move of
466 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
467 // which is a borrow.
468 assert_eq!(mutbl_b, hir::Mutability::Not); // can only coerce &T -> &U
469 return success(vec![], ty, obligations);
472 let InferOk { value: mut adjustments, obligations: o } =
473 self.adjust_steps_as_infer_ok(&autoderef);
474 obligations.extend(o);
475 obligations.extend(autoderef.into_obligations());
477 // Now apply the autoref. We have to extract the region out of
478 // the final ref type we got.
479 let ty::Ref(r_borrow, _, _) = ty.kind() else {
480 span_bug!(span, "expected a ref type, got {:?}", ty);
482 let mutbl = match mutbl_b {
483 hir::Mutability::Not => AutoBorrowMutability::Not,
484 hir::Mutability::Mut => {
485 AutoBorrowMutability::Mut { allow_two_phase_borrow: self.allow_two_phase }
488 adjustments.push(Adjustment {
489 kind: Adjust::Borrow(AutoBorrow::Ref(*r_borrow, mutbl)),
493 debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
495 success(adjustments, ty, obligations)
498 // &[T; n] or &mut [T; n] -> &[T]
499 // or &mut [T; n] -> &mut [T]
500 // or &Concrete -> &Trait, etc.
501 #[instrument(skip(self), level = "debug")]
502 fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
503 source = self.shallow_resolve(source);
504 target = self.shallow_resolve(target);
505 debug!(?source, ?target);
507 // These 'if' statements require some explanation.
508 // The `CoerceUnsized` trait is special - it is only
509 // possible to write `impl CoerceUnsized<B> for A` where
510 // A and B have 'matching' fields. This rules out the following
511 // two types of blanket impls:
513 // `impl<T> CoerceUnsized<T> for SomeType`
514 // `impl<T> CoerceUnsized<SomeType> for T`
516 // Both of these trigger a special `CoerceUnsized`-related error (E0376)
518 // We can take advantage of this fact to avoid performing unnecessary work.
519 // If either `source` or `target` is a type variable, then any applicable impl
520 // would need to be generic over the self-type (`impl<T> CoerceUnsized<SomeType> for T`)
521 // or generic over the `CoerceUnsized` type parameter (`impl<T> CoerceUnsized<T> for
524 // However, these are exactly the kinds of impls which are forbidden by
525 // the compiler! Therefore, we can be sure that coercion will always fail
526 // when either the source or target type is a type variable. This allows us
527 // to skip performing any trait selection, and immediately bail out.
528 if source.is_ty_var() {
529 debug!("coerce_unsized: source is a TyVar, bailing out");
530 return Err(TypeError::Mismatch);
532 if target.is_ty_var() {
533 debug!("coerce_unsized: target is a TyVar, bailing out");
534 return Err(TypeError::Mismatch);
538 (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
539 let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
540 debug!("missing Unsize or CoerceUnsized traits");
541 return Err(TypeError::Mismatch);
544 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
545 // a DST unless we have to. This currently comes out in the wash since
546 // we can't unify [T] with U. But to properly support DST, we need to allow
547 // that, at which point we will need extra checks on the target here.
549 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
550 let reborrow = match (source.kind(), target.kind()) {
551 (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
552 coerce_mutbls(mutbl_a, mutbl_b)?;
554 let coercion = Coercion(self.cause.span);
555 let r_borrow = self.next_region_var(coercion);
556 let mutbl = match mutbl_b {
557 hir::Mutability::Not => AutoBorrowMutability::Not,
558 hir::Mutability::Mut => AutoBorrowMutability::Mut {
559 // We don't allow two-phase borrows here, at least for initial
560 // implementation. If it happens that this coercion is a function argument,
561 // the reborrow in coerce_borrowed_ptr will pick it up.
562 allow_two_phase_borrow: AllowTwoPhase::No,
566 Adjustment { kind: Adjust::Deref(None), target: ty_a },
568 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
571 .mk_ref(r_borrow, ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a }),
575 (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
576 coerce_mutbls(mt_a, mt_b)?;
579 Adjustment { kind: Adjust::Deref(None), target: ty_a },
581 kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
582 target: self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mt_b, ty: ty_a }),
588 let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
590 // Setup either a subtyping or a LUB relationship between
591 // the `CoerceUnsized` target type and the expected type.
592 // We only have the latter, so we use an inference variable
593 // for the former and let type inference do the rest.
594 let origin = TypeVariableOrigin {
595 kind: TypeVariableOriginKind::MiscVariable,
596 span: self.cause.span,
598 let coerce_target = self.next_ty_var(origin);
599 let mut coercion = self.unify_and(coerce_target, target, |target| {
600 let unsize = Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target };
602 None => vec![unsize],
603 Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
607 let mut selcx = traits::SelectionContext::new(self);
609 // Create an obligation for `Source: CoerceUnsized<Target>`.
610 let cause = ObligationCause::new(
613 ObligationCauseCode::Coercion { source, target },
616 // Use a FIFO queue for this custom fulfillment procedure.
618 // A Vec (or SmallVec) is not a natural choice for a queue. However,
619 // this code path is hot, and this queue usually has a max length of 1
620 // and almost never more than 3. By using a SmallVec we avoid an
621 // allocation, at the (very small) cost of (occasionally) having to
622 // shift subsequent elements down when removing the front element.
623 let mut queue: SmallVec<[_; 4]> = smallvec![traits::predicate_for_trait_def(
630 &[coerce_target.into()]
633 let mut has_unsized_tuple_coercion = false;
634 let mut has_trait_upcasting_coercion = None;
636 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
637 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
638 // inference might unify those two inner type variables later.
639 let traits = [coerce_unsized_did, unsize_did];
640 while !queue.is_empty() {
641 let obligation = queue.remove(0);
642 debug!("coerce_unsized resolve step: {:?}", obligation);
643 let bound_predicate = obligation.predicate.kind();
644 let trait_pred = match bound_predicate.skip_binder() {
645 ty::PredicateKind::Trait(trait_pred) if traits.contains(&trait_pred.def_id()) => {
646 if unsize_did == trait_pred.def_id() {
647 let self_ty = trait_pred.self_ty();
648 let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty();
649 if let (ty::Dynamic(ref data_a, ..), ty::Dynamic(ref data_b, ..)) =
650 (self_ty.kind(), unsize_ty.kind())
651 && data_a.principal_def_id() != data_b.principal_def_id()
653 debug!("coerce_unsized: found trait upcasting coercion");
654 has_trait_upcasting_coercion = Some((self_ty, unsize_ty));
656 if let ty::Tuple(..) = unsize_ty.kind() {
657 debug!("coerce_unsized: found unsized tuple coercion");
658 has_unsized_tuple_coercion = true;
661 bound_predicate.rebind(trait_pred)
664 coercion.obligations.push(obligation);
668 match selcx.select(&obligation.with(trait_pred)) {
669 // Uncertain or unimplemented.
671 if trait_pred.def_id() == unsize_did {
672 let trait_pred = self.resolve_vars_if_possible(trait_pred);
673 let self_ty = trait_pred.skip_binder().self_ty();
674 let unsize_ty = trait_pred.skip_binder().trait_ref.substs[1].expect_ty();
675 debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
676 match (&self_ty.kind(), &unsize_ty.kind()) {
677 (ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
678 if self.type_var_is_sized(*v) =>
680 debug!("coerce_unsized: have sized infer {:?}", v);
681 coercion.obligations.push(obligation);
682 // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
686 // Some other case for `$0: Unsize<Something>`. Note that we
687 // hit this case even if `Something` is a sized type, so just
688 // don't do the coercion.
689 debug!("coerce_unsized: ambiguous unsize");
690 return Err(TypeError::Mismatch);
694 debug!("coerce_unsized: early return - ambiguous");
695 return Err(TypeError::Mismatch);
698 Err(traits::Unimplemented) => {
699 debug!("coerce_unsized: early return - can't prove obligation");
700 return Err(TypeError::Mismatch);
703 // Object safety violations or miscellaneous.
705 self.err_ctxt().report_selection_error(
711 // Treat this like an obligation and follow through
712 // with the unsizing - the lack of a coercion should
713 // be silent, as it causes a type mismatch later.
716 Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
720 if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
722 &self.tcx.sess.parse_sess,
723 sym::unsized_tuple_coercion,
725 "unsized tuple coercion is not stable enough for use and is subject to change",
730 if let Some((sub, sup)) = has_trait_upcasting_coercion
731 && !self.tcx().features().trait_upcasting
733 // Renders better when we erase regions, since they're not really the point here.
734 let (sub, sup) = self.tcx.erase_regions((sub, sup));
735 let mut err = feature_err(
736 &self.tcx.sess.parse_sess,
737 sym::trait_upcasting,
739 &format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
741 err.note(&format!("required when coercing `{source}` into `{target}`"));
748 fn coerce_from_safe_fn<F, G>(
751 fn_ty_a: ty::PolyFnSig<'tcx>,
755 ) -> CoerceResult<'tcx>
757 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
758 G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
760 self.commit_if_ok(|snapshot| {
761 let result = if let ty::FnPtr(fn_ty_b) = b.kind()
762 && let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) =
763 (fn_ty_a.unsafety(), fn_ty_b.unsafety())
765 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
766 self.unify_and(unsafe_a, b, to_unsafe)
768 self.unify_and(a, b, normal)
771 // FIXME(#73154): This is a hack. Currently LUB can generate
772 // unsolvable constraints. Additionally, it returns `a`
773 // unconditionally, even when the "LUB" is `b`. In the future, we
774 // want the coerced type to be the actual supertype of these two,
775 // but for now, we want to just error to ensure we don't lock
776 // ourselves into a specific behavior with NLL.
777 self.leak_check(false, snapshot)?;
783 fn coerce_from_fn_pointer(
786 fn_ty_a: ty::PolyFnSig<'tcx>,
788 ) -> CoerceResult<'tcx> {
789 //! Attempts to coerce from the type of a Rust function item
790 //! into a closure or a `proc`.
793 let b = self.shallow_resolve(b);
794 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
796 self.coerce_from_safe_fn(
800 simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)),
805 fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
806 //! Attempts to coerce from the type of a Rust function item
807 //! into a closure or a `proc`.
809 let b = self.shallow_resolve(b);
810 let InferOk { value: b, mut obligations } =
811 self.normalize_associated_types_in_as_infer_ok(self.cause.span, b);
812 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
815 ty::FnPtr(b_sig) => {
816 let a_sig = a.fn_sig(self.tcx);
817 if let ty::FnDef(def_id, _) = *a.kind() {
818 // Intrinsics are not coercible to function pointers
819 if self.tcx.is_intrinsic(def_id) {
820 return Err(TypeError::IntrinsicCast);
823 // Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).
825 if b_sig.unsafety() == hir::Unsafety::Normal
826 && !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
828 return Err(TypeError::TargetFeatureCast(def_id));
832 let InferOk { value: a_sig, obligations: o1 } =
833 self.normalize_associated_types_in_as_infer_ok(self.cause.span, a_sig);
834 obligations.extend(o1);
836 let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
837 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
844 kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
845 target: a_fn_pointer,
848 kind: Adjust::Pointer(PointerCast::UnsafeFnPointer),
853 simple(Adjust::Pointer(PointerCast::ReifyFnPointer)),
856 obligations.extend(o2);
857 Ok(InferOk { value, obligations })
859 _ => self.unify_and(a, b, identity),
863 fn coerce_closure_to_fn(
866 closure_def_id_a: DefId,
867 substs_a: SubstsRef<'tcx>,
869 ) -> CoerceResult<'tcx> {
870 //! Attempts to coerce from the type of a non-capturing closure
871 //! into a function pointer.
874 let b = self.shallow_resolve(b);
877 // At this point we haven't done capture analysis, which means
878 // that the ClosureSubsts just contains an inference variable instead
879 // of tuple of captured types.
881 // All we care here is if any variable is being captured and not the exact paths,
882 // so we check `upvars_mentioned` for root variables being captured.
886 .upvars_mentioned(closure_def_id_a.expect_local())
887 .map_or(true, |u| u.is_empty()) =>
889 // We coerce the closure, which has fn type
890 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
892 // `fn(arg0,arg1,...) -> _`
894 // `unsafe fn(arg0,arg1,...) -> _`
895 let closure_sig = substs_a.as_closure().sig();
896 let unsafety = fn_ty.unsafety();
898 self.tcx.mk_fn_ptr(self.tcx.signature_unclosure(closure_sig, unsafety));
899 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
903 simple(Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))),
906 _ => self.unify_and(a, b, identity),
910 fn coerce_unsafe_ptr(
914 mutbl_b: hir::Mutability,
915 ) -> CoerceResult<'tcx> {
916 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
918 let (is_ref, mt_a) = match *a.kind() {
919 ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
920 ty::RawPtr(mt) => (false, mt),
921 _ => return self.unify_and(a, b, identity),
923 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
925 // Check that the types which they point at are compatible.
926 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty });
927 // Although references and unsafe ptrs have the same
928 // representation, we still register an Adjust::DerefRef so that
929 // regionck knows that the region for `a` must be valid here.
931 self.unify_and(a_unsafe, b, |target| {
933 Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
934 Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
937 } else if mt_a.mutbl != mutbl_b {
938 self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer)))
940 self.unify_and(a_unsafe, b, identity)
945 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
946 /// Attempt to coerce an expression to a type, and return the
947 /// adjusted type of the expression, if successful.
948 /// Adjustments are only recorded if the coercion succeeded.
949 /// The expressions *must not* have any pre-existing adjustments.
952 expr: &hir::Expr<'_>,
955 allow_two_phase: AllowTwoPhase,
956 cause: Option<ObligationCause<'tcx>>,
957 ) -> RelateResult<'tcx, Ty<'tcx>> {
958 let source = self.resolve_vars_with_obligations(expr_ty);
959 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
962 cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
963 let coerce = Coerce::new(self, cause, allow_two_phase);
964 let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
966 let (adjustments, _) = self.register_infer_ok_obligations(ok);
967 self.apply_adjustments(expr, adjustments);
968 Ok(if expr_ty.references_error() { self.tcx.ty_error() } else { target })
971 /// Same as `try_coerce()`, but without side-effects.
973 /// Returns false if the coercion creates any obligations that result in
975 pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
976 let source = self.resolve_vars_with_obligations(expr_ty);
977 debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);
979 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
980 // We don't ever need two-phase here since we throw out the result of the coercion
981 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
983 let Ok(ok) = coerce.coerce(source, target) else {
986 let mut fcx = traits::FulfillmentContext::new_in_snapshot();
987 fcx.register_predicate_obligations(self, ok.obligations);
988 fcx.select_where_possible(&self).is_empty()
992 /// Given a type and a target type, this function will calculate and return
993 /// how many dereference steps needed to achieve `expr_ty <: target`. If
994 /// it's not possible, return `None`.
995 pub fn deref_steps(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> Option<usize> {
996 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
997 // We don't ever need two-phase here since we throw out the result of the coercion
998 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
1000 .autoderef(rustc_span::DUMMY_SP, expr_ty)
1001 .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
1004 /// Given a type, this function will calculate and return the type given
1005 /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
1007 /// This function is for diagnostics only, since it does not register
1008 /// trait or region sub-obligations. (presumably we could, but it's not
1009 /// particularly important for diagnostics...)
1010 pub fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
1011 self.autoderef(rustc_span::DUMMY_SP, expr_ty).nth(1).and_then(|(deref_ty, _)| {
1013 .type_implements_trait(
1014 self.tcx.lang_items().deref_mut_trait()?,
1024 /// Given some expressions, their known unified type and another expression,
1025 /// tries to unify the types, potentially inserting coercions on any of the
1026 /// provided expressions and returns their LUB (aka "common supertype").
1028 /// This is really an internal helper. From outside the coercion
1029 /// module, you should instantiate a `CoerceMany` instance.
1030 fn try_find_coercion_lub<E>(
1032 cause: &ObligationCause<'tcx>,
1035 new: &hir::Expr<'_>,
1037 ) -> RelateResult<'tcx, Ty<'tcx>>
1041 let prev_ty = self.resolve_vars_with_obligations(prev_ty);
1042 let new_ty = self.resolve_vars_with_obligations(new_ty);
1044 "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
1050 // The following check fixes #88097, where the compiler erroneously
1051 // attempted to coerce a closure type to itself via a function pointer.
1052 if prev_ty == new_ty {
1056 // Special-case that coercion alone cannot handle:
1057 // Function items or non-capturing closures of differing IDs or InternalSubsts.
1058 let (a_sig, b_sig) = {
1059 #[allow(rustc::usage_of_ty_tykind)]
1060 let is_capturing_closure = |ty: &ty::TyKind<'tcx>| {
1061 if let &ty::Closure(closure_def_id, _substs) = ty {
1062 self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some()
1067 if is_capturing_closure(prev_ty.kind()) || is_capturing_closure(new_ty.kind()) {
1070 match (prev_ty.kind(), new_ty.kind()) {
1071 (ty::FnDef(..), ty::FnDef(..)) => {
1072 // Don't reify if the function types have a LUB, i.e., they
1073 // are the same function and their parameters have a LUB.
1075 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1077 // We have a LUB of prev_ty and new_ty, just return it.
1078 Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
1080 (Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx)))
1084 (ty::Closure(_, substs), ty::FnDef(..)) => {
1085 let b_sig = new_ty.fn_sig(self.tcx);
1088 .signature_unclosure(substs.as_closure().sig(), b_sig.unsafety());
1089 (Some(a_sig), Some(b_sig))
1091 (ty::FnDef(..), ty::Closure(_, substs)) => {
1092 let a_sig = prev_ty.fn_sig(self.tcx);
1095 .signature_unclosure(substs.as_closure().sig(), a_sig.unsafety());
1096 (Some(a_sig), Some(b_sig))
1098 (ty::Closure(_, substs_a), ty::Closure(_, substs_b)) => (
1099 Some(self.tcx.signature_unclosure(
1100 substs_a.as_closure().sig(),
1101 hir::Unsafety::Normal,
1103 Some(self.tcx.signature_unclosure(
1104 substs_b.as_closure().sig(),
1105 hir::Unsafety::Normal,
1112 if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) {
1113 // Intrinsics are not coercible to function pointers.
1114 if a_sig.abi() == Abi::RustIntrinsic
1115 || a_sig.abi() == Abi::PlatformIntrinsic
1116 || b_sig.abi() == Abi::RustIntrinsic
1117 || b_sig.abi() == Abi::PlatformIntrinsic
1119 return Err(TypeError::IntrinsicCast);
1121 // The signature must match.
1122 let a_sig = self.normalize_associated_types_in(new.span, a_sig);
1123 let b_sig = self.normalize_associated_types_in(new.span, b_sig);
1125 .at(cause, self.param_env)
1126 .trace(prev_ty, new_ty)
1128 .map(|ok| self.register_infer_ok_obligations(ok))?;
1130 // Reify both sides and return the reified fn pointer type.
1131 let fn_ptr = self.tcx.mk_fn_ptr(sig);
1132 let prev_adjustment = match prev_ty.kind() {
1133 ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(a_sig.unsafety())),
1134 ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
1135 _ => unreachable!(),
1137 let next_adjustment = match new_ty.kind() {
1138 ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(b_sig.unsafety())),
1139 ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
1140 _ => unreachable!(),
1142 for expr in exprs.iter().map(|e| e.as_coercion_site()) {
1143 self.apply_adjustments(
1145 vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
1148 self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
1152 // Configure a Coerce instance to compute the LUB.
1153 // We don't allow two-phase borrows on any autorefs this creates since we
1154 // probably aren't processing function arguments here and even if we were,
1155 // they're going to get autorefed again anyway and we can apply 2-phase borrows
1157 let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
1158 coerce.use_lub = true;
1160 // First try to coerce the new expression to the type of the previous ones,
1161 // but only if the new expression has no coercion already applied to it.
1162 let mut first_error = None;
1163 if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
1164 let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
1167 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1168 self.apply_adjustments(new, adjustments);
1170 "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
1171 new_ty, prev_ty, target
1175 Err(e) => first_error = Some(e),
1179 // Then try to coerce the previous expressions to the type of the new one.
1180 // This requires ensuring there are no coercions applied to *any* of the
1181 // previous expressions, other than noop reborrows (ignoring lifetimes).
1183 let expr = expr.as_coercion_site();
1184 let noop = match self.typeck_results.borrow().expr_adjustments(expr) {
1186 Adjustment { kind: Adjust::Deref(_), .. },
1187 Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. },
1189 match *self.node_ty(expr.hir_id).kind() {
1190 ty::Ref(_, _, mt_orig) => {
1191 let mutbl_adj: hir::Mutability = mutbl_adj.into();
1192 // Reborrow that we can safely ignore, because
1193 // the next adjustment can only be a Deref
1194 // which will be merged into it.
1195 mutbl_adj == mt_orig
1200 &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
1206 "coercion::try_find_coercion_lub: older expression {:?} had adjustments, requiring LUB",
1211 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1212 .map(|ok| self.register_infer_ok_obligations(ok));
1216 match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
1218 // Avoid giving strange errors on failed attempts.
1219 if let Some(e) = first_error {
1222 self.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1223 .map(|ok| self.register_infer_ok_obligations(ok))
1227 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1229 let expr = expr.as_coercion_site();
1230 self.apply_adjustments(expr, adjustments.clone());
1233 "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
1234 prev_ty, new_ty, target
1242 /// CoerceMany encapsulates the pattern you should use when you have
1243 /// many expressions that are all getting coerced to a common
1244 /// type. This arises, for example, when you have a match (the result
1245 /// of each arm is coerced to a common type). It also arises in less
1246 /// obvious places, such as when you have many `break foo` expressions
1247 /// that target the same loop, or the various `return` expressions in
1250 /// The basic protocol is as follows:
1252 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
1253 /// This will also serve as the "starting LUB". The expectation is
1254 /// that this type is something which all of the expressions *must*
1255 /// be coercible to. Use a fresh type variable if needed.
1256 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
1257 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
1258 /// unit. This happens for example if you have a `break` with no expression,
1259 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
1260 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
1261 /// from you so that you don't have to worry your pretty head about it.
1262 /// But if an error is reported, the final type will be `err`.
1263 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
1264 /// previously coerced expressions.
1265 /// - When all done, invoke `complete()`. This will return the LUB of
1266 /// all your expressions.
1267 /// - WARNING: I don't believe this final type is guaranteed to be
1268 /// related to your initial `expected_ty` in any particular way,
1269 /// although it will typically be a subtype, so you should check it.
1270 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
1271 /// previously coerced expressions.
1275 /// ```ignore (illustrative)
1276 /// let mut coerce = CoerceMany::new(expected_ty);
1277 /// for expr in exprs {
1278 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
1279 /// coerce.coerce(fcx, &cause, expr, expr_ty);
1281 /// let final_ty = coerce.complete(fcx);
1283 pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
1284 expected_ty: Ty<'tcx>,
1285 final_ty: Option<Ty<'tcx>>,
1286 expressions: Expressions<'tcx, 'exprs, E>,
1290 /// The type of a `CoerceMany` that is storing up the expressions into
1291 /// a buffer. We use this in `check/mod.rs` for things like `break`.
1292 pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;
1294 enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
1295 Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
1296 UpFront(&'exprs [E]),
1299 impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
1300 /// The usual case; collect the set of expressions dynamically.
1301 /// If the full set of coercion sites is known before hand,
1302 /// consider `with_coercion_sites()` instead to avoid allocation.
1303 pub fn new(expected_ty: Ty<'tcx>) -> Self {
1304 Self::make(expected_ty, Expressions::Dynamic(vec![]))
1307 /// As an optimization, you can create a `CoerceMany` with a
1308 /// pre-existing slice of expressions. In this case, you are
1309 /// expected to pass each element in the slice to `coerce(...)` in
1310 /// order. This is used with arrays in particular to avoid
1311 /// needlessly cloning the slice.
1312 pub fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
1313 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
1316 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
1317 CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
1320 /// Returns the "expected type" with which this coercion was
1321 /// constructed. This represents the "downward propagated" type
1322 /// that was given to us at the start of typing whatever construct
1323 /// we are typing (e.g., the match expression).
1325 /// Typically, this is used as the expected type when
1326 /// type-checking each of the alternative expressions whose types
1327 /// we are trying to merge.
1328 pub fn expected_ty(&self) -> Ty<'tcx> {
1332 /// Returns the current "merged type", representing our best-guess
1333 /// at the LUB of the expressions we've seen so far (if any). This
1334 /// isn't *final* until you call `self.complete()`, which will return
1335 /// the merged type.
1336 pub fn merged_ty(&self) -> Ty<'tcx> {
1337 self.final_ty.unwrap_or(self.expected_ty)
1340 /// Indicates that the value generated by `expression`, which is
1341 /// of type `expression_ty`, is one of the possibilities that we
1342 /// could coerce from. This will record `expression`, and later
1343 /// calls to `coerce` may come back and add adjustments and things
1347 fcx: &FnCtxt<'a, 'tcx>,
1348 cause: &ObligationCause<'tcx>,
1349 expression: &'tcx hir::Expr<'tcx>,
1350 expression_ty: Ty<'tcx>,
1352 self.coerce_inner(fcx, cause, Some(expression), expression_ty, None, false)
1355 /// Indicates that one of the inputs is a "forced unit". This
1356 /// occurs in a case like `if foo { ... };`, where the missing else
1357 /// generates a "forced unit". Another example is a `loop { break;
1358 /// }`, where the `break` has no argument expression. We treat
1359 /// these cases slightly differently for error-reporting
1360 /// purposes. Note that these tend to correspond to cases where
1361 /// the `()` expression is implicit in the source, and hence we do
1362 /// not take an expression argument.
1364 /// The `augment_error` gives you a chance to extend the error
1365 /// message, in case any results (e.g., we use this to suggest
1366 /// removing a `;`).
1367 pub fn coerce_forced_unit<'a>(
1369 fcx: &FnCtxt<'a, 'tcx>,
1370 cause: &ObligationCause<'tcx>,
1371 augment_error: &mut dyn FnMut(&mut Diagnostic),
1372 label_unit_as_expected: bool,
1379 Some(augment_error),
1380 label_unit_as_expected,
1384 /// The inner coercion "engine". If `expression` is `None`, this
1385 /// is a forced-unit case, and hence `expression_ty` must be
1387 #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
1388 pub(crate) fn coerce_inner<'a>(
1390 fcx: &FnCtxt<'a, 'tcx>,
1391 cause: &ObligationCause<'tcx>,
1392 expression: Option<&'tcx hir::Expr<'tcx>>,
1393 mut expression_ty: Ty<'tcx>,
1394 augment_error: Option<&mut dyn FnMut(&mut Diagnostic)>,
1395 label_expression_as_expected: bool,
1397 // Incorporate whatever type inference information we have
1398 // until now; in principle we might also want to process
1399 // pending obligations, but doing so should only improve
1400 // compatibility (hopefully that is true) by helping us
1401 // uncover never types better.
1402 if expression_ty.is_ty_var() {
1403 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1406 // If we see any error types, just propagate that error
1408 if expression_ty.references_error() || self.merged_ty().references_error() {
1409 self.final_ty = Some(fcx.tcx.ty_error());
1413 // Handle the actual type unification etc.
1414 let result = if let Some(expression) = expression {
1415 if self.pushed == 0 {
1416 // Special-case the first expression we are coercing.
1417 // To be honest, I'm not entirely sure why we do this.
1418 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1424 Some(cause.clone()),
1427 match self.expressions {
1428 Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
1435 Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub(
1437 &coercion_sites[0..self.pushed],
1445 // this is a hack for cases where we default to `()` because
1446 // the expression etc has been omitted from the source. An
1447 // example is an `if let` without an else:
1449 // if let Some(x) = ... { }
1451 // we wind up with a second match arm that is like `_ =>
1452 // ()`. That is the case we are considering here. We take
1453 // a different path to get the right "expected, found"
1454 // message and so forth (and because we know that
1455 // `expression_ty` will be unit).
1457 // Another example is `break` with no argument expression.
1458 assert!(expression_ty.is_unit(), "if let hack without unit type");
1459 fcx.at(cause, fcx.param_env)
1460 .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
1462 fcx.register_infer_ok_obligations(infer_ok);
1470 self.final_ty = Some(v);
1471 if let Some(e) = expression {
1472 match self.expressions {
1473 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1474 Expressions::UpFront(coercion_sites) => {
1475 // if the user gave us an array to validate, check that we got
1476 // the next expression in the list, as expected
1478 coercion_sites[self.pushed].as_coercion_site().hir_id,
1486 Err(coercion_error) => {
1487 // Mark that we've failed to coerce the types here to suppress
1488 // any superfluous errors we might encounter while trying to
1489 // emit or provide suggestions on how to fix the initial error.
1490 fcx.set_tainted_by_errors();
1491 let (expected, found) = if label_expression_as_expected {
1492 // In the case where this is a "forced unit", like
1493 // `break`, we want to call the `()` "expected"
1494 // since it is implied by the syntax.
1495 // (Note: not all force-units work this way.)"
1496 (expression_ty, self.merged_ty())
1498 // Otherwise, the "expected" type for error
1499 // reporting is the current unification type,
1500 // which is basically the LUB of the expressions
1501 // we've seen so far (combined with the expected
1503 (self.merged_ty(), expression_ty)
1505 let (expected, found) = fcx.resolve_vars_if_possible((expected, found));
1508 let mut unsized_return = false;
1509 let mut visitor = CollectRetsVisitor { ret_exprs: vec![] };
1510 match *cause.code() {
1511 ObligationCauseCode::ReturnNoExpression => {
1512 err = struct_span_err!(
1516 "`return;` in a function whose return type is not `()`"
1518 err.span_label(cause.span, "return type is not `()`");
1520 ObligationCauseCode::BlockTailExpression(blk_id) => {
1521 let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
1522 err = self.report_return_mismatched_types(
1526 coercion_error.clone(),
1532 if !fcx.tcx.features().unsized_locals {
1533 unsized_return = self.is_return_ty_unsized(fcx, blk_id);
1535 if let Some(expression) = expression
1536 && let hir::ExprKind::Loop(loop_blk, ..) = expression.kind {
1537 intravisit::walk_block(& mut visitor, loop_blk);
1540 ObligationCauseCode::ReturnValue(id) => {
1541 err = self.report_return_mismatched_types(
1545 coercion_error.clone(),
1551 if !fcx.tcx.features().unsized_locals {
1552 let id = fcx.tcx.hir().get_parent_node(id);
1553 unsized_return = self.is_return_ty_unsized(fcx, id);
1557 err = fcx.err_ctxt().report_mismatched_types(
1561 coercion_error.clone(),
1566 if let Some(augment_error) = augment_error {
1567 augment_error(&mut err);
1570 let is_insufficiently_polymorphic =
1571 matches!(coercion_error, TypeError::RegionsInsufficientlyPolymorphic(..));
1573 if !is_insufficiently_polymorphic && let Some(expr) = expression {
1574 fcx.emit_coerce_suggestions(
1580 Some(coercion_error),
1584 if visitor.ret_exprs.len() > 0 && let Some(expr) = expression {
1585 self.note_unreachable_loop_return(&mut err, &expr, &visitor.ret_exprs);
1587 err.emit_unless(unsized_return);
1589 self.final_ty = Some(fcx.tcx.ty_error());
1593 fn note_unreachable_loop_return(
1595 err: &mut Diagnostic,
1596 expr: &hir::Expr<'tcx>,
1597 ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>,
1599 let hir::ExprKind::Loop(_, _, _, loop_span) = expr.kind else { return;};
1600 let mut span: MultiSpan = vec![loop_span].into();
1601 span.push_span_label(loop_span, "this might have zero elements to iterate on");
1602 const MAXITER: usize = 3;
1603 let iter = ret_exprs.iter().take(MAXITER);
1604 for ret_expr in iter {
1605 span.push_span_label(
1607 "if the loop doesn't execute, this value would never get returned",
1612 "the function expects a value to always be returned, but loops might run zero times",
1614 if MAXITER < ret_exprs.len() {
1616 "if the loop doesn't execute, {} other values would never get returned",
1617 ret_exprs.len() - MAXITER
1621 "return a value for the case when the loop has zero elements to iterate on, or \
1622 consider changing the return type to account for that possibility",
1626 fn report_return_mismatched_types<'a>(
1628 cause: &ObligationCause<'tcx>,
1631 ty_err: TypeError<'tcx>,
1632 fcx: &FnCtxt<'a, 'tcx>,
1634 expression: Option<&'tcx hir::Expr<'tcx>>,
1635 blk_id: Option<hir::HirId>,
1636 ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
1637 let mut err = fcx.err_ctxt().report_mismatched_types(cause, expected, found, ty_err);
1639 let mut pointing_at_return_type = false;
1640 let mut fn_output = None;
1642 let parent_id = fcx.tcx.hir().get_parent_node(id);
1643 let parent = fcx.tcx.hir().get(parent_id);
1644 if let Some(expr) = expression
1645 && let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(&hir::Closure { body, .. }), .. }) = parent
1646 && !matches!(fcx.tcx.hir().body(body).value.kind, hir::ExprKind::Block(..))
1648 fcx.suggest_missing_semicolon(&mut err, expr, expected, true);
1650 // Verify that this is a tail expression of a function, otherwise the
1651 // label pointing out the cause for the type coercion will be wrong
1652 // as prior return coercions would not be relevant (#57664).
1653 let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) {
1654 pointing_at_return_type =
1655 fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id);
1656 if let (Some(cond_expr), true, false) = (
1657 fcx.tcx.hir().get_if_cause(expr.hir_id),
1659 pointing_at_return_type,
1661 // If the block is from an external macro or try (`?`) desugaring, then
1662 // do not suggest adding a semicolon, because there's nowhere to put it.
1663 // See issues #81943 and #87051.
1665 cond_expr.span.desugaring_kind(),
1666 None | Some(DesugaringKind::WhileLoop)
1667 ) && !in_external_macro(fcx.tcx.sess, cond_expr.span)
1670 hir::ExprKind::Match(.., hir::MatchSource::TryDesugar)
1673 err.span_label(cond_expr.span, "expected this to be `()`");
1674 if expr.can_have_side_effects() {
1675 fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
1678 fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
1680 fcx.get_fn_decl(parent_id)
1683 if let Some((fn_decl, can_suggest)) = fn_decl {
1684 if blk_id.is_none() {
1685 pointing_at_return_type |= fcx.suggest_missing_return_type(
1691 fcx.tcx.hir().get_parent_item(id).into(),
1694 if !pointing_at_return_type {
1695 fn_output = Some(&fn_decl.output); // `impl Trait` return type
1699 let parent_id = fcx.tcx.hir().get_parent_item(id);
1700 let parent_item = fcx.tcx.hir().get_by_def_id(parent_id.def_id);
1702 if let (Some(expr), Some(_), Some((fn_decl, _, _))) =
1703 (expression, blk_id, fcx.get_node_fn_decl(parent_item))
1705 fcx.suggest_missing_break_or_return_expr(
1716 let ret_coercion_span = fcx.ret_coercion_span.get();
1718 if let Some(sp) = ret_coercion_span
1719 // If the closure has an explicit return type annotation, or if
1720 // the closure's return type has been inferred from outside
1721 // requirements (such as an Fn* trait bound), then a type error
1722 // may occur at the first return expression we see in the closure
1723 // (if it conflicts with the declared return type). Skip adding a
1724 // note in this case, since it would be incorrect.
1725 && !fcx.return_type_pre_known
1730 "return type inferred to be `{}` here",
1736 if let (Some(sp), Some(fn_output)) = (ret_coercion_span, fn_output) {
1737 self.add_impl_trait_explanation(&mut err, cause, fcx, expected, sp, fn_output);
1743 fn add_impl_trait_explanation<'a>(
1745 err: &mut Diagnostic,
1746 cause: &ObligationCause<'tcx>,
1747 fcx: &FnCtxt<'a, 'tcx>,
1750 fn_output: &hir::FnRetTy<'_>,
1752 let return_sp = fn_output.span();
1753 err.span_label(return_sp, "expected because this return type...");
1756 format!("...is found to be `{}` here", fcx.resolve_vars_with_obligations(expected)),
1758 let impl_trait_msg = "for information on `impl Trait`, see \
1759 <https://doc.rust-lang.org/book/ch10-02-traits.html\
1760 #returning-types-that-implement-traits>";
1761 let trait_obj_msg = "for information on trait objects, see \
1762 <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
1763 #using-trait-objects-that-allow-for-values-of-different-types>";
1764 err.note("to return `impl Trait`, all returned values must be of the same type");
1765 err.note(impl_trait_msg);
1770 .span_to_snippet(return_sp)
1771 .unwrap_or_else(|_| "dyn Trait".to_string());
1772 let mut snippet_iter = snippet.split_whitespace();
1773 let has_impl = snippet_iter.next().map_or(false, |s| s == "impl");
1774 // Only suggest `Box<dyn Trait>` if `Trait` in `impl Trait` is object safe.
1775 let mut is_object_safe = false;
1776 if let hir::FnRetTy::Return(ty) = fn_output
1777 // Get the return type.
1778 && let hir::TyKind::OpaqueDef(..) = ty.kind
1780 let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty);
1781 // Get the `impl Trait`'s `DefId`.
1782 if let ty::Opaque(def_id, _) = ty.kind()
1783 // Get the `impl Trait`'s `Item` so that we can get its trait bounds and
1784 // get the `Trait`'s `DefId`.
1785 && let hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, .. }) =
1786 fcx.tcx.hir().expect_item(def_id.expect_local()).kind
1788 // Are of this `impl Trait`'s traits object safe?
1789 is_object_safe = bounds.iter().all(|bound| {
1792 .and_then(|t| t.trait_def_id())
1793 .map_or(false, |def_id| {
1794 fcx.tcx.object_safety_violations(def_id).is_empty()
1801 err.multipart_suggestion(
1802 "you could change the return type to be a boxed trait object",
1804 (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()),
1805 (return_sp.shrink_to_hi(), ">".to_string()),
1807 Applicability::MachineApplicable,
1809 let sugg = [sp, cause.span]
1813 (sp.shrink_to_lo(), "Box::new(".to_string()),
1814 (sp.shrink_to_hi(), ")".to_string()),
1818 .collect::<Vec<_>>();
1819 err.multipart_suggestion(
1820 "if you change the return type to expect trait objects, box the returned \
1823 Applicability::MaybeIncorrect,
1827 "if the trait `{}` were object safe, you could return a boxed trait object",
1831 err.note(trait_obj_msg);
1833 err.help("you could instead create a new `enum` with a variant for each returned type");
1836 fn is_return_ty_unsized<'a>(&self, fcx: &FnCtxt<'a, 'tcx>, blk_id: hir::HirId) -> bool {
1837 if let Some((fn_decl, _)) = fcx.get_fn_decl(blk_id)
1838 && let hir::FnRetTy::Return(ty) = fn_decl.output
1839 && let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty)
1840 && let ty::Dynamic(..) = ty.kind()
1847 pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
1848 if let Some(final_ty) = self.final_ty {
1851 // If we only had inputs that were of type `!` (or no
1852 // inputs at all), then the final type is `!`.
1853 assert_eq!(self.pushed, 0);
1859 /// Something that can be converted into an expression to which we can
1860 /// apply a coercion.
1861 pub trait AsCoercionSite {
1862 fn as_coercion_site(&self) -> &hir::Expr<'_>;
1865 impl AsCoercionSite for hir::Expr<'_> {
1866 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1871 impl<'a, T> AsCoercionSite for &'a T
1875 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1876 (**self).as_coercion_site()
1880 impl AsCoercionSite for ! {
1881 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1886 impl AsCoercionSite for hir::Arm<'_> {
1887 fn as_coercion_site(&self) -> &hir::Expr<'_> {