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`.
40 struct_span_err, Applicability, Diagnostic, DiagnosticBuilder, ErrorGuaranteed, MultiSpan,
43 use rustc_hir::def_id::DefId;
44 use rustc_hir::intravisit::{self, Visitor};
46 use rustc_hir_analysis::astconv::AstConv;
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, 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::TraitEngineExt as _;
66 use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode};
68 use smallvec::{smallvec, SmallVec};
71 struct Coerce<'a, 'tcx> {
72 fcx: &'a FnCtxt<'a, 'tcx>,
73 cause: ObligationCause<'tcx>,
75 /// Determines whether or not allow_two_phase_borrow is set on any
76 /// autoref adjustments we create while coercing. We don't want to
77 /// allow deref coercions to create two-phase borrows, at least initially,
78 /// but we do need two-phase borrows for function argument reborrows.
79 /// See #47489 and #48598
80 /// See docs on the "AllowTwoPhase" type for a more detailed discussion
81 allow_two_phase: AllowTwoPhase,
84 impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
85 type Target = FnCtxt<'a, 'tcx>;
86 fn deref(&self) -> &Self::Target {
91 type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
93 struct CollectRetsVisitor<'tcx> {
94 ret_exprs: Vec<&'tcx hir::Expr<'tcx>>,
97 impl<'tcx> Visitor<'tcx> for CollectRetsVisitor<'tcx> {
98 fn visit_expr(&mut self, expr: &'tcx Expr<'tcx>) {
99 if let hir::ExprKind::Ret(_) = expr.kind {
100 self.ret_exprs.push(expr);
102 intravisit::walk_expr(self, expr);
106 /// Coercing a mutable reference to an immutable works, while
107 /// coercing `&T` to `&mut T` should be forbidden.
108 fn coerce_mutbls<'tcx>(
109 from_mutbl: hir::Mutability,
110 to_mutbl: hir::Mutability,
111 ) -> RelateResult<'tcx, ()> {
112 match (from_mutbl, to_mutbl) {
113 (hir::Mutability::Mut, hir::Mutability::Mut | hir::Mutability::Not)
114 | (hir::Mutability::Not, hir::Mutability::Not) => Ok(()),
115 (hir::Mutability::Not, hir::Mutability::Mut) => Err(TypeError::Mutability),
119 /// Do not require any adjustments, i.e. coerce `x -> x`.
120 fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> {
124 fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
125 move |target| vec![Adjustment { kind, target }]
128 /// This always returns `Ok(...)`.
130 adj: Vec<Adjustment<'tcx>>,
132 obligations: traits::PredicateObligations<'tcx>,
133 ) -> CoerceResult<'tcx> {
134 Ok(InferOk { value: (adj, target), obligations })
137 impl<'f, 'tcx> Coerce<'f, 'tcx> {
139 fcx: &'f FnCtxt<'f, 'tcx>,
140 cause: ObligationCause<'tcx>,
141 allow_two_phase: AllowTwoPhase,
143 Coerce { fcx, cause, allow_two_phase, use_lub: false }
146 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
147 debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
148 self.commit_if_ok(|_| {
150 self.at(&self.cause, self.fcx.param_env).lub(b, a)
152 self.at(&self.cause, self.fcx.param_env)
154 .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
159 /// Unify two types (using sub or lub) and produce a specific coercion.
160 fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
162 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
165 .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
168 #[instrument(skip(self))]
169 fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
170 // First, remove any resolved type variables (at the top level, at least):
171 let a = self.shallow_resolve(a);
172 let b = self.shallow_resolve(b);
173 debug!("Coerce.tys({:?} => {:?})", a, b);
175 // Just ignore error types.
176 if a.references_error() || b.references_error() {
177 return success(vec![], self.fcx.tcx.ty_error(), vec![]);
180 // Coercing from `!` to any type is allowed:
182 return success(simple(Adjust::NeverToAny)(b), b, vec![]);
185 // Coercing *from* an unresolved inference variable means that
186 // we have no information about the source type. This will always
187 // ultimately fall back to some form of subtyping.
189 return self.coerce_from_inference_variable(a, b, identity);
192 // Consider coercing the subtype to a DST
194 // NOTE: this is wrapped in a `commit_if_ok` because it creates
195 // a "spurious" type variable, and we don't want to have that
196 // type variable in memory if the coercion fails.
197 let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
200 debug!("coerce: unsize successful");
203 Err(TypeError::ObjectUnsafeCoercion(did)) => {
204 debug!("coerce: unsize not object safe");
205 return Err(TypeError::ObjectUnsafeCoercion(did));
208 debug!(?error, "coerce: unsize failed");
212 // Examine the supertype and consider auto-borrowing.
214 ty::RawPtr(mt_b) => {
215 return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
217 ty::Ref(r_b, _, mutbl_b) => {
218 return self.coerce_borrowed_pointer(a, b, r_b, mutbl_b);
220 ty::Dynamic(predicates, region, ty::DynStar) if self.tcx.features().dyn_star => {
221 return self.coerce_dyn_star(a, b, predicates, region);
228 // Function items are coercible to any closure
229 // type; function pointers are not (that would
230 // require double indirection).
231 // Additionally, we permit coercion of function
232 // items to drop the unsafe qualifier.
233 self.coerce_from_fn_item(a, b)
236 // We permit coercion of fn pointers to drop the
238 self.coerce_from_fn_pointer(a, a_f, b)
240 ty::Closure(closure_def_id_a, substs_a) => {
241 // Non-capturing closures are coercible to
242 // function pointers or unsafe function pointers.
243 // It cannot convert closures that require unsafe.
244 self.coerce_closure_to_fn(a, closure_def_id_a, substs_a, b)
247 // Otherwise, just use unification rules.
248 self.unify_and(a, b, identity)
253 /// Coercing *from* an inference variable. In this case, we have no information
254 /// about the source type, so we can't really do a true coercion and we always
255 /// fall back to subtyping (`unify_and`).
256 fn coerce_from_inference_variable(
260 make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
261 ) -> CoerceResult<'tcx> {
262 debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
263 assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
264 assert!(self.shallow_resolve(b) == b);
267 // Two unresolved type variables: create a `Coerce` predicate.
268 let target_ty = if self.use_lub {
269 self.next_ty_var(TypeVariableOrigin {
270 kind: TypeVariableOriginKind::LatticeVariable,
271 span: self.cause.span,
277 let mut obligations = Vec::with_capacity(2);
278 for &source_ty in &[a, b] {
279 if source_ty != target_ty {
280 obligations.push(Obligation::new(
284 ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate {
293 "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
294 target_ty, obligations
296 let adjustments = make_adjustments(target_ty);
297 InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
299 // One unresolved type variable: just apply subtyping, we may be able
300 // to do something useful.
301 self.unify_and(a, b, make_adjustments)
305 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
306 /// To match `A` with `B`, autoderef will be performed,
307 /// calling `deref`/`deref_mut` where necessary.
308 fn coerce_borrowed_pointer(
312 r_b: ty::Region<'tcx>,
313 mutbl_b: hir::Mutability,
314 ) -> CoerceResult<'tcx> {
315 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
317 // If we have a parameter of type `&M T_a` and the value
318 // provided is `expr`, we will be adding an implicit borrow,
319 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
320 // to type check, we will construct the type that `&M*expr` would
323 let (r_a, mt_a) = match *a.kind() {
324 ty::Ref(r_a, ty, mutbl) => {
325 let mt_a = ty::TypeAndMut { ty, mutbl };
326 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
329 _ => return self.unify_and(a, b, identity),
332 let span = self.cause.span;
334 let mut first_error = None;
335 let mut r_borrow_var = None;
336 let mut autoderef = self.autoderef(span, a);
337 let mut found = None;
339 for (referent_ty, autoderefs) in autoderef.by_ref() {
341 // Don't let this pass, otherwise it would cause
342 // &T to autoref to &&T.
346 // At this point, we have deref'd `a` to `referent_ty`. So
347 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
348 // In the autoderef loop for `&'a mut Vec<T>`, we would get
351 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
352 // - `Vec<T>` -- 1 deref
353 // - `[T]` -- 2 deref
355 // At each point after the first callback, we want to
356 // check to see whether this would match out target type
357 // (`&'b mut [T]`) if we autoref'd it. We can't just
358 // compare the referent types, though, because we still
359 // have to consider the mutability. E.g., in the case
360 // we've been considering, we have an `&mut` reference, so
361 // the `T` in `[T]` needs to be unified with equality.
363 // Therefore, we construct reference types reflecting what
364 // the types will be after we do the final auto-ref and
365 // compare those. Note that this means we use the target
366 // mutability [1], since it may be that we are coercing
367 // from `&mut T` to `&U`.
369 // One fine point concerns the region that we use. We
370 // choose the region such that the region of the final
371 // type that results from `unify` will be the region we
372 // want for the autoref:
374 // - if in sub mode, that means we want to use `'b` (the
375 // region from the target reference) for both
376 // pointers [2]. This is because sub mode (somewhat
377 // arbitrarily) returns the subtype region. In the case
378 // where we are coercing to a target type, we know we
379 // want to use that target type region (`'b`) because --
380 // for the program to type-check -- it must be the
381 // smaller of the two.
382 // - One fine point. It may be surprising that we can
383 // use `'b` without relating `'a` and `'b`. The reason
384 // that this is ok is that what we produce is
385 // effectively a `&'b *x` expression (if you could
386 // annotate the region of a borrow), and regionck has
387 // code that adds edges from the region of a borrow
388 // (`'b`, here) into the regions in the borrowed
389 // expression (`*x`, here). (Search for "link".)
390 // - if in lub mode, things can get fairly complicated. The
391 // easiest thing is just to make a fresh
392 // region variable [4], which effectively means we defer
393 // the decision to region inference (and regionck, which will add
394 // some more edges to this variable). However, this can wind up
395 // creating a crippling number of variables in some cases --
396 // e.g., #32278 -- so we optimize one particular case [3].
397 // Let me try to explain with some examples:
398 // - The "running example" above represents the simple case,
399 // where we have one `&` reference at the outer level and
400 // ownership all the rest of the way down. In this case,
401 // we want `LUB('a, 'b)` as the resulting region.
402 // - However, if there are nested borrows, that region is
403 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
404 // `&'b T`. In this case, `'a` is actually irrelevant.
405 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
406 // we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
407 // (The errors actually show up in borrowck, typically, because
408 // this extra edge causes the region `'a` to be inferred to something
409 // too big, which then results in borrowck errors.)
410 // - We could track the innermost shared reference, but there is already
411 // code in regionck that has the job of creating links between
412 // the region of a borrow and the regions in the thing being
413 // borrowed (here, `'a` and `'x`), and it knows how to handle
414 // all the various cases. So instead we just make a region variable
415 // and let regionck figure it out.
416 let r = if !self.use_lub {
418 } else if autoderefs == 1 {
421 if r_borrow_var.is_none() {
422 // create var lazily, at most once
423 let coercion = Coercion(span);
424 let r = self.next_region_var(coercion);
425 r_borrow_var = Some(r); // [4] above
427 r_borrow_var.unwrap()
429 let derefd_ty_a = self.tcx.mk_ref(
433 mutbl: mutbl_b, // [1] above
436 match self.unify(derefd_ty_a, b) {
442 if first_error.is_none() {
443 first_error = Some(err);
449 // Extract type or return an error. We return the first error
450 // we got, which should be from relating the "base" type
451 // (e.g., in example above, the failure from relating `Vec<T>`
452 // to the target type), since that should be the least
454 let Some(InferOk { value: ty, mut obligations }) = found else {
455 let err = first_error.expect("coerce_borrowed_pointer had no error");
456 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
460 if ty == a && mt_a.mutbl == hir::Mutability::Not && autoderef.step_count() == 1 {
461 // As a special case, if we would produce `&'a *x`, that's
462 // a total no-op. We end up with the type `&'a T` just as
463 // we started with. In that case, just skip it
464 // altogether. This is just an optimization.
466 // Note that for `&mut`, we DO want to reborrow --
467 // otherwise, this would be a move, which might be an
468 // error. For example `foo(self.x)` where `self` and
469 // `self.x` both have `&mut `type would be a move of
470 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
471 // which is a borrow.
472 assert_eq!(mutbl_b, hir::Mutability::Not); // can only coerce &T -> &U
473 return success(vec![], ty, obligations);
476 let InferOk { value: mut adjustments, obligations: o } =
477 self.adjust_steps_as_infer_ok(&autoderef);
478 obligations.extend(o);
479 obligations.extend(autoderef.into_obligations());
481 // Now apply the autoref. We have to extract the region out of
482 // the final ref type we got.
483 let ty::Ref(r_borrow, _, _) = ty.kind() else {
484 span_bug!(span, "expected a ref type, got {:?}", ty);
486 let mutbl = match mutbl_b {
487 hir::Mutability::Not => AutoBorrowMutability::Not,
488 hir::Mutability::Mut => {
489 AutoBorrowMutability::Mut { allow_two_phase_borrow: self.allow_two_phase }
492 adjustments.push(Adjustment {
493 kind: Adjust::Borrow(AutoBorrow::Ref(*r_borrow, mutbl)),
497 debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
499 success(adjustments, ty, obligations)
502 // &[T; n] or &mut [T; n] -> &[T]
503 // or &mut [T; n] -> &mut [T]
504 // or &Concrete -> &Trait, etc.
505 #[instrument(skip(self), level = "debug")]
506 fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
507 source = self.shallow_resolve(source);
508 target = self.shallow_resolve(target);
509 debug!(?source, ?target);
511 // These 'if' statements require some explanation.
512 // The `CoerceUnsized` trait is special - it is only
513 // possible to write `impl CoerceUnsized<B> for A` where
514 // A and B have 'matching' fields. This rules out the following
515 // two types of blanket impls:
517 // `impl<T> CoerceUnsized<T> for SomeType`
518 // `impl<T> CoerceUnsized<SomeType> for T`
520 // Both of these trigger a special `CoerceUnsized`-related error (E0376)
522 // We can take advantage of this fact to avoid performing unnecessary work.
523 // If either `source` or `target` is a type variable, then any applicable impl
524 // would need to be generic over the self-type (`impl<T> CoerceUnsized<SomeType> for T`)
525 // or generic over the `CoerceUnsized` type parameter (`impl<T> CoerceUnsized<T> for
528 // However, these are exactly the kinds of impls which are forbidden by
529 // the compiler! Therefore, we can be sure that coercion will always fail
530 // when either the source or target type is a type variable. This allows us
531 // to skip performing any trait selection, and immediately bail out.
532 if source.is_ty_var() {
533 debug!("coerce_unsized: source is a TyVar, bailing out");
534 return Err(TypeError::Mismatch);
536 if target.is_ty_var() {
537 debug!("coerce_unsized: target is a TyVar, bailing out");
538 return Err(TypeError::Mismatch);
542 (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
543 let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
544 debug!("missing Unsize or CoerceUnsized traits");
545 return Err(TypeError::Mismatch);
548 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
549 // a DST unless we have to. This currently comes out in the wash since
550 // we can't unify [T] with U. But to properly support DST, we need to allow
551 // that, at which point we will need extra checks on the target here.
553 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
554 let reborrow = match (source.kind(), target.kind()) {
555 (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
556 coerce_mutbls(mutbl_a, mutbl_b)?;
558 let coercion = Coercion(self.cause.span);
559 let r_borrow = self.next_region_var(coercion);
560 let mutbl = match mutbl_b {
561 hir::Mutability::Not => AutoBorrowMutability::Not,
562 hir::Mutability::Mut => AutoBorrowMutability::Mut {
563 // We don't allow two-phase borrows here, at least for initial
564 // implementation. If it happens that this coercion is a function argument,
565 // the reborrow in coerce_borrowed_ptr will pick it up.
566 allow_two_phase_borrow: AllowTwoPhase::No,
570 Adjustment { kind: Adjust::Deref(None), target: ty_a },
572 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
575 .mk_ref(r_borrow, ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a }),
579 (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
580 coerce_mutbls(mt_a, mt_b)?;
583 Adjustment { kind: Adjust::Deref(None), target: ty_a },
585 kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
586 target: self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mt_b, ty: ty_a }),
592 let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
594 // Setup either a subtyping or a LUB relationship between
595 // the `CoerceUnsized` target type and the expected type.
596 // We only have the latter, so we use an inference variable
597 // for the former and let type inference do the rest.
598 let origin = TypeVariableOrigin {
599 kind: TypeVariableOriginKind::MiscVariable,
600 span: self.cause.span,
602 let coerce_target = self.next_ty_var(origin);
603 let mut coercion = self.unify_and(coerce_target, target, |target| {
604 let unsize = Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target };
606 None => vec![unsize],
607 Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
611 let mut selcx = traits::SelectionContext::new(self);
613 // Create an obligation for `Source: CoerceUnsized<Target>`.
614 let cause = ObligationCause::new(
617 ObligationCauseCode::Coercion { source, target },
620 // Use a FIFO queue for this custom fulfillment procedure.
622 // A Vec (or SmallVec) is not a natural choice for a queue. However,
623 // this code path is hot, and this queue usually has a max length of 1
624 // and almost never more than 3. By using a SmallVec we avoid an
625 // allocation, at the (very small) cost of (occasionally) having to
626 // shift subsequent elements down when removing the front element.
627 let mut queue: SmallVec<[_; 4]> = smallvec![traits::predicate_for_trait_def(
634 &[coerce_target.into()]
637 let mut has_unsized_tuple_coercion = false;
638 let mut has_trait_upcasting_coercion = None;
640 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
641 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
642 // inference might unify those two inner type variables later.
643 let traits = [coerce_unsized_did, unsize_did];
644 while !queue.is_empty() {
645 let obligation = queue.remove(0);
646 debug!("coerce_unsized resolve step: {:?}", obligation);
647 let bound_predicate = obligation.predicate.kind();
648 let trait_pred = match bound_predicate.skip_binder() {
649 ty::PredicateKind::Trait(trait_pred) if traits.contains(&trait_pred.def_id()) => {
650 if unsize_did == trait_pred.def_id() {
651 let self_ty = trait_pred.self_ty();
652 let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty();
653 if let (ty::Dynamic(ref data_a, ..), ty::Dynamic(ref data_b, ..)) =
654 (self_ty.kind(), unsize_ty.kind())
655 && data_a.principal_def_id() != data_b.principal_def_id()
657 debug!("coerce_unsized: found trait upcasting coercion");
658 has_trait_upcasting_coercion = Some((self_ty, unsize_ty));
660 if let ty::Tuple(..) = unsize_ty.kind() {
661 debug!("coerce_unsized: found unsized tuple coercion");
662 has_unsized_tuple_coercion = true;
665 bound_predicate.rebind(trait_pred)
668 coercion.obligations.push(obligation);
672 match selcx.select(&obligation.with(selcx.tcx(), trait_pred)) {
673 // Uncertain or unimplemented.
675 if trait_pred.def_id() == unsize_did {
676 let trait_pred = self.resolve_vars_if_possible(trait_pred);
677 let self_ty = trait_pred.skip_binder().self_ty();
678 let unsize_ty = trait_pred.skip_binder().trait_ref.substs[1].expect_ty();
679 debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
680 match (&self_ty.kind(), &unsize_ty.kind()) {
681 (ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
682 if self.type_var_is_sized(*v) =>
684 debug!("coerce_unsized: have sized infer {:?}", v);
685 coercion.obligations.push(obligation);
686 // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
690 // Some other case for `$0: Unsize<Something>`. Note that we
691 // hit this case even if `Something` is a sized type, so just
692 // don't do the coercion.
693 debug!("coerce_unsized: ambiguous unsize");
694 return Err(TypeError::Mismatch);
698 debug!("coerce_unsized: early return - ambiguous");
699 return Err(TypeError::Mismatch);
702 Err(traits::Unimplemented) => {
703 debug!("coerce_unsized: early return - can't prove obligation");
704 return Err(TypeError::Mismatch);
707 // Object safety violations or miscellaneous.
709 self.err_ctxt().report_selection_error(obligation.clone(), &obligation, &err);
710 // Treat this like an obligation and follow through
711 // with the unsizing - the lack of a coercion should
712 // be silent, as it causes a type mismatch later.
715 Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
719 if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
721 &self.tcx.sess.parse_sess,
722 sym::unsized_tuple_coercion,
724 "unsized tuple coercion is not stable enough for use and is subject to change",
729 if let Some((sub, sup)) = has_trait_upcasting_coercion
730 && !self.tcx().features().trait_upcasting
732 // Renders better when we erase regions, since they're not really the point here.
733 let (sub, sup) = self.tcx.erase_regions((sub, sup));
734 let mut err = feature_err(
735 &self.tcx.sess.parse_sess,
736 sym::trait_upcasting,
738 &format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
740 err.note(&format!("required when coercing `{source}` into `{target}`"));
751 predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
752 b_region: ty::Region<'tcx>,
753 ) -> CoerceResult<'tcx> {
754 if !self.tcx.features().dyn_star {
755 return Err(TypeError::Mismatch);
758 if let ty::Dynamic(a_data, _, _) = a.kind()
759 && let ty::Dynamic(b_data, _, _) = b.kind()
761 if a_data.principal_def_id() == b_data.principal_def_id() {
762 return self.unify_and(a, b, |_| vec![]);
763 } else if !self.tcx().features().trait_upcasting {
764 let mut err = feature_err(
765 &self.tcx.sess.parse_sess,
766 sym::trait_upcasting,
769 "cannot cast `{a}` to `{b}`, trait upcasting coercion is experimental"
776 // Check the obligations of the cast -- for example, when casting
777 // `usize` to `dyn* Clone + 'static`:
778 let mut obligations: Vec<_> = predicates
781 // For each existential predicate (e.g., `?Self: Clone`) substitute
782 // the type of the expression (e.g., `usize` in our example above)
783 // and then require that the resulting predicate (e.g., `usize: Clone`)
785 let predicate = predicate.with_self_ty(self.tcx, a);
786 Obligation::new(self.tcx, self.cause.clone(), self.param_env, predicate)
789 // Enforce the region bound (e.g., `usize: 'static`, in our example).
794 ty::Binder::dummy(ty::PredicateKind::TypeOutlives(ty::OutlivesPredicate(
801 // Enforce that the type is `usize`/pointer-sized. For now, only those
802 // can be coerced to `dyn*`, except for `dyn* -> dyn*` upcasts.
803 if !a.is_dyn_star() {
804 obligations.push(Obligation::new(
808 ty::Binder::dummy(ty::TraitRef::new(
809 self.tcx.require_lang_item(hir::LangItem::PointerSized, Some(self.cause.span)),
810 self.tcx.mk_substs_trait(a, &[]),
812 .to_poly_trait_predicate(),
817 value: (vec![Adjustment { kind: Adjust::DynStar, target: b }], b),
822 fn coerce_from_safe_fn<F, G>(
825 fn_ty_a: ty::PolyFnSig<'tcx>,
829 ) -> CoerceResult<'tcx>
831 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
832 G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
834 self.commit_if_ok(|snapshot| {
835 let result = if let ty::FnPtr(fn_ty_b) = b.kind()
836 && let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) =
837 (fn_ty_a.unsafety(), fn_ty_b.unsafety())
839 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
840 self.unify_and(unsafe_a, b, to_unsafe)
842 self.unify_and(a, b, normal)
845 // FIXME(#73154): This is a hack. Currently LUB can generate
846 // unsolvable constraints. Additionally, it returns `a`
847 // unconditionally, even when the "LUB" is `b`. In the future, we
848 // want the coerced type to be the actual supertype of these two,
849 // but for now, we want to just error to ensure we don't lock
850 // ourselves into a specific behavior with NLL.
851 self.leak_check(false, snapshot)?;
857 fn coerce_from_fn_pointer(
860 fn_ty_a: ty::PolyFnSig<'tcx>,
862 ) -> CoerceResult<'tcx> {
863 //! Attempts to coerce from the type of a Rust function item
864 //! into a closure or a `proc`.
867 let b = self.shallow_resolve(b);
868 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
870 self.coerce_from_safe_fn(
874 simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)),
879 fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
880 //! Attempts to coerce from the type of a Rust function item
881 //! into a closure or a `proc`.
883 let b = self.shallow_resolve(b);
884 let InferOk { value: b, mut obligations } =
885 self.normalize_associated_types_in_as_infer_ok(self.cause.span, b);
886 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
889 ty::FnPtr(b_sig) => {
890 let a_sig = a.fn_sig(self.tcx);
891 if let ty::FnDef(def_id, _) = *a.kind() {
892 // Intrinsics are not coercible to function pointers
893 if self.tcx.is_intrinsic(def_id) {
894 return Err(TypeError::IntrinsicCast);
897 // Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).
899 if b_sig.unsafety() == hir::Unsafety::Normal
900 && !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
902 return Err(TypeError::TargetFeatureCast(def_id));
906 let InferOk { value: a_sig, obligations: o1 } =
907 self.normalize_associated_types_in_as_infer_ok(self.cause.span, a_sig);
908 obligations.extend(o1);
910 let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
911 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
918 kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
919 target: a_fn_pointer,
922 kind: Adjust::Pointer(PointerCast::UnsafeFnPointer),
927 simple(Adjust::Pointer(PointerCast::ReifyFnPointer)),
930 obligations.extend(o2);
931 Ok(InferOk { value, obligations })
933 _ => self.unify_and(a, b, identity),
937 fn coerce_closure_to_fn(
940 closure_def_id_a: DefId,
941 substs_a: SubstsRef<'tcx>,
943 ) -> CoerceResult<'tcx> {
944 //! Attempts to coerce from the type of a non-capturing closure
945 //! into a function pointer.
948 let b = self.shallow_resolve(b);
951 // At this point we haven't done capture analysis, which means
952 // that the ClosureSubsts just contains an inference variable instead
953 // of tuple of captured types.
955 // All we care here is if any variable is being captured and not the exact paths,
956 // so we check `upvars_mentioned` for root variables being captured.
960 .upvars_mentioned(closure_def_id_a.expect_local())
961 .map_or(true, |u| u.is_empty()) =>
963 // We coerce the closure, which has fn type
964 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
966 // `fn(arg0,arg1,...) -> _`
968 // `unsafe fn(arg0,arg1,...) -> _`
969 let closure_sig = substs_a.as_closure().sig();
970 let unsafety = fn_ty.unsafety();
972 self.tcx.mk_fn_ptr(self.tcx.signature_unclosure(closure_sig, unsafety));
973 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
977 simple(Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))),
980 _ => self.unify_and(a, b, identity),
984 fn coerce_unsafe_ptr(
988 mutbl_b: hir::Mutability,
989 ) -> CoerceResult<'tcx> {
990 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
992 let (is_ref, mt_a) = match *a.kind() {
993 ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
994 ty::RawPtr(mt) => (false, mt),
995 _ => return self.unify_and(a, b, identity),
997 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
999 // Check that the types which they point at are compatible.
1000 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty });
1001 // Although references and unsafe ptrs have the same
1002 // representation, we still register an Adjust::DerefRef so that
1003 // regionck knows that the region for `a` must be valid here.
1005 self.unify_and(a_unsafe, b, |target| {
1007 Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
1008 Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
1011 } else if mt_a.mutbl != mutbl_b {
1012 self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer)))
1014 self.unify_and(a_unsafe, b, identity)
1019 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
1020 /// Attempt to coerce an expression to a type, and return the
1021 /// adjusted type of the expression, if successful.
1022 /// Adjustments are only recorded if the coercion succeeded.
1023 /// The expressions *must not* have any pre-existing adjustments.
1026 expr: &hir::Expr<'_>,
1029 allow_two_phase: AllowTwoPhase,
1030 cause: Option<ObligationCause<'tcx>>,
1031 ) -> RelateResult<'tcx, Ty<'tcx>> {
1032 let source = self.resolve_vars_with_obligations(expr_ty);
1033 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
1036 cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
1037 let coerce = Coerce::new(self, cause, allow_two_phase);
1038 let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
1040 let (adjustments, _) = self.register_infer_ok_obligations(ok);
1041 self.apply_adjustments(expr, adjustments);
1042 Ok(if expr_ty.references_error() { self.tcx.ty_error() } else { target })
1045 /// Same as `try_coerce()`, but without side-effects.
1047 /// Returns false if the coercion creates any obligations that result in
1049 pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
1050 let source = self.resolve_vars_with_obligations(expr_ty);
1051 debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);
1053 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
1054 // We don't ever need two-phase here since we throw out the result of the coercion
1055 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
1057 let Ok(ok) = coerce.coerce(source, target) else {
1060 let mut fcx = <dyn TraitEngine<'tcx>>::new_in_snapshot(self.tcx);
1061 fcx.register_predicate_obligations(self, ok.obligations);
1062 fcx.select_where_possible(&self).is_empty()
1066 /// Given a type and a target type, this function will calculate and return
1067 /// how many dereference steps needed to achieve `expr_ty <: target`. If
1068 /// it's not possible, return `None`.
1069 pub fn deref_steps(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> Option<usize> {
1070 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
1071 // We don't ever need two-phase here since we throw out the result of the coercion
1072 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
1074 .autoderef(rustc_span::DUMMY_SP, expr_ty)
1075 .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
1078 /// Given a type, this function will calculate and return the type given
1079 /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
1081 /// This function is for diagnostics only, since it does not register
1082 /// trait or region sub-obligations. (presumably we could, but it's not
1083 /// particularly important for diagnostics...)
1084 pub fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
1085 self.autoderef(rustc_span::DUMMY_SP, expr_ty).nth(1).and_then(|(deref_ty, _)| {
1087 .type_implements_trait(
1088 self.tcx.lang_items().deref_mut_trait()?,
1098 /// Given some expressions, their known unified type and another expression,
1099 /// tries to unify the types, potentially inserting coercions on any of the
1100 /// provided expressions and returns their LUB (aka "common supertype").
1102 /// This is really an internal helper. From outside the coercion
1103 /// module, you should instantiate a `CoerceMany` instance.
1104 fn try_find_coercion_lub<E>(
1106 cause: &ObligationCause<'tcx>,
1109 new: &hir::Expr<'_>,
1111 ) -> RelateResult<'tcx, Ty<'tcx>>
1115 let prev_ty = self.resolve_vars_with_obligations(prev_ty);
1116 let new_ty = self.resolve_vars_with_obligations(new_ty);
1118 "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
1124 // The following check fixes #88097, where the compiler erroneously
1125 // attempted to coerce a closure type to itself via a function pointer.
1126 if prev_ty == new_ty {
1130 // Special-case that coercion alone cannot handle:
1131 // Function items or non-capturing closures of differing IDs or InternalSubsts.
1132 let (a_sig, b_sig) = {
1133 #[allow(rustc::usage_of_ty_tykind)]
1134 let is_capturing_closure = |ty: &ty::TyKind<'tcx>| {
1135 if let &ty::Closure(closure_def_id, _substs) = ty {
1136 self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some()
1141 if is_capturing_closure(prev_ty.kind()) || is_capturing_closure(new_ty.kind()) {
1144 match (prev_ty.kind(), new_ty.kind()) {
1145 (ty::FnDef(..), ty::FnDef(..)) => {
1146 // Don't reify if the function types have a LUB, i.e., they
1147 // are the same function and their parameters have a LUB.
1149 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1151 // We have a LUB of prev_ty and new_ty, just return it.
1152 Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
1154 (Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx)))
1158 (ty::Closure(_, substs), ty::FnDef(..)) => {
1159 let b_sig = new_ty.fn_sig(self.tcx);
1162 .signature_unclosure(substs.as_closure().sig(), b_sig.unsafety());
1163 (Some(a_sig), Some(b_sig))
1165 (ty::FnDef(..), ty::Closure(_, substs)) => {
1166 let a_sig = prev_ty.fn_sig(self.tcx);
1169 .signature_unclosure(substs.as_closure().sig(), a_sig.unsafety());
1170 (Some(a_sig), Some(b_sig))
1172 (ty::Closure(_, substs_a), ty::Closure(_, substs_b)) => (
1173 Some(self.tcx.signature_unclosure(
1174 substs_a.as_closure().sig(),
1175 hir::Unsafety::Normal,
1177 Some(self.tcx.signature_unclosure(
1178 substs_b.as_closure().sig(),
1179 hir::Unsafety::Normal,
1186 if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) {
1187 // Intrinsics are not coercible to function pointers.
1188 if a_sig.abi() == Abi::RustIntrinsic
1189 || a_sig.abi() == Abi::PlatformIntrinsic
1190 || b_sig.abi() == Abi::RustIntrinsic
1191 || b_sig.abi() == Abi::PlatformIntrinsic
1193 return Err(TypeError::IntrinsicCast);
1195 // The signature must match.
1196 let a_sig = self.normalize_associated_types_in(new.span, a_sig);
1197 let b_sig = self.normalize_associated_types_in(new.span, b_sig);
1199 .at(cause, self.param_env)
1200 .trace(prev_ty, new_ty)
1202 .map(|ok| self.register_infer_ok_obligations(ok))?;
1204 // Reify both sides and return the reified fn pointer type.
1205 let fn_ptr = self.tcx.mk_fn_ptr(sig);
1206 let prev_adjustment = match prev_ty.kind() {
1207 ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(a_sig.unsafety())),
1208 ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
1209 _ => unreachable!(),
1211 let next_adjustment = match new_ty.kind() {
1212 ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(b_sig.unsafety())),
1213 ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
1214 _ => unreachable!(),
1216 for expr in exprs.iter().map(|e| e.as_coercion_site()) {
1217 self.apply_adjustments(
1219 vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
1222 self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
1226 // Configure a Coerce instance to compute the LUB.
1227 // We don't allow two-phase borrows on any autorefs this creates since we
1228 // probably aren't processing function arguments here and even if we were,
1229 // they're going to get autorefed again anyway and we can apply 2-phase borrows
1231 let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
1232 coerce.use_lub = true;
1234 // First try to coerce the new expression to the type of the previous ones,
1235 // but only if the new expression has no coercion already applied to it.
1236 let mut first_error = None;
1237 if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
1238 let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
1241 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1242 self.apply_adjustments(new, adjustments);
1244 "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
1245 new_ty, prev_ty, target
1249 Err(e) => first_error = Some(e),
1253 // Then try to coerce the previous expressions to the type of the new one.
1254 // This requires ensuring there are no coercions applied to *any* of the
1255 // previous expressions, other than noop reborrows (ignoring lifetimes).
1257 let expr = expr.as_coercion_site();
1258 let noop = match self.typeck_results.borrow().expr_adjustments(expr) {
1260 Adjustment { kind: Adjust::Deref(_), .. },
1261 Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. },
1263 match *self.node_ty(expr.hir_id).kind() {
1264 ty::Ref(_, _, mt_orig) => {
1265 let mutbl_adj: hir::Mutability = mutbl_adj.into();
1266 // Reborrow that we can safely ignore, because
1267 // the next adjustment can only be a Deref
1268 // which will be merged into it.
1269 mutbl_adj == mt_orig
1274 &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
1280 "coercion::try_find_coercion_lub: older expression {:?} had adjustments, requiring LUB",
1285 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1286 .map(|ok| self.register_infer_ok_obligations(ok));
1290 match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
1292 // Avoid giving strange errors on failed attempts.
1293 if let Some(e) = first_error {
1296 self.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1297 .map(|ok| self.register_infer_ok_obligations(ok))
1301 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1303 let expr = expr.as_coercion_site();
1304 self.apply_adjustments(expr, adjustments.clone());
1307 "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
1308 prev_ty, new_ty, target
1316 /// CoerceMany encapsulates the pattern you should use when you have
1317 /// many expressions that are all getting coerced to a common
1318 /// type. This arises, for example, when you have a match (the result
1319 /// of each arm is coerced to a common type). It also arises in less
1320 /// obvious places, such as when you have many `break foo` expressions
1321 /// that target the same loop, or the various `return` expressions in
1324 /// The basic protocol is as follows:
1326 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
1327 /// This will also serve as the "starting LUB". The expectation is
1328 /// that this type is something which all of the expressions *must*
1329 /// be coercible to. Use a fresh type variable if needed.
1330 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
1331 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
1332 /// unit. This happens for example if you have a `break` with no expression,
1333 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
1334 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
1335 /// from you so that you don't have to worry your pretty head about it.
1336 /// But if an error is reported, the final type will be `err`.
1337 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
1338 /// previously coerced expressions.
1339 /// - When all done, invoke `complete()`. This will return the LUB of
1340 /// all your expressions.
1341 /// - WARNING: I don't believe this final type is guaranteed to be
1342 /// related to your initial `expected_ty` in any particular way,
1343 /// although it will typically be a subtype, so you should check it.
1344 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
1345 /// previously coerced expressions.
1349 /// ```ignore (illustrative)
1350 /// let mut coerce = CoerceMany::new(expected_ty);
1351 /// for expr in exprs {
1352 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
1353 /// coerce.coerce(fcx, &cause, expr, expr_ty);
1355 /// let final_ty = coerce.complete(fcx);
1357 pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
1358 expected_ty: Ty<'tcx>,
1359 final_ty: Option<Ty<'tcx>>,
1360 expressions: Expressions<'tcx, 'exprs, E>,
1364 /// The type of a `CoerceMany` that is storing up the expressions into
1365 /// a buffer. We use this in `check/mod.rs` for things like `break`.
1366 pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;
1368 enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
1369 Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
1370 UpFront(&'exprs [E]),
1373 impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
1374 /// The usual case; collect the set of expressions dynamically.
1375 /// If the full set of coercion sites is known before hand,
1376 /// consider `with_coercion_sites()` instead to avoid allocation.
1377 pub fn new(expected_ty: Ty<'tcx>) -> Self {
1378 Self::make(expected_ty, Expressions::Dynamic(vec![]))
1381 /// As an optimization, you can create a `CoerceMany` with a
1382 /// pre-existing slice of expressions. In this case, you are
1383 /// expected to pass each element in the slice to `coerce(...)` in
1384 /// order. This is used with arrays in particular to avoid
1385 /// needlessly cloning the slice.
1386 pub fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
1387 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
1390 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
1391 CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
1394 /// Returns the "expected type" with which this coercion was
1395 /// constructed. This represents the "downward propagated" type
1396 /// that was given to us at the start of typing whatever construct
1397 /// we are typing (e.g., the match expression).
1399 /// Typically, this is used as the expected type when
1400 /// type-checking each of the alternative expressions whose types
1401 /// we are trying to merge.
1402 pub fn expected_ty(&self) -> Ty<'tcx> {
1406 /// Returns the current "merged type", representing our best-guess
1407 /// at the LUB of the expressions we've seen so far (if any). This
1408 /// isn't *final* until you call `self.complete()`, which will return
1409 /// the merged type.
1410 pub fn merged_ty(&self) -> Ty<'tcx> {
1411 self.final_ty.unwrap_or(self.expected_ty)
1414 /// Indicates that the value generated by `expression`, which is
1415 /// of type `expression_ty`, is one of the possibilities that we
1416 /// could coerce from. This will record `expression`, and later
1417 /// calls to `coerce` may come back and add adjustments and things
1421 fcx: &FnCtxt<'a, 'tcx>,
1422 cause: &ObligationCause<'tcx>,
1423 expression: &'tcx hir::Expr<'tcx>,
1424 expression_ty: Ty<'tcx>,
1426 self.coerce_inner(fcx, cause, Some(expression), expression_ty, None, false)
1429 /// Indicates that one of the inputs is a "forced unit". This
1430 /// occurs in a case like `if foo { ... };`, where the missing else
1431 /// generates a "forced unit". Another example is a `loop { break;
1432 /// }`, where the `break` has no argument expression. We treat
1433 /// these cases slightly differently for error-reporting
1434 /// purposes. Note that these tend to correspond to cases where
1435 /// the `()` expression is implicit in the source, and hence we do
1436 /// not take an expression argument.
1438 /// The `augment_error` gives you a chance to extend the error
1439 /// message, in case any results (e.g., we use this to suggest
1440 /// removing a `;`).
1441 pub fn coerce_forced_unit<'a>(
1443 fcx: &FnCtxt<'a, 'tcx>,
1444 cause: &ObligationCause<'tcx>,
1445 augment_error: &mut dyn FnMut(&mut Diagnostic),
1446 label_unit_as_expected: bool,
1453 Some(augment_error),
1454 label_unit_as_expected,
1458 /// The inner coercion "engine". If `expression` is `None`, this
1459 /// is a forced-unit case, and hence `expression_ty` must be
1461 #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
1462 pub(crate) fn coerce_inner<'a>(
1464 fcx: &FnCtxt<'a, 'tcx>,
1465 cause: &ObligationCause<'tcx>,
1466 expression: Option<&'tcx hir::Expr<'tcx>>,
1467 mut expression_ty: Ty<'tcx>,
1468 augment_error: Option<&mut dyn FnMut(&mut Diagnostic)>,
1469 label_expression_as_expected: bool,
1471 // Incorporate whatever type inference information we have
1472 // until now; in principle we might also want to process
1473 // pending obligations, but doing so should only improve
1474 // compatibility (hopefully that is true) by helping us
1475 // uncover never types better.
1476 if expression_ty.is_ty_var() {
1477 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1480 // If we see any error types, just propagate that error
1482 if expression_ty.references_error() || self.merged_ty().references_error() {
1483 self.final_ty = Some(fcx.tcx.ty_error());
1487 // Handle the actual type unification etc.
1488 let result = if let Some(expression) = expression {
1489 if self.pushed == 0 {
1490 // Special-case the first expression we are coercing.
1491 // To be honest, I'm not entirely sure why we do this.
1492 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1498 Some(cause.clone()),
1501 match self.expressions {
1502 Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
1509 Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub(
1511 &coercion_sites[0..self.pushed],
1519 // this is a hack for cases where we default to `()` because
1520 // the expression etc has been omitted from the source. An
1521 // example is an `if let` without an else:
1523 // if let Some(x) = ... { }
1525 // we wind up with a second match arm that is like `_ =>
1526 // ()`. That is the case we are considering here. We take
1527 // a different path to get the right "expected, found"
1528 // message and so forth (and because we know that
1529 // `expression_ty` will be unit).
1531 // Another example is `break` with no argument expression.
1532 assert!(expression_ty.is_unit(), "if let hack without unit type");
1533 fcx.at(cause, fcx.param_env)
1534 .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
1536 fcx.register_infer_ok_obligations(infer_ok);
1544 self.final_ty = Some(v);
1545 if let Some(e) = expression {
1546 match self.expressions {
1547 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1548 Expressions::UpFront(coercion_sites) => {
1549 // if the user gave us an array to validate, check that we got
1550 // the next expression in the list, as expected
1552 coercion_sites[self.pushed].as_coercion_site().hir_id,
1560 Err(coercion_error) => {
1561 // Mark that we've failed to coerce the types here to suppress
1562 // any superfluous errors we might encounter while trying to
1563 // emit or provide suggestions on how to fix the initial error.
1564 fcx.set_tainted_by_errors();
1565 let (expected, found) = if label_expression_as_expected {
1566 // In the case where this is a "forced unit", like
1567 // `break`, we want to call the `()` "expected"
1568 // since it is implied by the syntax.
1569 // (Note: not all force-units work this way.)"
1570 (expression_ty, self.merged_ty())
1572 // Otherwise, the "expected" type for error
1573 // reporting is the current unification type,
1574 // which is basically the LUB of the expressions
1575 // we've seen so far (combined with the expected
1577 (self.merged_ty(), expression_ty)
1579 let (expected, found) = fcx.resolve_vars_if_possible((expected, found));
1582 let mut unsized_return = false;
1583 let mut visitor = CollectRetsVisitor { ret_exprs: vec![] };
1584 match *cause.code() {
1585 ObligationCauseCode::ReturnNoExpression => {
1586 err = struct_span_err!(
1590 "`return;` in a function whose return type is not `()`"
1592 err.span_label(cause.span, "return type is not `()`");
1594 ObligationCauseCode::BlockTailExpression(blk_id) => {
1595 let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
1596 err = self.report_return_mismatched_types(
1600 coercion_error.clone(),
1606 if !fcx.tcx.features().unsized_locals {
1607 unsized_return = self.is_return_ty_unsized(fcx, blk_id);
1609 if let Some(expression) = expression
1610 && let hir::ExprKind::Loop(loop_blk, ..) = expression.kind {
1611 intravisit::walk_block(& mut visitor, loop_blk);
1614 ObligationCauseCode::ReturnValue(id) => {
1615 err = self.report_return_mismatched_types(
1619 coercion_error.clone(),
1625 if !fcx.tcx.features().unsized_locals {
1626 let id = fcx.tcx.hir().get_parent_node(id);
1627 unsized_return = self.is_return_ty_unsized(fcx, id);
1631 err = fcx.err_ctxt().report_mismatched_types(
1635 coercion_error.clone(),
1640 if let Some(augment_error) = augment_error {
1641 augment_error(&mut err);
1644 let is_insufficiently_polymorphic =
1645 matches!(coercion_error, TypeError::RegionsInsufficientlyPolymorphic(..));
1647 if !is_insufficiently_polymorphic && let Some(expr) = expression {
1648 fcx.emit_coerce_suggestions(
1654 Some(coercion_error),
1658 if visitor.ret_exprs.len() > 0 && let Some(expr) = expression {
1659 self.note_unreachable_loop_return(&mut err, &expr, &visitor.ret_exprs);
1661 let reported = err.emit_unless(unsized_return);
1663 self.final_ty = Some(fcx.tcx.ty_error_with_guaranteed(reported));
1667 fn note_unreachable_loop_return(
1669 err: &mut Diagnostic,
1670 expr: &hir::Expr<'tcx>,
1671 ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>,
1673 let hir::ExprKind::Loop(_, _, _, loop_span) = expr.kind else { return;};
1674 let mut span: MultiSpan = vec![loop_span].into();
1675 span.push_span_label(loop_span, "this might have zero elements to iterate on");
1676 const MAXITER: usize = 3;
1677 let iter = ret_exprs.iter().take(MAXITER);
1678 for ret_expr in iter {
1679 span.push_span_label(
1681 "if the loop doesn't execute, this value would never get returned",
1686 "the function expects a value to always be returned, but loops might run zero times",
1688 if MAXITER < ret_exprs.len() {
1690 "if the loop doesn't execute, {} other values would never get returned",
1691 ret_exprs.len() - MAXITER
1695 "return a value for the case when the loop has zero elements to iterate on, or \
1696 consider changing the return type to account for that possibility",
1700 fn report_return_mismatched_types<'a>(
1702 cause: &ObligationCause<'tcx>,
1705 ty_err: TypeError<'tcx>,
1706 fcx: &FnCtxt<'a, 'tcx>,
1708 expression: Option<&'tcx hir::Expr<'tcx>>,
1709 blk_id: Option<hir::HirId>,
1710 ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
1711 let mut err = fcx.err_ctxt().report_mismatched_types(cause, expected, found, ty_err);
1713 let mut pointing_at_return_type = false;
1714 let mut fn_output = None;
1716 let parent_id = fcx.tcx.hir().get_parent_node(id);
1717 let parent = fcx.tcx.hir().get(parent_id);
1718 if let Some(expr) = expression
1719 && let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(&hir::Closure { body, .. }), .. }) = parent
1720 && !matches!(fcx.tcx.hir().body(body).value.kind, hir::ExprKind::Block(..))
1722 fcx.suggest_missing_semicolon(&mut err, expr, expected, true);
1724 // Verify that this is a tail expression of a function, otherwise the
1725 // label pointing out the cause for the type coercion will be wrong
1726 // as prior return coercions would not be relevant (#57664).
1727 let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) {
1728 pointing_at_return_type =
1729 fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id);
1730 if let (Some(cond_expr), true, false) = (
1731 fcx.tcx.hir().get_if_cause(expr.hir_id),
1733 pointing_at_return_type,
1735 // If the block is from an external macro or try (`?`) desugaring, then
1736 // do not suggest adding a semicolon, because there's nowhere to put it.
1737 // See issues #81943 and #87051.
1739 cond_expr.span.desugaring_kind(),
1740 None | Some(DesugaringKind::WhileLoop)
1741 ) && !in_external_macro(fcx.tcx.sess, cond_expr.span)
1744 hir::ExprKind::Match(.., hir::MatchSource::TryDesugar)
1747 err.span_label(cond_expr.span, "expected this to be `()`");
1748 if expr.can_have_side_effects() {
1749 fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
1752 fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
1754 fcx.get_fn_decl(parent_id)
1757 if let Some((fn_decl, can_suggest)) = fn_decl {
1758 if blk_id.is_none() {
1759 pointing_at_return_type |= fcx.suggest_missing_return_type(
1765 fcx.tcx.hir().get_parent_item(id).into(),
1768 if !pointing_at_return_type {
1769 fn_output = Some(&fn_decl.output); // `impl Trait` return type
1773 let parent_id = fcx.tcx.hir().get_parent_item(id);
1774 let parent_item = fcx.tcx.hir().get_by_def_id(parent_id.def_id);
1776 if let (Some(expr), Some(_), Some((fn_decl, _, _))) =
1777 (expression, blk_id, fcx.get_node_fn_decl(parent_item))
1779 fcx.suggest_missing_break_or_return_expr(
1790 let ret_coercion_span = fcx.ret_coercion_span.get();
1792 if let Some(sp) = ret_coercion_span
1793 // If the closure has an explicit return type annotation, or if
1794 // the closure's return type has been inferred from outside
1795 // requirements (such as an Fn* trait bound), then a type error
1796 // may occur at the first return expression we see in the closure
1797 // (if it conflicts with the declared return type). Skip adding a
1798 // note in this case, since it would be incorrect.
1799 && let Some(fn_sig) = fcx.body_fn_sig()
1800 && fn_sig.output().is_ty_var()
1805 "return type inferred to be `{}` here",
1811 if let (Some(sp), Some(fn_output)) = (ret_coercion_span, fn_output) {
1812 self.add_impl_trait_explanation(&mut err, cause, fcx, expected, sp, fn_output);
1818 fn add_impl_trait_explanation<'a>(
1820 err: &mut Diagnostic,
1821 cause: &ObligationCause<'tcx>,
1822 fcx: &FnCtxt<'a, 'tcx>,
1825 fn_output: &hir::FnRetTy<'_>,
1827 let return_sp = fn_output.span();
1828 err.span_label(return_sp, "expected because this return type...");
1831 format!("...is found to be `{}` here", fcx.resolve_vars_with_obligations(expected)),
1833 let impl_trait_msg = "for information on `impl Trait`, see \
1834 <https://doc.rust-lang.org/book/ch10-02-traits.html\
1835 #returning-types-that-implement-traits>";
1836 let trait_obj_msg = "for information on trait objects, see \
1837 <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
1838 #using-trait-objects-that-allow-for-values-of-different-types>";
1839 err.note("to return `impl Trait`, all returned values must be of the same type");
1840 err.note(impl_trait_msg);
1845 .span_to_snippet(return_sp)
1846 .unwrap_or_else(|_| "dyn Trait".to_string());
1847 let mut snippet_iter = snippet.split_whitespace();
1848 let has_impl = snippet_iter.next().map_or(false, |s| s == "impl");
1849 // Only suggest `Box<dyn Trait>` if `Trait` in `impl Trait` is object safe.
1850 let mut is_object_safe = false;
1851 if let hir::FnRetTy::Return(ty) = fn_output
1852 // Get the return type.
1853 && let hir::TyKind::OpaqueDef(..) = ty.kind
1855 let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty);
1856 // Get the `impl Trait`'s `DefId`.
1857 if let ty::Opaque(def_id, _) = ty.kind()
1858 // Get the `impl Trait`'s `Item` so that we can get its trait bounds and
1859 // get the `Trait`'s `DefId`.
1860 && let hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, .. }) =
1861 fcx.tcx.hir().expect_item(def_id.expect_local()).kind
1863 // Are of this `impl Trait`'s traits object safe?
1864 is_object_safe = bounds.iter().all(|bound| {
1867 .and_then(|t| t.trait_def_id())
1868 .map_or(false, |def_id| {
1869 fcx.tcx.object_safety_violations(def_id).is_empty()
1876 err.multipart_suggestion(
1877 "you could change the return type to be a boxed trait object",
1879 (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()),
1880 (return_sp.shrink_to_hi(), ">".to_string()),
1882 Applicability::MachineApplicable,
1884 let sugg = [sp, cause.span]
1888 (sp.shrink_to_lo(), "Box::new(".to_string()),
1889 (sp.shrink_to_hi(), ")".to_string()),
1893 .collect::<Vec<_>>();
1894 err.multipart_suggestion(
1895 "if you change the return type to expect trait objects, box the returned \
1898 Applicability::MaybeIncorrect,
1902 "if the trait `{}` were object safe, you could return a boxed trait object",
1906 err.note(trait_obj_msg);
1908 err.help("you could instead create a new `enum` with a variant for each returned type");
1911 fn is_return_ty_unsized<'a>(&self, fcx: &FnCtxt<'a, 'tcx>, blk_id: hir::HirId) -> bool {
1912 if let Some((fn_decl, _)) = fcx.get_fn_decl(blk_id)
1913 && let hir::FnRetTy::Return(ty) = fn_decl.output
1914 && let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty)
1915 && let ty::Dynamic(..) = ty.kind()
1922 pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
1923 if let Some(final_ty) = self.final_ty {
1926 // If we only had inputs that were of type `!` (or no
1927 // inputs at all), then the final type is `!`.
1928 assert_eq!(self.pushed, 0);
1934 /// Something that can be converted into an expression to which we can
1935 /// apply a coercion.
1936 pub trait AsCoercionSite {
1937 fn as_coercion_site(&self) -> &hir::Expr<'_>;
1940 impl AsCoercionSite for hir::Expr<'_> {
1941 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1946 impl<'a, T> AsCoercionSite for &'a T
1950 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1951 (**self).as_coercion_site()
1955 impl AsCoercionSite for ! {
1956 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1961 impl AsCoercionSite for hir::Arm<'_> {
1962 fn as_coercion_site(&self) -> &hir::Expr<'_> {