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, 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);
219 ty::Dynamic(predicates, region, ty::DynStar) if self.tcx.features().dyn_star => {
220 return self.coerce_dyn_star(a, b, predicates, region);
227 // Function items are coercible to any closure
228 // type; function pointers are not (that would
229 // require double indirection).
230 // Additionally, we permit coercion of function
231 // items to drop the unsafe qualifier.
232 self.coerce_from_fn_item(a, b)
235 // We permit coercion of fn pointers to drop the
237 self.coerce_from_fn_pointer(a, a_f, b)
239 ty::Closure(closure_def_id_a, substs_a) => {
240 // Non-capturing closures are coercible to
241 // function pointers or unsafe function pointers.
242 // It cannot convert closures that require unsafe.
243 self.coerce_closure_to_fn(a, closure_def_id_a, substs_a, b)
246 // Otherwise, just use unification rules.
247 self.unify_and(a, b, identity)
252 /// Coercing *from* an inference variable. In this case, we have no information
253 /// about the source type, so we can't really do a true coercion and we always
254 /// fall back to subtyping (`unify_and`).
255 fn coerce_from_inference_variable(
259 make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
260 ) -> CoerceResult<'tcx> {
261 debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
262 assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
263 assert!(self.shallow_resolve(b) == b);
266 // Two unresolved type variables: create a `Coerce` predicate.
267 let target_ty = if self.use_lub {
268 self.next_ty_var(TypeVariableOrigin {
269 kind: TypeVariableOriginKind::LatticeVariable,
270 span: self.cause.span,
276 let mut obligations = Vec::with_capacity(2);
277 for &source_ty in &[a, b] {
278 if source_ty != target_ty {
279 obligations.push(Obligation::new(
282 ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate {
286 .to_predicate(self.tcx()),
292 "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
293 target_ty, obligations
295 let adjustments = make_adjustments(target_ty);
296 InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
298 // One unresolved type variable: just apply subtyping, we may be able
299 // to do something useful.
300 self.unify_and(a, b, make_adjustments)
304 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
305 /// To match `A` with `B`, autoderef will be performed,
306 /// calling `deref`/`deref_mut` where necessary.
307 fn coerce_borrowed_pointer(
311 r_b: ty::Region<'tcx>,
312 mutbl_b: hir::Mutability,
313 ) -> CoerceResult<'tcx> {
314 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
316 // If we have a parameter of type `&M T_a` and the value
317 // provided is `expr`, we will be adding an implicit borrow,
318 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
319 // to type check, we will construct the type that `&M*expr` would
322 let (r_a, mt_a) = match *a.kind() {
323 ty::Ref(r_a, ty, mutbl) => {
324 let mt_a = ty::TypeAndMut { ty, mutbl };
325 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
328 _ => return self.unify_and(a, b, identity),
331 let span = self.cause.span;
333 let mut first_error = None;
334 let mut r_borrow_var = None;
335 let mut autoderef = self.autoderef(span, a);
336 let mut found = None;
338 for (referent_ty, autoderefs) in autoderef.by_ref() {
340 // Don't let this pass, otherwise it would cause
341 // &T to autoref to &&T.
345 // At this point, we have deref'd `a` to `referent_ty`. So
346 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
347 // In the autoderef loop for `&'a mut Vec<T>`, we would get
350 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
351 // - `Vec<T>` -- 1 deref
352 // - `[T]` -- 2 deref
354 // At each point after the first callback, we want to
355 // check to see whether this would match out target type
356 // (`&'b mut [T]`) if we autoref'd it. We can't just
357 // compare the referent types, though, because we still
358 // have to consider the mutability. E.g., in the case
359 // we've been considering, we have an `&mut` reference, so
360 // the `T` in `[T]` needs to be unified with equality.
362 // Therefore, we construct reference types reflecting what
363 // the types will be after we do the final auto-ref and
364 // compare those. Note that this means we use the target
365 // mutability [1], since it may be that we are coercing
366 // from `&mut T` to `&U`.
368 // One fine point concerns the region that we use. We
369 // choose the region such that the region of the final
370 // type that results from `unify` will be the region we
371 // want for the autoref:
373 // - if in sub mode, that means we want to use `'b` (the
374 // region from the target reference) for both
375 // pointers [2]. This is because sub mode (somewhat
376 // arbitrarily) returns the subtype region. In the case
377 // where we are coercing to a target type, we know we
378 // want to use that target type region (`'b`) because --
379 // for the program to type-check -- it must be the
380 // smaller of the two.
381 // - One fine point. It may be surprising that we can
382 // use `'b` without relating `'a` and `'b`. The reason
383 // that this is ok is that what we produce is
384 // effectively a `&'b *x` expression (if you could
385 // annotate the region of a borrow), and regionck has
386 // code that adds edges from the region of a borrow
387 // (`'b`, here) into the regions in the borrowed
388 // expression (`*x`, here). (Search for "link".)
389 // - if in lub mode, things can get fairly complicated. The
390 // easiest thing is just to make a fresh
391 // region variable [4], which effectively means we defer
392 // the decision to region inference (and regionck, which will add
393 // some more edges to this variable). However, this can wind up
394 // creating a crippling number of variables in some cases --
395 // e.g., #32278 -- so we optimize one particular case [3].
396 // Let me try to explain with some examples:
397 // - The "running example" above represents the simple case,
398 // where we have one `&` reference at the outer level and
399 // ownership all the rest of the way down. In this case,
400 // we want `LUB('a, 'b)` as the resulting region.
401 // - However, if there are nested borrows, that region is
402 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
403 // `&'b T`. In this case, `'a` is actually irrelevant.
404 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
405 // we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
406 // (The errors actually show up in borrowck, typically, because
407 // this extra edge causes the region `'a` to be inferred to something
408 // too big, which then results in borrowck errors.)
409 // - We could track the innermost shared reference, but there is already
410 // code in regionck that has the job of creating links between
411 // the region of a borrow and the regions in the thing being
412 // borrowed (here, `'a` and `'x`), and it knows how to handle
413 // all the various cases. So instead we just make a region variable
414 // and let regionck figure it out.
415 let r = if !self.use_lub {
417 } else if autoderefs == 1 {
420 if r_borrow_var.is_none() {
421 // create var lazily, at most once
422 let coercion = Coercion(span);
423 let r = self.next_region_var(coercion);
424 r_borrow_var = Some(r); // [4] above
426 r_borrow_var.unwrap()
428 let derefd_ty_a = self.tcx.mk_ref(
432 mutbl: mutbl_b, // [1] above
435 match self.unify(derefd_ty_a, b) {
441 if first_error.is_none() {
442 first_error = Some(err);
448 // Extract type or return an error. We return the first error
449 // we got, which should be from relating the "base" type
450 // (e.g., in example above, the failure from relating `Vec<T>`
451 // to the target type), since that should be the least
453 let Some(InferOk { value: ty, mut obligations }) = found else {
454 let err = first_error.expect("coerce_borrowed_pointer had no error");
455 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
459 if ty == a && mt_a.mutbl == hir::Mutability::Not && autoderef.step_count() == 1 {
460 // As a special case, if we would produce `&'a *x`, that's
461 // a total no-op. We end up with the type `&'a T` just as
462 // we started with. In that case, just skip it
463 // altogether. This is just an optimization.
465 // Note that for `&mut`, we DO want to reborrow --
466 // otherwise, this would be a move, which might be an
467 // error. For example `foo(self.x)` where `self` and
468 // `self.x` both have `&mut `type would be a move of
469 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
470 // which is a borrow.
471 assert_eq!(mutbl_b, hir::Mutability::Not); // can only coerce &T -> &U
472 return success(vec![], ty, obligations);
475 let InferOk { value: mut adjustments, obligations: o } =
476 self.adjust_steps_as_infer_ok(&autoderef);
477 obligations.extend(o);
478 obligations.extend(autoderef.into_obligations());
480 // Now apply the autoref. We have to extract the region out of
481 // the final ref type we got.
482 let ty::Ref(r_borrow, _, _) = ty.kind() else {
483 span_bug!(span, "expected a ref type, got {:?}", ty);
485 let mutbl = match mutbl_b {
486 hir::Mutability::Not => AutoBorrowMutability::Not,
487 hir::Mutability::Mut => {
488 AutoBorrowMutability::Mut { allow_two_phase_borrow: self.allow_two_phase }
491 adjustments.push(Adjustment {
492 kind: Adjust::Borrow(AutoBorrow::Ref(*r_borrow, mutbl)),
496 debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
498 success(adjustments, ty, obligations)
501 // &[T; n] or &mut [T; n] -> &[T]
502 // or &mut [T; n] -> &mut [T]
503 // or &Concrete -> &Trait, etc.
504 #[instrument(skip(self), level = "debug")]
505 fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
506 source = self.shallow_resolve(source);
507 target = self.shallow_resolve(target);
508 debug!(?source, ?target);
510 // These 'if' statements require some explanation.
511 // The `CoerceUnsized` trait is special - it is only
512 // possible to write `impl CoerceUnsized<B> for A` where
513 // A and B have 'matching' fields. This rules out the following
514 // two types of blanket impls:
516 // `impl<T> CoerceUnsized<T> for SomeType`
517 // `impl<T> CoerceUnsized<SomeType> for T`
519 // Both of these trigger a special `CoerceUnsized`-related error (E0376)
521 // We can take advantage of this fact to avoid performing unnecessary work.
522 // If either `source` or `target` is a type variable, then any applicable impl
523 // would need to be generic over the self-type (`impl<T> CoerceUnsized<SomeType> for T`)
524 // or generic over the `CoerceUnsized` type parameter (`impl<T> CoerceUnsized<T> for
527 // However, these are exactly the kinds of impls which are forbidden by
528 // the compiler! Therefore, we can be sure that coercion will always fail
529 // when either the source or target type is a type variable. This allows us
530 // to skip performing any trait selection, and immediately bail out.
531 if source.is_ty_var() {
532 debug!("coerce_unsized: source is a TyVar, bailing out");
533 return Err(TypeError::Mismatch);
535 if target.is_ty_var() {
536 debug!("coerce_unsized: target is a TyVar, bailing out");
537 return Err(TypeError::Mismatch);
541 (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
542 let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
543 debug!("missing Unsize or CoerceUnsized traits");
544 return Err(TypeError::Mismatch);
547 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
548 // a DST unless we have to. This currently comes out in the wash since
549 // we can't unify [T] with U. But to properly support DST, we need to allow
550 // that, at which point we will need extra checks on the target here.
552 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
553 let reborrow = match (source.kind(), target.kind()) {
554 (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
555 coerce_mutbls(mutbl_a, mutbl_b)?;
557 let coercion = Coercion(self.cause.span);
558 let r_borrow = self.next_region_var(coercion);
559 let mutbl = match mutbl_b {
560 hir::Mutability::Not => AutoBorrowMutability::Not,
561 hir::Mutability::Mut => AutoBorrowMutability::Mut {
562 // We don't allow two-phase borrows here, at least for initial
563 // implementation. If it happens that this coercion is a function argument,
564 // the reborrow in coerce_borrowed_ptr will pick it up.
565 allow_two_phase_borrow: AllowTwoPhase::No,
569 Adjustment { kind: Adjust::Deref(None), target: ty_a },
571 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
574 .mk_ref(r_borrow, ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a }),
578 (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
579 coerce_mutbls(mt_a, mt_b)?;
582 Adjustment { kind: Adjust::Deref(None), target: ty_a },
584 kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
585 target: self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mt_b, ty: ty_a }),
591 let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
593 // Setup either a subtyping or a LUB relationship between
594 // the `CoerceUnsized` target type and the expected type.
595 // We only have the latter, so we use an inference variable
596 // for the former and let type inference do the rest.
597 let origin = TypeVariableOrigin {
598 kind: TypeVariableOriginKind::MiscVariable,
599 span: self.cause.span,
601 let coerce_target = self.next_ty_var(origin);
602 let mut coercion = self.unify_and(coerce_target, target, |target| {
603 let unsize = Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target };
605 None => vec![unsize],
606 Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
610 let mut selcx = traits::SelectionContext::new(self);
612 // Create an obligation for `Source: CoerceUnsized<Target>`.
613 let cause = ObligationCause::new(
616 ObligationCauseCode::Coercion { source, target },
619 // Use a FIFO queue for this custom fulfillment procedure.
621 // A Vec (or SmallVec) is not a natural choice for a queue. However,
622 // this code path is hot, and this queue usually has a max length of 1
623 // and almost never more than 3. By using a SmallVec we avoid an
624 // allocation, at the (very small) cost of (occasionally) having to
625 // shift subsequent elements down when removing the front element.
626 let mut queue: SmallVec<[_; 4]> = smallvec![traits::predicate_for_trait_def(
633 &[coerce_target.into()]
636 let mut has_unsized_tuple_coercion = false;
637 let mut has_trait_upcasting_coercion = None;
639 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
640 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
641 // inference might unify those two inner type variables later.
642 let traits = [coerce_unsized_did, unsize_did];
643 while !queue.is_empty() {
644 let obligation = queue.remove(0);
645 debug!("coerce_unsized resolve step: {:?}", obligation);
646 let bound_predicate = obligation.predicate.kind();
647 let trait_pred = match bound_predicate.skip_binder() {
648 ty::PredicateKind::Trait(trait_pred) if traits.contains(&trait_pred.def_id()) => {
649 if unsize_did == trait_pred.def_id() {
650 let self_ty = trait_pred.self_ty();
651 let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty();
652 if let (ty::Dynamic(ref data_a, ..), ty::Dynamic(ref data_b, ..)) =
653 (self_ty.kind(), unsize_ty.kind())
654 && data_a.principal_def_id() != data_b.principal_def_id()
656 debug!("coerce_unsized: found trait upcasting coercion");
657 has_trait_upcasting_coercion = Some((self_ty, unsize_ty));
659 if let ty::Tuple(..) = unsize_ty.kind() {
660 debug!("coerce_unsized: found unsized tuple coercion");
661 has_unsized_tuple_coercion = true;
664 bound_predicate.rebind(trait_pred)
667 coercion.obligations.push(obligation);
671 match selcx.select(&obligation.with(trait_pred)) {
672 // Uncertain or unimplemented.
674 if trait_pred.def_id() == unsize_did {
675 let trait_pred = self.resolve_vars_if_possible(trait_pred);
676 let self_ty = trait_pred.skip_binder().self_ty();
677 let unsize_ty = trait_pred.skip_binder().trait_ref.substs[1].expect_ty();
678 debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
679 match (&self_ty.kind(), &unsize_ty.kind()) {
680 (ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
681 if self.type_var_is_sized(*v) =>
683 debug!("coerce_unsized: have sized infer {:?}", v);
684 coercion.obligations.push(obligation);
685 // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
689 // Some other case for `$0: Unsize<Something>`. Note that we
690 // hit this case even if `Something` is a sized type, so just
691 // don't do the coercion.
692 debug!("coerce_unsized: ambiguous unsize");
693 return Err(TypeError::Mismatch);
697 debug!("coerce_unsized: early return - ambiguous");
698 return Err(TypeError::Mismatch);
701 Err(traits::Unimplemented) => {
702 debug!("coerce_unsized: early return - can't prove obligation");
703 return Err(TypeError::Mismatch);
706 // Object safety violations or miscellaneous.
708 self.err_ctxt().report_selection_error(obligation.clone(), &obligation, &err);
709 // Treat this like an obligation and follow through
710 // with the unsizing - the lack of a coercion should
711 // be silent, as it causes a type mismatch later.
714 Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
718 if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
720 &self.tcx.sess.parse_sess,
721 sym::unsized_tuple_coercion,
723 "unsized tuple coercion is not stable enough for use and is subject to change",
728 if let Some((sub, sup)) = has_trait_upcasting_coercion
729 && !self.tcx().features().trait_upcasting
731 // Renders better when we erase regions, since they're not really the point here.
732 let (sub, sup) = self.tcx.erase_regions((sub, sup));
733 let mut err = feature_err(
734 &self.tcx.sess.parse_sess,
735 sym::trait_upcasting,
737 &format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
739 err.note(&format!("required when coercing `{source}` into `{target}`"));
750 predicates: &'tcx ty::List<ty::Binder<'tcx, ty::ExistentialPredicate<'tcx>>>,
751 b_region: ty::Region<'tcx>,
752 ) -> CoerceResult<'tcx> {
753 if !self.tcx.features().dyn_star {
754 return Err(TypeError::Mismatch);
757 if let ty::Dynamic(a_data, _, _) = a.kind()
758 && let ty::Dynamic(b_data, _, _) = b.kind()
760 if a_data.principal_def_id() == b_data.principal_def_id() {
761 return self.unify_and(a, b, |_| vec![]);
762 } else if !self.tcx().features().trait_upcasting {
763 let mut err = feature_err(
764 &self.tcx.sess.parse_sess,
765 sym::trait_upcasting,
768 "cannot cast `{a}` to `{b}`, trait upcasting coercion is experimental"
775 // Check the obligations of the cast -- for example, when casting
776 // `usize` to `dyn* Clone + 'static`:
777 let obligations = predicates
780 // For each existential predicate (e.g., `?Self: Clone`) substitute
781 // the type of the expression (e.g., `usize` in our example above)
782 // and then require that the resulting predicate (e.g., `usize: Clone`)
784 let predicate = predicate.with_self_ty(self.tcx, a);
785 Obligation::new(self.cause.clone(), self.param_env, predicate)
787 // Enforce the region bound (e.g., `usize: 'static`, in our example).
788 .chain([Obligation::new(
791 self.tcx.mk_predicate(ty::Binder::dummy(ty::PredicateKind::TypeOutlives(
792 ty::OutlivesPredicate(a, b_region),
798 value: (vec![Adjustment { kind: Adjust::DynStar, target: b }], b),
803 fn coerce_from_safe_fn<F, G>(
806 fn_ty_a: ty::PolyFnSig<'tcx>,
810 ) -> CoerceResult<'tcx>
812 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
813 G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
815 self.commit_if_ok(|snapshot| {
816 let result = if let ty::FnPtr(fn_ty_b) = b.kind()
817 && let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) =
818 (fn_ty_a.unsafety(), fn_ty_b.unsafety())
820 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
821 self.unify_and(unsafe_a, b, to_unsafe)
823 self.unify_and(a, b, normal)
826 // FIXME(#73154): This is a hack. Currently LUB can generate
827 // unsolvable constraints. Additionally, it returns `a`
828 // unconditionally, even when the "LUB" is `b`. In the future, we
829 // want the coerced type to be the actual supertype of these two,
830 // but for now, we want to just error to ensure we don't lock
831 // ourselves into a specific behavior with NLL.
832 self.leak_check(false, snapshot)?;
838 fn coerce_from_fn_pointer(
841 fn_ty_a: ty::PolyFnSig<'tcx>,
843 ) -> CoerceResult<'tcx> {
844 //! Attempts to coerce from the type of a Rust function item
845 //! into a closure or a `proc`.
848 let b = self.shallow_resolve(b);
849 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
851 self.coerce_from_safe_fn(
855 simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)),
860 fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
861 //! Attempts to coerce from the type of a Rust function item
862 //! into a closure or a `proc`.
864 let b = self.shallow_resolve(b);
865 let InferOk { value: b, mut obligations } =
866 self.normalize_associated_types_in_as_infer_ok(self.cause.span, b);
867 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
870 ty::FnPtr(b_sig) => {
871 let a_sig = a.fn_sig(self.tcx);
872 if let ty::FnDef(def_id, _) = *a.kind() {
873 // Intrinsics are not coercible to function pointers
874 if self.tcx.is_intrinsic(def_id) {
875 return Err(TypeError::IntrinsicCast);
878 // Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).
880 if b_sig.unsafety() == hir::Unsafety::Normal
881 && !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
883 return Err(TypeError::TargetFeatureCast(def_id));
887 let InferOk { value: a_sig, obligations: o1 } =
888 self.normalize_associated_types_in_as_infer_ok(self.cause.span, a_sig);
889 obligations.extend(o1);
891 let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
892 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
899 kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
900 target: a_fn_pointer,
903 kind: Adjust::Pointer(PointerCast::UnsafeFnPointer),
908 simple(Adjust::Pointer(PointerCast::ReifyFnPointer)),
911 obligations.extend(o2);
912 Ok(InferOk { value, obligations })
914 _ => self.unify_and(a, b, identity),
918 fn coerce_closure_to_fn(
921 closure_def_id_a: DefId,
922 substs_a: SubstsRef<'tcx>,
924 ) -> CoerceResult<'tcx> {
925 //! Attempts to coerce from the type of a non-capturing closure
926 //! into a function pointer.
929 let b = self.shallow_resolve(b);
932 // At this point we haven't done capture analysis, which means
933 // that the ClosureSubsts just contains an inference variable instead
934 // of tuple of captured types.
936 // All we care here is if any variable is being captured and not the exact paths,
937 // so we check `upvars_mentioned` for root variables being captured.
941 .upvars_mentioned(closure_def_id_a.expect_local())
942 .map_or(true, |u| u.is_empty()) =>
944 // We coerce the closure, which has fn type
945 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
947 // `fn(arg0,arg1,...) -> _`
949 // `unsafe fn(arg0,arg1,...) -> _`
950 let closure_sig = substs_a.as_closure().sig();
951 let unsafety = fn_ty.unsafety();
953 self.tcx.mk_fn_ptr(self.tcx.signature_unclosure(closure_sig, unsafety));
954 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
958 simple(Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))),
961 _ => self.unify_and(a, b, identity),
965 fn coerce_unsafe_ptr(
969 mutbl_b: hir::Mutability,
970 ) -> CoerceResult<'tcx> {
971 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
973 let (is_ref, mt_a) = match *a.kind() {
974 ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
975 ty::RawPtr(mt) => (false, mt),
976 _ => return self.unify_and(a, b, identity),
978 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
980 // Check that the types which they point at are compatible.
981 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty });
982 // Although references and unsafe ptrs have the same
983 // representation, we still register an Adjust::DerefRef so that
984 // regionck knows that the region for `a` must be valid here.
986 self.unify_and(a_unsafe, b, |target| {
988 Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
989 Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
992 } else if mt_a.mutbl != mutbl_b {
993 self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer)))
995 self.unify_and(a_unsafe, b, identity)
1000 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
1001 /// Attempt to coerce an expression to a type, and return the
1002 /// adjusted type of the expression, if successful.
1003 /// Adjustments are only recorded if the coercion succeeded.
1004 /// The expressions *must not* have any pre-existing adjustments.
1007 expr: &hir::Expr<'_>,
1010 allow_two_phase: AllowTwoPhase,
1011 cause: Option<ObligationCause<'tcx>>,
1012 ) -> RelateResult<'tcx, Ty<'tcx>> {
1013 let source = self.resolve_vars_with_obligations(expr_ty);
1014 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
1017 cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
1018 let coerce = Coerce::new(self, cause, allow_two_phase);
1019 let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
1021 let (adjustments, _) = self.register_infer_ok_obligations(ok);
1022 self.apply_adjustments(expr, adjustments);
1023 Ok(if expr_ty.references_error() { self.tcx.ty_error() } else { target })
1026 /// Same as `try_coerce()`, but without side-effects.
1028 /// Returns false if the coercion creates any obligations that result in
1030 pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
1031 let source = self.resolve_vars_with_obligations(expr_ty);
1032 debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);
1034 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
1035 // We don't ever need two-phase here since we throw out the result of the coercion
1036 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
1038 let Ok(ok) = coerce.coerce(source, target) else {
1041 let mut fcx = traits::FulfillmentContext::new_in_snapshot();
1042 fcx.register_predicate_obligations(self, ok.obligations);
1043 fcx.select_where_possible(&self).is_empty()
1047 /// Given a type and a target type, this function will calculate and return
1048 /// how many dereference steps needed to achieve `expr_ty <: target`. If
1049 /// it's not possible, return `None`.
1050 pub fn deref_steps(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> Option<usize> {
1051 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
1052 // We don't ever need two-phase here since we throw out the result of the coercion
1053 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
1055 .autoderef(rustc_span::DUMMY_SP, expr_ty)
1056 .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
1059 /// Given a type, this function will calculate and return the type given
1060 /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
1062 /// This function is for diagnostics only, since it does not register
1063 /// trait or region sub-obligations. (presumably we could, but it's not
1064 /// particularly important for diagnostics...)
1065 pub fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
1066 self.autoderef(rustc_span::DUMMY_SP, expr_ty).nth(1).and_then(|(deref_ty, _)| {
1068 .type_implements_trait(
1069 self.tcx.lang_items().deref_mut_trait()?,
1079 /// Given some expressions, their known unified type and another expression,
1080 /// tries to unify the types, potentially inserting coercions on any of the
1081 /// provided expressions and returns their LUB (aka "common supertype").
1083 /// This is really an internal helper. From outside the coercion
1084 /// module, you should instantiate a `CoerceMany` instance.
1085 fn try_find_coercion_lub<E>(
1087 cause: &ObligationCause<'tcx>,
1090 new: &hir::Expr<'_>,
1092 ) -> RelateResult<'tcx, Ty<'tcx>>
1096 let prev_ty = self.resolve_vars_with_obligations(prev_ty);
1097 let new_ty = self.resolve_vars_with_obligations(new_ty);
1099 "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
1105 // The following check fixes #88097, where the compiler erroneously
1106 // attempted to coerce a closure type to itself via a function pointer.
1107 if prev_ty == new_ty {
1111 // Special-case that coercion alone cannot handle:
1112 // Function items or non-capturing closures of differing IDs or InternalSubsts.
1113 let (a_sig, b_sig) = {
1114 #[allow(rustc::usage_of_ty_tykind)]
1115 let is_capturing_closure = |ty: &ty::TyKind<'tcx>| {
1116 if let &ty::Closure(closure_def_id, _substs) = ty {
1117 self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some()
1122 if is_capturing_closure(prev_ty.kind()) || is_capturing_closure(new_ty.kind()) {
1125 match (prev_ty.kind(), new_ty.kind()) {
1126 (ty::FnDef(..), ty::FnDef(..)) => {
1127 // Don't reify if the function types have a LUB, i.e., they
1128 // are the same function and their parameters have a LUB.
1130 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1132 // We have a LUB of prev_ty and new_ty, just return it.
1133 Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
1135 (Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx)))
1139 (ty::Closure(_, substs), ty::FnDef(..)) => {
1140 let b_sig = new_ty.fn_sig(self.tcx);
1143 .signature_unclosure(substs.as_closure().sig(), b_sig.unsafety());
1144 (Some(a_sig), Some(b_sig))
1146 (ty::FnDef(..), ty::Closure(_, substs)) => {
1147 let a_sig = prev_ty.fn_sig(self.tcx);
1150 .signature_unclosure(substs.as_closure().sig(), a_sig.unsafety());
1151 (Some(a_sig), Some(b_sig))
1153 (ty::Closure(_, substs_a), ty::Closure(_, substs_b)) => (
1154 Some(self.tcx.signature_unclosure(
1155 substs_a.as_closure().sig(),
1156 hir::Unsafety::Normal,
1158 Some(self.tcx.signature_unclosure(
1159 substs_b.as_closure().sig(),
1160 hir::Unsafety::Normal,
1167 if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) {
1168 // Intrinsics are not coercible to function pointers.
1169 if a_sig.abi() == Abi::RustIntrinsic
1170 || a_sig.abi() == Abi::PlatformIntrinsic
1171 || b_sig.abi() == Abi::RustIntrinsic
1172 || b_sig.abi() == Abi::PlatformIntrinsic
1174 return Err(TypeError::IntrinsicCast);
1176 // The signature must match.
1177 let a_sig = self.normalize_associated_types_in(new.span, a_sig);
1178 let b_sig = self.normalize_associated_types_in(new.span, b_sig);
1180 .at(cause, self.param_env)
1181 .trace(prev_ty, new_ty)
1183 .map(|ok| self.register_infer_ok_obligations(ok))?;
1185 // Reify both sides and return the reified fn pointer type.
1186 let fn_ptr = self.tcx.mk_fn_ptr(sig);
1187 let prev_adjustment = match prev_ty.kind() {
1188 ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(a_sig.unsafety())),
1189 ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
1190 _ => unreachable!(),
1192 let next_adjustment = match new_ty.kind() {
1193 ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(b_sig.unsafety())),
1194 ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
1195 _ => unreachable!(),
1197 for expr in exprs.iter().map(|e| e.as_coercion_site()) {
1198 self.apply_adjustments(
1200 vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
1203 self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
1207 // Configure a Coerce instance to compute the LUB.
1208 // We don't allow two-phase borrows on any autorefs this creates since we
1209 // probably aren't processing function arguments here and even if we were,
1210 // they're going to get autorefed again anyway and we can apply 2-phase borrows
1212 let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
1213 coerce.use_lub = true;
1215 // First try to coerce the new expression to the type of the previous ones,
1216 // but only if the new expression has no coercion already applied to it.
1217 let mut first_error = None;
1218 if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
1219 let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
1222 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1223 self.apply_adjustments(new, adjustments);
1225 "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
1226 new_ty, prev_ty, target
1230 Err(e) => first_error = Some(e),
1234 // Then try to coerce the previous expressions to the type of the new one.
1235 // This requires ensuring there are no coercions applied to *any* of the
1236 // previous expressions, other than noop reborrows (ignoring lifetimes).
1238 let expr = expr.as_coercion_site();
1239 let noop = match self.typeck_results.borrow().expr_adjustments(expr) {
1241 Adjustment { kind: Adjust::Deref(_), .. },
1242 Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. },
1244 match *self.node_ty(expr.hir_id).kind() {
1245 ty::Ref(_, _, mt_orig) => {
1246 let mutbl_adj: hir::Mutability = mutbl_adj.into();
1247 // Reborrow that we can safely ignore, because
1248 // the next adjustment can only be a Deref
1249 // which will be merged into it.
1250 mutbl_adj == mt_orig
1255 &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
1261 "coercion::try_find_coercion_lub: older expression {:?} had adjustments, requiring LUB",
1266 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1267 .map(|ok| self.register_infer_ok_obligations(ok));
1271 match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
1273 // Avoid giving strange errors on failed attempts.
1274 if let Some(e) = first_error {
1277 self.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1278 .map(|ok| self.register_infer_ok_obligations(ok))
1282 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1284 let expr = expr.as_coercion_site();
1285 self.apply_adjustments(expr, adjustments.clone());
1288 "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
1289 prev_ty, new_ty, target
1297 /// CoerceMany encapsulates the pattern you should use when you have
1298 /// many expressions that are all getting coerced to a common
1299 /// type. This arises, for example, when you have a match (the result
1300 /// of each arm is coerced to a common type). It also arises in less
1301 /// obvious places, such as when you have many `break foo` expressions
1302 /// that target the same loop, or the various `return` expressions in
1305 /// The basic protocol is as follows:
1307 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
1308 /// This will also serve as the "starting LUB". The expectation is
1309 /// that this type is something which all of the expressions *must*
1310 /// be coercible to. Use a fresh type variable if needed.
1311 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
1312 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
1313 /// unit. This happens for example if you have a `break` with no expression,
1314 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
1315 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
1316 /// from you so that you don't have to worry your pretty head about it.
1317 /// But if an error is reported, the final type will be `err`.
1318 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
1319 /// previously coerced expressions.
1320 /// - When all done, invoke `complete()`. This will return the LUB of
1321 /// all your expressions.
1322 /// - WARNING: I don't believe this final type is guaranteed to be
1323 /// related to your initial `expected_ty` in any particular way,
1324 /// although it will typically be a subtype, so you should check it.
1325 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
1326 /// previously coerced expressions.
1330 /// ```ignore (illustrative)
1331 /// let mut coerce = CoerceMany::new(expected_ty);
1332 /// for expr in exprs {
1333 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
1334 /// coerce.coerce(fcx, &cause, expr, expr_ty);
1336 /// let final_ty = coerce.complete(fcx);
1338 pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
1339 expected_ty: Ty<'tcx>,
1340 final_ty: Option<Ty<'tcx>>,
1341 expressions: Expressions<'tcx, 'exprs, E>,
1345 /// The type of a `CoerceMany` that is storing up the expressions into
1346 /// a buffer. We use this in `check/mod.rs` for things like `break`.
1347 pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;
1349 enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
1350 Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
1351 UpFront(&'exprs [E]),
1354 impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
1355 /// The usual case; collect the set of expressions dynamically.
1356 /// If the full set of coercion sites is known before hand,
1357 /// consider `with_coercion_sites()` instead to avoid allocation.
1358 pub fn new(expected_ty: Ty<'tcx>) -> Self {
1359 Self::make(expected_ty, Expressions::Dynamic(vec![]))
1362 /// As an optimization, you can create a `CoerceMany` with a
1363 /// pre-existing slice of expressions. In this case, you are
1364 /// expected to pass each element in the slice to `coerce(...)` in
1365 /// order. This is used with arrays in particular to avoid
1366 /// needlessly cloning the slice.
1367 pub fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
1368 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
1371 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
1372 CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
1375 /// Returns the "expected type" with which this coercion was
1376 /// constructed. This represents the "downward propagated" type
1377 /// that was given to us at the start of typing whatever construct
1378 /// we are typing (e.g., the match expression).
1380 /// Typically, this is used as the expected type when
1381 /// type-checking each of the alternative expressions whose types
1382 /// we are trying to merge.
1383 pub fn expected_ty(&self) -> Ty<'tcx> {
1387 /// Returns the current "merged type", representing our best-guess
1388 /// at the LUB of the expressions we've seen so far (if any). This
1389 /// isn't *final* until you call `self.complete()`, which will return
1390 /// the merged type.
1391 pub fn merged_ty(&self) -> Ty<'tcx> {
1392 self.final_ty.unwrap_or(self.expected_ty)
1395 /// Indicates that the value generated by `expression`, which is
1396 /// of type `expression_ty`, is one of the possibilities that we
1397 /// could coerce from. This will record `expression`, and later
1398 /// calls to `coerce` may come back and add adjustments and things
1402 fcx: &FnCtxt<'a, 'tcx>,
1403 cause: &ObligationCause<'tcx>,
1404 expression: &'tcx hir::Expr<'tcx>,
1405 expression_ty: Ty<'tcx>,
1407 self.coerce_inner(fcx, cause, Some(expression), expression_ty, None, false)
1410 /// Indicates that one of the inputs is a "forced unit". This
1411 /// occurs in a case like `if foo { ... };`, where the missing else
1412 /// generates a "forced unit". Another example is a `loop { break;
1413 /// }`, where the `break` has no argument expression. We treat
1414 /// these cases slightly differently for error-reporting
1415 /// purposes. Note that these tend to correspond to cases where
1416 /// the `()` expression is implicit in the source, and hence we do
1417 /// not take an expression argument.
1419 /// The `augment_error` gives you a chance to extend the error
1420 /// message, in case any results (e.g., we use this to suggest
1421 /// removing a `;`).
1422 pub fn coerce_forced_unit<'a>(
1424 fcx: &FnCtxt<'a, 'tcx>,
1425 cause: &ObligationCause<'tcx>,
1426 augment_error: &mut dyn FnMut(&mut Diagnostic),
1427 label_unit_as_expected: bool,
1434 Some(augment_error),
1435 label_unit_as_expected,
1439 /// The inner coercion "engine". If `expression` is `None`, this
1440 /// is a forced-unit case, and hence `expression_ty` must be
1442 #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
1443 pub(crate) fn coerce_inner<'a>(
1445 fcx: &FnCtxt<'a, 'tcx>,
1446 cause: &ObligationCause<'tcx>,
1447 expression: Option<&'tcx hir::Expr<'tcx>>,
1448 mut expression_ty: Ty<'tcx>,
1449 augment_error: Option<&mut dyn FnMut(&mut Diagnostic)>,
1450 label_expression_as_expected: bool,
1452 // Incorporate whatever type inference information we have
1453 // until now; in principle we might also want to process
1454 // pending obligations, but doing so should only improve
1455 // compatibility (hopefully that is true) by helping us
1456 // uncover never types better.
1457 if expression_ty.is_ty_var() {
1458 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1461 // If we see any error types, just propagate that error
1463 if expression_ty.references_error() || self.merged_ty().references_error() {
1464 self.final_ty = Some(fcx.tcx.ty_error());
1468 // Handle the actual type unification etc.
1469 let result = if let Some(expression) = expression {
1470 if self.pushed == 0 {
1471 // Special-case the first expression we are coercing.
1472 // To be honest, I'm not entirely sure why we do this.
1473 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1479 Some(cause.clone()),
1482 match self.expressions {
1483 Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
1490 Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub(
1492 &coercion_sites[0..self.pushed],
1500 // this is a hack for cases where we default to `()` because
1501 // the expression etc has been omitted from the source. An
1502 // example is an `if let` without an else:
1504 // if let Some(x) = ... { }
1506 // we wind up with a second match arm that is like `_ =>
1507 // ()`. That is the case we are considering here. We take
1508 // a different path to get the right "expected, found"
1509 // message and so forth (and because we know that
1510 // `expression_ty` will be unit).
1512 // Another example is `break` with no argument expression.
1513 assert!(expression_ty.is_unit(), "if let hack without unit type");
1514 fcx.at(cause, fcx.param_env)
1515 .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
1517 fcx.register_infer_ok_obligations(infer_ok);
1525 self.final_ty = Some(v);
1526 if let Some(e) = expression {
1527 match self.expressions {
1528 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1529 Expressions::UpFront(coercion_sites) => {
1530 // if the user gave us an array to validate, check that we got
1531 // the next expression in the list, as expected
1533 coercion_sites[self.pushed].as_coercion_site().hir_id,
1541 Err(coercion_error) => {
1542 // Mark that we've failed to coerce the types here to suppress
1543 // any superfluous errors we might encounter while trying to
1544 // emit or provide suggestions on how to fix the initial error.
1545 fcx.set_tainted_by_errors();
1546 let (expected, found) = if label_expression_as_expected {
1547 // In the case where this is a "forced unit", like
1548 // `break`, we want to call the `()` "expected"
1549 // since it is implied by the syntax.
1550 // (Note: not all force-units work this way.)"
1551 (expression_ty, self.merged_ty())
1553 // Otherwise, the "expected" type for error
1554 // reporting is the current unification type,
1555 // which is basically the LUB of the expressions
1556 // we've seen so far (combined with the expected
1558 (self.merged_ty(), expression_ty)
1560 let (expected, found) = fcx.resolve_vars_if_possible((expected, found));
1563 let mut unsized_return = false;
1564 let mut visitor = CollectRetsVisitor { ret_exprs: vec![] };
1565 match *cause.code() {
1566 ObligationCauseCode::ReturnNoExpression => {
1567 err = struct_span_err!(
1571 "`return;` in a function whose return type is not `()`"
1573 err.span_label(cause.span, "return type is not `()`");
1575 ObligationCauseCode::BlockTailExpression(blk_id) => {
1576 let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
1577 err = self.report_return_mismatched_types(
1581 coercion_error.clone(),
1587 if !fcx.tcx.features().unsized_locals {
1588 unsized_return = self.is_return_ty_unsized(fcx, blk_id);
1590 if let Some(expression) = expression
1591 && let hir::ExprKind::Loop(loop_blk, ..) = expression.kind {
1592 intravisit::walk_block(& mut visitor, loop_blk);
1595 ObligationCauseCode::ReturnValue(id) => {
1596 err = self.report_return_mismatched_types(
1600 coercion_error.clone(),
1606 if !fcx.tcx.features().unsized_locals {
1607 let id = fcx.tcx.hir().get_parent_node(id);
1608 unsized_return = self.is_return_ty_unsized(fcx, id);
1612 err = fcx.err_ctxt().report_mismatched_types(
1616 coercion_error.clone(),
1621 if let Some(augment_error) = augment_error {
1622 augment_error(&mut err);
1625 let is_insufficiently_polymorphic =
1626 matches!(coercion_error, TypeError::RegionsInsufficientlyPolymorphic(..));
1628 if !is_insufficiently_polymorphic && let Some(expr) = expression {
1629 fcx.emit_coerce_suggestions(
1635 Some(coercion_error),
1639 if visitor.ret_exprs.len() > 0 && let Some(expr) = expression {
1640 self.note_unreachable_loop_return(&mut err, &expr, &visitor.ret_exprs);
1642 err.emit_unless(unsized_return);
1644 self.final_ty = Some(fcx.tcx.ty_error());
1648 fn note_unreachable_loop_return(
1650 err: &mut Diagnostic,
1651 expr: &hir::Expr<'tcx>,
1652 ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>,
1654 let hir::ExprKind::Loop(_, _, _, loop_span) = expr.kind else { return;};
1655 let mut span: MultiSpan = vec![loop_span].into();
1656 span.push_span_label(loop_span, "this might have zero elements to iterate on");
1657 const MAXITER: usize = 3;
1658 let iter = ret_exprs.iter().take(MAXITER);
1659 for ret_expr in iter {
1660 span.push_span_label(
1662 "if the loop doesn't execute, this value would never get returned",
1667 "the function expects a value to always be returned, but loops might run zero times",
1669 if MAXITER < ret_exprs.len() {
1671 "if the loop doesn't execute, {} other values would never get returned",
1672 ret_exprs.len() - MAXITER
1676 "return a value for the case when the loop has zero elements to iterate on, or \
1677 consider changing the return type to account for that possibility",
1681 fn report_return_mismatched_types<'a>(
1683 cause: &ObligationCause<'tcx>,
1686 ty_err: TypeError<'tcx>,
1687 fcx: &FnCtxt<'a, 'tcx>,
1689 expression: Option<&'tcx hir::Expr<'tcx>>,
1690 blk_id: Option<hir::HirId>,
1691 ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
1692 let mut err = fcx.err_ctxt().report_mismatched_types(cause, expected, found, ty_err);
1694 let mut pointing_at_return_type = false;
1695 let mut fn_output = None;
1697 let parent_id = fcx.tcx.hir().get_parent_node(id);
1698 let parent = fcx.tcx.hir().get(parent_id);
1699 if let Some(expr) = expression
1700 && let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(&hir::Closure { body, .. }), .. }) = parent
1701 && !matches!(fcx.tcx.hir().body(body).value.kind, hir::ExprKind::Block(..))
1703 fcx.suggest_missing_semicolon(&mut err, expr, expected, true);
1705 // Verify that this is a tail expression of a function, otherwise the
1706 // label pointing out the cause for the type coercion will be wrong
1707 // as prior return coercions would not be relevant (#57664).
1708 let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) {
1709 pointing_at_return_type =
1710 fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id);
1711 if let (Some(cond_expr), true, false) = (
1712 fcx.tcx.hir().get_if_cause(expr.hir_id),
1714 pointing_at_return_type,
1716 // If the block is from an external macro or try (`?`) desugaring, then
1717 // do not suggest adding a semicolon, because there's nowhere to put it.
1718 // See issues #81943 and #87051.
1720 cond_expr.span.desugaring_kind(),
1721 None | Some(DesugaringKind::WhileLoop)
1722 ) && !in_external_macro(fcx.tcx.sess, cond_expr.span)
1725 hir::ExprKind::Match(.., hir::MatchSource::TryDesugar)
1728 err.span_label(cond_expr.span, "expected this to be `()`");
1729 if expr.can_have_side_effects() {
1730 fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
1733 fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
1735 fcx.get_fn_decl(parent_id)
1738 if let Some((fn_decl, can_suggest)) = fn_decl {
1739 if blk_id.is_none() {
1740 pointing_at_return_type |= fcx.suggest_missing_return_type(
1746 fcx.tcx.hir().get_parent_item(id).into(),
1749 if !pointing_at_return_type {
1750 fn_output = Some(&fn_decl.output); // `impl Trait` return type
1754 let parent_id = fcx.tcx.hir().get_parent_item(id);
1755 let parent_item = fcx.tcx.hir().get_by_def_id(parent_id.def_id);
1757 if let (Some(expr), Some(_), Some((fn_decl, _, _))) =
1758 (expression, blk_id, fcx.get_node_fn_decl(parent_item))
1760 fcx.suggest_missing_break_or_return_expr(
1771 let ret_coercion_span = fcx.ret_coercion_span.get();
1773 if let Some(sp) = ret_coercion_span
1774 // If the closure has an explicit return type annotation, or if
1775 // the closure's return type has been inferred from outside
1776 // requirements (such as an Fn* trait bound), then a type error
1777 // may occur at the first return expression we see in the closure
1778 // (if it conflicts with the declared return type). Skip adding a
1779 // note in this case, since it would be incorrect.
1780 && let Some(fn_sig) = fcx.body_fn_sig()
1781 && fn_sig.output().is_ty_var()
1786 "return type inferred to be `{}` here",
1792 if let (Some(sp), Some(fn_output)) = (ret_coercion_span, fn_output) {
1793 self.add_impl_trait_explanation(&mut err, cause, fcx, expected, sp, fn_output);
1799 fn add_impl_trait_explanation<'a>(
1801 err: &mut Diagnostic,
1802 cause: &ObligationCause<'tcx>,
1803 fcx: &FnCtxt<'a, 'tcx>,
1806 fn_output: &hir::FnRetTy<'_>,
1808 let return_sp = fn_output.span();
1809 err.span_label(return_sp, "expected because this return type...");
1812 format!("...is found to be `{}` here", fcx.resolve_vars_with_obligations(expected)),
1814 let impl_trait_msg = "for information on `impl Trait`, see \
1815 <https://doc.rust-lang.org/book/ch10-02-traits.html\
1816 #returning-types-that-implement-traits>";
1817 let trait_obj_msg = "for information on trait objects, see \
1818 <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
1819 #using-trait-objects-that-allow-for-values-of-different-types>";
1820 err.note("to return `impl Trait`, all returned values must be of the same type");
1821 err.note(impl_trait_msg);
1826 .span_to_snippet(return_sp)
1827 .unwrap_or_else(|_| "dyn Trait".to_string());
1828 let mut snippet_iter = snippet.split_whitespace();
1829 let has_impl = snippet_iter.next().map_or(false, |s| s == "impl");
1830 // Only suggest `Box<dyn Trait>` if `Trait` in `impl Trait` is object safe.
1831 let mut is_object_safe = false;
1832 if let hir::FnRetTy::Return(ty) = fn_output
1833 // Get the return type.
1834 && let hir::TyKind::OpaqueDef(..) = ty.kind
1836 let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty);
1837 // Get the `impl Trait`'s `DefId`.
1838 if let ty::Opaque(def_id, _) = ty.kind()
1839 // Get the `impl Trait`'s `Item` so that we can get its trait bounds and
1840 // get the `Trait`'s `DefId`.
1841 && let hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, .. }) =
1842 fcx.tcx.hir().expect_item(def_id.expect_local()).kind
1844 // Are of this `impl Trait`'s traits object safe?
1845 is_object_safe = bounds.iter().all(|bound| {
1848 .and_then(|t| t.trait_def_id())
1849 .map_or(false, |def_id| {
1850 fcx.tcx.object_safety_violations(def_id).is_empty()
1857 err.multipart_suggestion(
1858 "you could change the return type to be a boxed trait object",
1860 (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()),
1861 (return_sp.shrink_to_hi(), ">".to_string()),
1863 Applicability::MachineApplicable,
1865 let sugg = [sp, cause.span]
1869 (sp.shrink_to_lo(), "Box::new(".to_string()),
1870 (sp.shrink_to_hi(), ")".to_string()),
1874 .collect::<Vec<_>>();
1875 err.multipart_suggestion(
1876 "if you change the return type to expect trait objects, box the returned \
1879 Applicability::MaybeIncorrect,
1883 "if the trait `{}` were object safe, you could return a boxed trait object",
1887 err.note(trait_obj_msg);
1889 err.help("you could instead create a new `enum` with a variant for each returned type");
1892 fn is_return_ty_unsized<'a>(&self, fcx: &FnCtxt<'a, 'tcx>, blk_id: hir::HirId) -> bool {
1893 if let Some((fn_decl, _)) = fcx.get_fn_decl(blk_id)
1894 && let hir::FnRetTy::Return(ty) = fn_decl.output
1895 && let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty)
1896 && let ty::Dynamic(..) = ty.kind()
1903 pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
1904 if let Some(final_ty) = self.final_ty {
1907 // If we only had inputs that were of type `!` (or no
1908 // inputs at all), then the final type is `!`.
1909 assert_eq!(self.pushed, 0);
1915 /// Something that can be converted into an expression to which we can
1916 /// apply a coercion.
1917 pub trait AsCoercionSite {
1918 fn as_coercion_site(&self) -> &hir::Expr<'_>;
1921 impl AsCoercionSite for hir::Expr<'_> {
1922 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1927 impl<'a, T> AsCoercionSite for &'a T
1931 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1932 (**self).as_coercion_site()
1936 impl AsCoercionSite for ! {
1937 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1942 impl AsCoercionSite for hir::Arm<'_> {
1943 fn as_coercion_site(&self) -> &hir::Expr<'_> {