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;
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::{self, ObligationCause, ObligationCauseCode, ObligationCtxt};
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 if from_mutbl >= to_mutbl { Ok(()) } else { Err(TypeError::Mutability) }
114 /// Do not require any adjustments, i.e. coerce `x -> x`.
115 fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> {
119 fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
120 move |target| vec![Adjustment { kind, target }]
123 /// This always returns `Ok(...)`.
125 adj: Vec<Adjustment<'tcx>>,
127 obligations: traits::PredicateObligations<'tcx>,
128 ) -> CoerceResult<'tcx> {
129 Ok(InferOk { value: (adj, target), obligations })
132 impl<'f, 'tcx> Coerce<'f, 'tcx> {
134 fcx: &'f FnCtxt<'f, 'tcx>,
135 cause: ObligationCause<'tcx>,
136 allow_two_phase: AllowTwoPhase,
138 Coerce { fcx, cause, allow_two_phase, use_lub: false }
141 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
142 debug!("unify(a: {:?}, b: {:?}, use_lub: {})", a, b, self.use_lub);
143 self.commit_if_ok(|_| {
145 self.at(&self.cause, self.fcx.param_env).lub(b, a)
147 self.at(&self.cause, self.fcx.param_env)
149 .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
154 /// Unify two types (using sub or lub) and produce a specific coercion.
155 fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
157 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
160 .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
163 #[instrument(skip(self))]
164 fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
165 // First, remove any resolved type variables (at the top level, at least):
166 let a = self.shallow_resolve(a);
167 let b = self.shallow_resolve(b);
168 debug!("Coerce.tys({:?} => {:?})", a, b);
170 // Just ignore error types.
171 if a.references_error() || b.references_error() {
172 return success(vec![], self.fcx.tcx.ty_error(), vec![]);
175 // Coercing from `!` to any type is allowed:
177 return success(simple(Adjust::NeverToAny)(b), b, vec![]);
180 // Coercing *from* an unresolved inference variable means that
181 // we have no information about the source type. This will always
182 // ultimately fall back to some form of subtyping.
184 return self.coerce_from_inference_variable(a, b, identity);
187 // Consider coercing the subtype to a DST
189 // NOTE: this is wrapped in a `commit_if_ok` because it creates
190 // a "spurious" type variable, and we don't want to have that
191 // type variable in memory if the coercion fails.
192 let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
195 debug!("coerce: unsize successful");
198 Err(TypeError::ObjectUnsafeCoercion(did)) => {
199 debug!("coerce: unsize not object safe");
200 return Err(TypeError::ObjectUnsafeCoercion(did));
203 debug!(?error, "coerce: unsize failed");
207 // Examine the supertype and consider auto-borrowing.
209 ty::RawPtr(mt_b) => {
210 return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
212 ty::Ref(r_b, _, mutbl_b) => {
213 return self.coerce_borrowed_pointer(a, b, r_b, mutbl_b);
215 ty::Dynamic(predicates, region, ty::DynStar) if self.tcx.features().dyn_star => {
216 return self.coerce_dyn_star(a, b, predicates, region);
223 // Function items are coercible to any closure
224 // type; function pointers are not (that would
225 // require double indirection).
226 // Additionally, we permit coercion of function
227 // items to drop the unsafe qualifier.
228 self.coerce_from_fn_item(a, b)
231 // We permit coercion of fn pointers to drop the
233 self.coerce_from_fn_pointer(a, a_f, b)
235 ty::Closure(closure_def_id_a, substs_a) => {
236 // Non-capturing closures are coercible to
237 // function pointers or unsafe function pointers.
238 // It cannot convert closures that require unsafe.
239 self.coerce_closure_to_fn(a, closure_def_id_a, substs_a, b)
242 // Otherwise, just use unification rules.
243 self.unify_and(a, b, identity)
248 /// Coercing *from* an inference variable. In this case, we have no information
249 /// about the source type, so we can't really do a true coercion and we always
250 /// fall back to subtyping (`unify_and`).
251 fn coerce_from_inference_variable(
255 make_adjustments: impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
256 ) -> CoerceResult<'tcx> {
257 debug!("coerce_from_inference_variable(a={:?}, b={:?})", a, b);
258 assert!(a.is_ty_var() && self.shallow_resolve(a) == a);
259 assert!(self.shallow_resolve(b) == b);
262 // Two unresolved type variables: create a `Coerce` predicate.
263 let target_ty = if self.use_lub {
264 self.next_ty_var(TypeVariableOrigin {
265 kind: TypeVariableOriginKind::LatticeVariable,
266 span: self.cause.span,
272 let mut obligations = Vec::with_capacity(2);
273 for &source_ty in &[a, b] {
274 if source_ty != target_ty {
275 obligations.push(Obligation::new(
279 ty::Binder::dummy(ty::PredicateKind::Coerce(ty::CoercePredicate {
288 "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
289 target_ty, obligations
291 let adjustments = make_adjustments(target_ty);
292 InferResult::Ok(InferOk { value: (adjustments, target_ty), obligations })
294 // One unresolved type variable: just apply subtyping, we may be able
295 // to do something useful.
296 self.unify_and(a, b, make_adjustments)
300 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
301 /// To match `A` with `B`, autoderef will be performed,
302 /// calling `deref`/`deref_mut` where necessary.
303 fn coerce_borrowed_pointer(
307 r_b: ty::Region<'tcx>,
308 mutbl_b: hir::Mutability,
309 ) -> CoerceResult<'tcx> {
310 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
312 // If we have a parameter of type `&M T_a` and the value
313 // provided is `expr`, we will be adding an implicit borrow,
314 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
315 // to type check, we will construct the type that `&M*expr` would
318 let (r_a, mt_a) = match *a.kind() {
319 ty::Ref(r_a, ty, mutbl) => {
320 let mt_a = ty::TypeAndMut { ty, mutbl };
321 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
324 _ => return self.unify_and(a, b, identity),
327 let span = self.cause.span;
329 let mut first_error = None;
330 let mut r_borrow_var = None;
331 let mut autoderef = self.autoderef(span, a);
332 let mut found = None;
334 for (referent_ty, autoderefs) in autoderef.by_ref() {
336 // Don't let this pass, otherwise it would cause
337 // &T to autoref to &&T.
341 // At this point, we have deref'd `a` to `referent_ty`. So
342 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
343 // In the autoderef loop for `&'a mut Vec<T>`, we would get
346 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
347 // - `Vec<T>` -- 1 deref
348 // - `[T]` -- 2 deref
350 // At each point after the first callback, we want to
351 // check to see whether this would match out target type
352 // (`&'b mut [T]`) if we autoref'd it. We can't just
353 // compare the referent types, though, because we still
354 // have to consider the mutability. E.g., in the case
355 // we've been considering, we have an `&mut` reference, so
356 // the `T` in `[T]` needs to be unified with equality.
358 // Therefore, we construct reference types reflecting what
359 // the types will be after we do the final auto-ref and
360 // compare those. Note that this means we use the target
361 // mutability [1], since it may be that we are coercing
362 // from `&mut T` to `&U`.
364 // One fine point concerns the region that we use. We
365 // choose the region such that the region of the final
366 // type that results from `unify` will be the region we
367 // want for the autoref:
369 // - if in sub mode, that means we want to use `'b` (the
370 // region from the target reference) for both
371 // pointers [2]. This is because sub mode (somewhat
372 // arbitrarily) returns the subtype region. In the case
373 // where we are coercing to a target type, we know we
374 // want to use that target type region (`'b`) because --
375 // for the program to type-check -- it must be the
376 // smaller of the two.
377 // - One fine point. It may be surprising that we can
378 // use `'b` without relating `'a` and `'b`. The reason
379 // that this is ok is that what we produce is
380 // effectively a `&'b *x` expression (if you could
381 // annotate the region of a borrow), and regionck has
382 // code that adds edges from the region of a borrow
383 // (`'b`, here) into the regions in the borrowed
384 // expression (`*x`, here). (Search for "link".)
385 // - if in lub mode, things can get fairly complicated. The
386 // easiest thing is just to make a fresh
387 // region variable [4], which effectively means we defer
388 // the decision to region inference (and regionck, which will add
389 // some more edges to this variable). However, this can wind up
390 // creating a crippling number of variables in some cases --
391 // e.g., #32278 -- so we optimize one particular case [3].
392 // Let me try to explain with some examples:
393 // - The "running example" above represents the simple case,
394 // where we have one `&` reference at the outer level and
395 // ownership all the rest of the way down. In this case,
396 // we want `LUB('a, 'b)` as the resulting region.
397 // - However, if there are nested borrows, that region is
398 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
399 // `&'b T`. In this case, `'a` is actually irrelevant.
400 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
401 // we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
402 // (The errors actually show up in borrowck, typically, because
403 // this extra edge causes the region `'a` to be inferred to something
404 // too big, which then results in borrowck errors.)
405 // - We could track the innermost shared reference, but there is already
406 // code in regionck that has the job of creating links between
407 // the region of a borrow and the regions in the thing being
408 // borrowed (here, `'a` and `'x`), and it knows how to handle
409 // all the various cases. So instead we just make a region variable
410 // and let regionck figure it out.
411 let r = if !self.use_lub {
413 } else if autoderefs == 1 {
416 if r_borrow_var.is_none() {
417 // create var lazily, at most once
418 let coercion = Coercion(span);
419 let r = self.next_region_var(coercion);
420 r_borrow_var = Some(r); // [4] above
422 r_borrow_var.unwrap()
424 let derefd_ty_a = self.tcx.mk_ref(
428 mutbl: mutbl_b, // [1] above
431 match self.unify(derefd_ty_a, b) {
437 if first_error.is_none() {
438 first_error = Some(err);
444 // Extract type or return an error. We return the first error
445 // we got, which should be from relating the "base" type
446 // (e.g., in example above, the failure from relating `Vec<T>`
447 // to the target type), since that should be the least
449 let Some(InferOk { value: ty, mut obligations }) = found else {
450 let err = first_error.expect("coerce_borrowed_pointer had no error");
451 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
455 if ty == a && mt_a.mutbl.is_not() && autoderef.step_count() == 1 {
456 // As a special case, if we would produce `&'a *x`, that's
457 // a total no-op. We end up with the type `&'a T` just as
458 // we started with. In that case, just skip it
459 // altogether. This is just an optimization.
461 // Note that for `&mut`, we DO want to reborrow --
462 // otherwise, this would be a move, which might be an
463 // error. For example `foo(self.x)` where `self` and
464 // `self.x` both have `&mut `type would be a move of
465 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
466 // which is a borrow.
467 assert!(mutbl_b.is_not()); // can only coerce &T -> &U
468 return success(vec![], ty, obligations);
471 let InferOk { value: mut adjustments, obligations: o } =
472 self.adjust_steps_as_infer_ok(&autoderef);
473 obligations.extend(o);
474 obligations.extend(autoderef.into_obligations());
476 // Now apply the autoref. We have to extract the region out of
477 // the final ref type we got.
478 let ty::Ref(r_borrow, _, _) = ty.kind() else {
479 span_bug!(span, "expected a ref type, got {:?}", ty);
481 let mutbl = AutoBorrowMutability::new(mutbl_b, self.allow_two_phase);
482 adjustments.push(Adjustment {
483 kind: Adjust::Borrow(AutoBorrow::Ref(*r_borrow, mutbl)),
487 debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
489 success(adjustments, ty, obligations)
492 // &[T; n] or &mut [T; n] -> &[T]
493 // or &mut [T; n] -> &mut [T]
494 // or &Concrete -> &Trait, etc.
495 #[instrument(skip(self), level = "debug")]
496 fn coerce_unsized(&self, mut source: Ty<'tcx>, mut target: Ty<'tcx>) -> CoerceResult<'tcx> {
497 source = self.shallow_resolve(source);
498 target = self.shallow_resolve(target);
499 debug!(?source, ?target);
501 // These 'if' statements require some explanation.
502 // The `CoerceUnsized` trait is special - it is only
503 // possible to write `impl CoerceUnsized<B> for A` where
504 // A and B have 'matching' fields. This rules out the following
505 // two types of blanket impls:
507 // `impl<T> CoerceUnsized<T> for SomeType`
508 // `impl<T> CoerceUnsized<SomeType> for T`
510 // Both of these trigger a special `CoerceUnsized`-related error (E0376)
512 // We can take advantage of this fact to avoid performing unnecessary work.
513 // If either `source` or `target` is a type variable, then any applicable impl
514 // would need to be generic over the self-type (`impl<T> CoerceUnsized<SomeType> for T`)
515 // or generic over the `CoerceUnsized` type parameter (`impl<T> CoerceUnsized<T> for
518 // However, these are exactly the kinds of impls which are forbidden by
519 // the compiler! Therefore, we can be sure that coercion will always fail
520 // when either the source or target type is a type variable. This allows us
521 // to skip performing any trait selection, and immediately bail out.
522 if source.is_ty_var() {
523 debug!("coerce_unsized: source is a TyVar, bailing out");
524 return Err(TypeError::Mismatch);
526 if target.is_ty_var() {
527 debug!("coerce_unsized: target is a TyVar, bailing out");
528 return Err(TypeError::Mismatch);
532 (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
533 let (Some(unsize_did), Some(coerce_unsized_did)) = traits else {
534 debug!("missing Unsize or CoerceUnsized traits");
535 return Err(TypeError::Mismatch);
538 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
539 // a DST unless we have to. This currently comes out in the wash since
540 // we can't unify [T] with U. But to properly support DST, we need to allow
541 // that, at which point we will need extra checks on the target here.
543 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
544 let reborrow = match (source.kind(), target.kind()) {
545 (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
546 coerce_mutbls(mutbl_a, mutbl_b)?;
548 let coercion = Coercion(self.cause.span);
549 let r_borrow = self.next_region_var(coercion);
551 // We don't allow two-phase borrows here, at least for initial
552 // implementation. If it happens that this coercion is a function argument,
553 // the reborrow in coerce_borrowed_ptr will pick it up.
554 let mutbl = AutoBorrowMutability::new(mutbl_b, AllowTwoPhase::No);
557 Adjustment { kind: Adjust::Deref(None), target: ty_a },
559 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
562 .mk_ref(r_borrow, ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a }),
566 (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
567 coerce_mutbls(mt_a, mt_b)?;
570 Adjustment { kind: Adjust::Deref(None), target: ty_a },
572 kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
573 target: self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mt_b, ty: ty_a }),
579 let coerce_source = reborrow.as_ref().map_or(source, |(_, r)| r.target);
581 // Setup either a subtyping or a LUB relationship between
582 // the `CoerceUnsized` target type and the expected type.
583 // We only have the latter, so we use an inference variable
584 // for the former and let type inference do the rest.
585 let origin = TypeVariableOrigin {
586 kind: TypeVariableOriginKind::MiscVariable,
587 span: self.cause.span,
589 let coerce_target = self.next_ty_var(origin);
590 let mut coercion = self.unify_and(coerce_target, target, |target| {
591 let unsize = Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target };
593 None => vec![unsize],
594 Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
598 let mut selcx = traits::SelectionContext::new(self);
600 // Create an obligation for `Source: CoerceUnsized<Target>`.
601 let cause = ObligationCause::new(
604 ObligationCauseCode::Coercion { source, target },
607 // Use a FIFO queue for this custom fulfillment procedure.
609 // A Vec (or SmallVec) is not a natural choice for a queue. However,
610 // this code path is hot, and this queue usually has a max length of 1
611 // and almost never more than 3. By using a SmallVec we avoid an
612 // allocation, at the (very small) cost of (occasionally) having to
613 // shift subsequent elements down when removing the front element.
614 let mut queue: SmallVec<[_; 4]> = smallvec![traits::predicate_for_trait_def(
620 [coerce_source, coerce_target]
623 let mut has_unsized_tuple_coercion = false;
624 let mut has_trait_upcasting_coercion = None;
626 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
627 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
628 // inference might unify those two inner type variables later.
629 let traits = [coerce_unsized_did, unsize_did];
630 while !queue.is_empty() {
631 let obligation = queue.remove(0);
632 debug!("coerce_unsized resolve step: {:?}", obligation);
633 let bound_predicate = obligation.predicate.kind();
634 let trait_pred = match bound_predicate.skip_binder() {
635 ty::PredicateKind::Clause(ty::Clause::Trait(trait_pred))
636 if traits.contains(&trait_pred.def_id()) =>
638 if unsize_did == trait_pred.def_id() {
639 let self_ty = trait_pred.self_ty();
640 let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty();
641 if let (ty::Dynamic(ref data_a, ..), ty::Dynamic(ref data_b, ..)) =
642 (self_ty.kind(), unsize_ty.kind())
643 && data_a.principal_def_id() != data_b.principal_def_id()
645 debug!("coerce_unsized: found trait upcasting coercion");
646 has_trait_upcasting_coercion = Some((self_ty, unsize_ty));
648 if let ty::Tuple(..) = unsize_ty.kind() {
649 debug!("coerce_unsized: found unsized tuple coercion");
650 has_unsized_tuple_coercion = true;
653 bound_predicate.rebind(trait_pred)
656 coercion.obligations.push(obligation);
660 match selcx.select(&obligation.with(selcx.tcx(), trait_pred)) {
661 // Uncertain or unimplemented.
663 if trait_pred.def_id() == unsize_did {
664 let trait_pred = self.resolve_vars_if_possible(trait_pred);
665 let self_ty = trait_pred.skip_binder().self_ty();
666 let unsize_ty = trait_pred.skip_binder().trait_ref.substs[1].expect_ty();
667 debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred);
668 match (&self_ty.kind(), &unsize_ty.kind()) {
669 (ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
670 if self.type_var_is_sized(*v) =>
672 debug!("coerce_unsized: have sized infer {:?}", v);
673 coercion.obligations.push(obligation);
674 // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
678 // Some other case for `$0: Unsize<Something>`. Note that we
679 // hit this case even if `Something` is a sized type, so just
680 // don't do the coercion.
681 debug!("coerce_unsized: ambiguous unsize");
682 return Err(TypeError::Mismatch);
686 debug!("coerce_unsized: early return - ambiguous");
687 return Err(TypeError::Mismatch);
690 Err(traits::Unimplemented) => {
691 debug!("coerce_unsized: early return - can't prove obligation");
692 return Err(TypeError::Mismatch);
695 // Object safety violations or miscellaneous.
697 self.err_ctxt().report_selection_error(obligation.clone(), &obligation, &err);
698 // Treat this like an obligation and follow through
699 // with the unsizing - the lack of a coercion should
700 // be silent, as it causes a type mismatch later.
703 Ok(Some(impl_source)) => queue.extend(impl_source.nested_obligations()),
707 if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
709 &self.tcx.sess.parse_sess,
710 sym::unsized_tuple_coercion,
712 "unsized tuple coercion is not stable enough for use and is subject to change",
717 if let Some((sub, sup)) = has_trait_upcasting_coercion
718 && !self.tcx().features().trait_upcasting
720 // Renders better when we erase regions, since they're not really the point here.
721 let (sub, sup) = self.tcx.erase_regions((sub, sup));
722 let mut err = feature_err(
723 &self.tcx.sess.parse_sess,
724 sym::trait_upcasting,
726 &format!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
728 err.note(&format!("required when coercing `{source}` into `{target}`"));
739 predicates: &'tcx ty::List<ty::PolyExistentialPredicate<'tcx>>,
740 b_region: ty::Region<'tcx>,
741 ) -> CoerceResult<'tcx> {
742 if !self.tcx.features().dyn_star {
743 return Err(TypeError::Mismatch);
746 if let ty::Dynamic(a_data, _, _) = a.kind()
747 && let ty::Dynamic(b_data, _, _) = b.kind()
748 && a_data.principal_def_id() == b_data.principal_def_id()
750 return self.unify_and(a, b, |_| vec![]);
753 // Check the obligations of the cast -- for example, when casting
754 // `usize` to `dyn* Clone + 'static`:
755 let mut obligations: Vec<_> = predicates
758 // For each existential predicate (e.g., `?Self: Clone`) substitute
759 // the type of the expression (e.g., `usize` in our example above)
760 // and then require that the resulting predicate (e.g., `usize: Clone`)
762 let predicate = predicate.with_self_ty(self.tcx, a);
763 Obligation::new(self.tcx, self.cause.clone(), self.param_env, predicate)
766 // Enforce the region bound (e.g., `usize: 'static`, in our example).
771 ty::Binder::dummy(ty::PredicateKind::Clause(ty::Clause::TypeOutlives(
772 ty::OutlivesPredicate(a, b_region),
778 // Enforce that the type is `usize`/pointer-sized.
779 obligations.push(Obligation::new(
784 self.tcx.at(self.cause.span).mk_trait_ref(hir::LangItem::PointerSized, [a]),
789 value: (vec![Adjustment { kind: Adjust::DynStar, target: b }], b),
794 fn coerce_from_safe_fn<F, G>(
797 fn_ty_a: ty::PolyFnSig<'tcx>,
801 ) -> CoerceResult<'tcx>
803 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
804 G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
806 self.commit_if_ok(|snapshot| {
807 let result = if let ty::FnPtr(fn_ty_b) = b.kind()
808 && let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) =
809 (fn_ty_a.unsafety(), fn_ty_b.unsafety())
811 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
812 self.unify_and(unsafe_a, b, to_unsafe)
814 self.unify_and(a, b, normal)
817 // FIXME(#73154): This is a hack. Currently LUB can generate
818 // unsolvable constraints. Additionally, it returns `a`
819 // unconditionally, even when the "LUB" is `b`. In the future, we
820 // want the coerced type to be the actual supertype of these two,
821 // but for now, we want to just error to ensure we don't lock
822 // ourselves into a specific behavior with NLL.
823 self.leak_check(false, snapshot)?;
829 fn coerce_from_fn_pointer(
832 fn_ty_a: ty::PolyFnSig<'tcx>,
834 ) -> CoerceResult<'tcx> {
835 //! Attempts to coerce from the type of a Rust function item
836 //! into a closure or a `proc`.
839 let b = self.shallow_resolve(b);
840 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
842 self.coerce_from_safe_fn(
846 simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)),
851 fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
852 //! Attempts to coerce from the type of a Rust function item
853 //! into a closure or a `proc`.
855 let b = self.shallow_resolve(b);
856 let InferOk { value: b, mut obligations } =
857 self.normalize_associated_types_in_as_infer_ok(self.cause.span, b);
858 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
861 ty::FnPtr(b_sig) => {
862 let a_sig = a.fn_sig(self.tcx);
863 if let ty::FnDef(def_id, _) = *a.kind() {
864 // Intrinsics are not coercible to function pointers
865 if self.tcx.is_intrinsic(def_id) {
866 return Err(TypeError::IntrinsicCast);
869 // Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).
871 if b_sig.unsafety() == hir::Unsafety::Normal
872 && !self.tcx.codegen_fn_attrs(def_id).target_features.is_empty()
874 return Err(TypeError::TargetFeatureCast(def_id));
878 let InferOk { value: a_sig, obligations: o1 } =
879 self.normalize_associated_types_in_as_infer_ok(self.cause.span, a_sig);
880 obligations.extend(o1);
882 let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
883 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
890 kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
891 target: a_fn_pointer,
894 kind: Adjust::Pointer(PointerCast::UnsafeFnPointer),
899 simple(Adjust::Pointer(PointerCast::ReifyFnPointer)),
902 obligations.extend(o2);
903 Ok(InferOk { value, obligations })
905 _ => self.unify_and(a, b, identity),
909 fn coerce_closure_to_fn(
912 closure_def_id_a: DefId,
913 substs_a: SubstsRef<'tcx>,
915 ) -> CoerceResult<'tcx> {
916 //! Attempts to coerce from the type of a non-capturing closure
917 //! into a function pointer.
920 let b = self.shallow_resolve(b);
923 // At this point we haven't done capture analysis, which means
924 // that the ClosureSubsts just contains an inference variable instead
925 // of tuple of captured types.
927 // All we care here is if any variable is being captured and not the exact paths,
928 // so we check `upvars_mentioned` for root variables being captured.
932 .upvars_mentioned(closure_def_id_a.expect_local())
933 .map_or(true, |u| u.is_empty()) =>
935 // We coerce the closure, which has fn type
936 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
938 // `fn(arg0,arg1,...) -> _`
940 // `unsafe fn(arg0,arg1,...) -> _`
941 let closure_sig = substs_a.as_closure().sig();
942 let unsafety = fn_ty.unsafety();
944 self.tcx.mk_fn_ptr(self.tcx.signature_unclosure(closure_sig, unsafety));
945 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
949 simple(Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))),
952 _ => self.unify_and(a, b, identity),
956 fn coerce_unsafe_ptr(
960 mutbl_b: hir::Mutability,
961 ) -> CoerceResult<'tcx> {
962 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
964 let (is_ref, mt_a) = match *a.kind() {
965 ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
966 ty::RawPtr(mt) => (false, mt),
967 _ => return self.unify_and(a, b, identity),
969 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
971 // Check that the types which they point at are compatible.
972 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty });
973 // Although references and unsafe ptrs have the same
974 // representation, we still register an Adjust::DerefRef so that
975 // regionck knows that the region for `a` must be valid here.
977 self.unify_and(a_unsafe, b, |target| {
979 Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
980 Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
983 } else if mt_a.mutbl != mutbl_b {
984 self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer)))
986 self.unify_and(a_unsafe, b, identity)
991 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
992 /// Attempt to coerce an expression to a type, and return the
993 /// adjusted type of the expression, if successful.
994 /// Adjustments are only recorded if the coercion succeeded.
995 /// The expressions *must not* have any pre-existing adjustments.
998 expr: &hir::Expr<'_>,
1001 allow_two_phase: AllowTwoPhase,
1002 cause: Option<ObligationCause<'tcx>>,
1003 ) -> RelateResult<'tcx, Ty<'tcx>> {
1004 let source = self.resolve_vars_with_obligations(expr_ty);
1005 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
1008 cause.unwrap_or_else(|| self.cause(expr.span, ObligationCauseCode::ExprAssignable));
1009 let coerce = Coerce::new(self, cause, allow_two_phase);
1010 let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
1012 let (adjustments, _) = self.register_infer_ok_obligations(ok);
1013 self.apply_adjustments(expr, adjustments);
1014 Ok(if expr_ty.references_error() { self.tcx.ty_error() } else { target })
1017 /// Same as `try_coerce()`, but without side-effects.
1019 /// Returns false if the coercion creates any obligations that result in
1021 pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
1022 let source = self.resolve_vars_with_obligations(expr_ty);
1023 debug!("coercion::can_with_predicates({:?} -> {:?})", source, target);
1025 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
1026 // We don't ever need two-phase here since we throw out the result of the coercion
1027 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
1029 let Ok(ok) = coerce.coerce(source, target) else {
1032 let ocx = ObligationCtxt::new_in_snapshot(self);
1033 ocx.register_obligations(ok.obligations);
1034 ocx.select_where_possible().is_empty()
1038 /// Given a type and a target type, this function will calculate and return
1039 /// how many dereference steps needed to achieve `expr_ty <: target`. If
1040 /// it's not possible, return `None`.
1041 pub fn deref_steps(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> Option<usize> {
1042 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
1043 // We don't ever need two-phase here since we throw out the result of the coercion
1044 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
1046 .autoderef(rustc_span::DUMMY_SP, expr_ty)
1047 .find_map(|(ty, steps)| self.probe(|_| coerce.unify(ty, target)).ok().map(|_| steps))
1050 /// Given a type, this function will calculate and return the type given
1051 /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
1053 /// This function is for diagnostics only, since it does not register
1054 /// trait or region sub-obligations. (presumably we could, but it's not
1055 /// particularly important for diagnostics...)
1056 pub fn deref_once_mutably_for_diagnostic(&self, expr_ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
1057 self.autoderef(rustc_span::DUMMY_SP, expr_ty).nth(1).and_then(|(deref_ty, _)| {
1059 .type_implements_trait(
1060 self.tcx.lang_items().deref_mut_trait()?,
1069 /// Given some expressions, their known unified type and another expression,
1070 /// tries to unify the types, potentially inserting coercions on any of the
1071 /// provided expressions and returns their LUB (aka "common supertype").
1073 /// This is really an internal helper. From outside the coercion
1074 /// module, you should instantiate a `CoerceMany` instance.
1075 fn try_find_coercion_lub<E>(
1077 cause: &ObligationCause<'tcx>,
1080 new: &hir::Expr<'_>,
1082 ) -> RelateResult<'tcx, Ty<'tcx>>
1086 let prev_ty = self.resolve_vars_with_obligations(prev_ty);
1087 let new_ty = self.resolve_vars_with_obligations(new_ty);
1089 "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
1095 // The following check fixes #88097, where the compiler erroneously
1096 // attempted to coerce a closure type to itself via a function pointer.
1097 if prev_ty == new_ty {
1101 // Special-case that coercion alone cannot handle:
1102 // Function items or non-capturing closures of differing IDs or InternalSubsts.
1103 let (a_sig, b_sig) = {
1104 #[allow(rustc::usage_of_ty_tykind)]
1105 let is_capturing_closure = |ty: &ty::TyKind<'tcx>| {
1106 if let &ty::Closure(closure_def_id, _substs) = ty {
1107 self.tcx.upvars_mentioned(closure_def_id.expect_local()).is_some()
1112 if is_capturing_closure(prev_ty.kind()) || is_capturing_closure(new_ty.kind()) {
1115 match (prev_ty.kind(), new_ty.kind()) {
1116 (ty::FnDef(..), ty::FnDef(..)) => {
1117 // Don't reify if the function types have a LUB, i.e., they
1118 // are the same function and their parameters have a LUB.
1120 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1122 // We have a LUB of prev_ty and new_ty, just return it.
1123 Ok(ok) => return Ok(self.register_infer_ok_obligations(ok)),
1125 (Some(prev_ty.fn_sig(self.tcx)), Some(new_ty.fn_sig(self.tcx)))
1129 (ty::Closure(_, substs), ty::FnDef(..)) => {
1130 let b_sig = new_ty.fn_sig(self.tcx);
1133 .signature_unclosure(substs.as_closure().sig(), b_sig.unsafety());
1134 (Some(a_sig), Some(b_sig))
1136 (ty::FnDef(..), ty::Closure(_, substs)) => {
1137 let a_sig = prev_ty.fn_sig(self.tcx);
1140 .signature_unclosure(substs.as_closure().sig(), a_sig.unsafety());
1141 (Some(a_sig), Some(b_sig))
1143 (ty::Closure(_, substs_a), ty::Closure(_, substs_b)) => (
1144 Some(self.tcx.signature_unclosure(
1145 substs_a.as_closure().sig(),
1146 hir::Unsafety::Normal,
1148 Some(self.tcx.signature_unclosure(
1149 substs_b.as_closure().sig(),
1150 hir::Unsafety::Normal,
1157 if let (Some(a_sig), Some(b_sig)) = (a_sig, b_sig) {
1158 // Intrinsics are not coercible to function pointers.
1159 if a_sig.abi() == Abi::RustIntrinsic
1160 || a_sig.abi() == Abi::PlatformIntrinsic
1161 || b_sig.abi() == Abi::RustIntrinsic
1162 || b_sig.abi() == Abi::PlatformIntrinsic
1164 return Err(TypeError::IntrinsicCast);
1166 // The signature must match.
1167 let a_sig = self.normalize_associated_types_in(new.span, a_sig);
1168 let b_sig = self.normalize_associated_types_in(new.span, b_sig);
1170 .at(cause, self.param_env)
1171 .trace(prev_ty, new_ty)
1173 .map(|ok| self.register_infer_ok_obligations(ok))?;
1175 // Reify both sides and return the reified fn pointer type.
1176 let fn_ptr = self.tcx.mk_fn_ptr(sig);
1177 let prev_adjustment = match prev_ty.kind() {
1178 ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(a_sig.unsafety())),
1179 ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
1180 _ => unreachable!(),
1182 let next_adjustment = match new_ty.kind() {
1183 ty::Closure(..) => Adjust::Pointer(PointerCast::ClosureFnPointer(b_sig.unsafety())),
1184 ty::FnDef(..) => Adjust::Pointer(PointerCast::ReifyFnPointer),
1185 _ => unreachable!(),
1187 for expr in exprs.iter().map(|e| e.as_coercion_site()) {
1188 self.apply_adjustments(
1190 vec![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }],
1193 self.apply_adjustments(new, vec![Adjustment { kind: next_adjustment, target: fn_ptr }]);
1197 // Configure a Coerce instance to compute the LUB.
1198 // We don't allow two-phase borrows on any autorefs this creates since we
1199 // probably aren't processing function arguments here and even if we were,
1200 // they're going to get autorefed again anyway and we can apply 2-phase borrows
1202 let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
1203 coerce.use_lub = true;
1205 // First try to coerce the new expression to the type of the previous ones,
1206 // but only if the new expression has no coercion already applied to it.
1207 let mut first_error = None;
1208 if !self.typeck_results.borrow().adjustments().contains_key(new.hir_id) {
1209 let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
1212 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1213 self.apply_adjustments(new, adjustments);
1215 "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
1216 new_ty, prev_ty, target
1220 Err(e) => first_error = Some(e),
1224 // Then try to coerce the previous expressions to the type of the new one.
1225 // This requires ensuring there are no coercions applied to *any* of the
1226 // previous expressions, other than noop reborrows (ignoring lifetimes).
1228 let expr = expr.as_coercion_site();
1229 let noop = match self.typeck_results.borrow().expr_adjustments(expr) {
1231 Adjustment { kind: Adjust::Deref(_), .. },
1232 Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. },
1234 match *self.node_ty(expr.hir_id).kind() {
1235 ty::Ref(_, _, mt_orig) => {
1236 let mutbl_adj: hir::Mutability = mutbl_adj.into();
1237 // Reborrow that we can safely ignore, because
1238 // the next adjustment can only be a Deref
1239 // which will be merged into it.
1240 mutbl_adj == mt_orig
1245 &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
1251 "coercion::try_find_coercion_lub: older expression {:?} had adjustments, requiring LUB",
1256 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1257 .map(|ok| self.register_infer_ok_obligations(ok));
1261 match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
1263 // Avoid giving strange errors on failed attempts.
1264 if let Some(e) = first_error {
1267 self.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
1268 .map(|ok| self.register_infer_ok_obligations(ok))
1272 let (adjustments, target) = self.register_infer_ok_obligations(ok);
1274 let expr = expr.as_coercion_site();
1275 self.apply_adjustments(expr, adjustments.clone());
1278 "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
1279 prev_ty, new_ty, target
1287 /// CoerceMany encapsulates the pattern you should use when you have
1288 /// many expressions that are all getting coerced to a common
1289 /// type. This arises, for example, when you have a match (the result
1290 /// of each arm is coerced to a common type). It also arises in less
1291 /// obvious places, such as when you have many `break foo` expressions
1292 /// that target the same loop, or the various `return` expressions in
1295 /// The basic protocol is as follows:
1297 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
1298 /// This will also serve as the "starting LUB". The expectation is
1299 /// that this type is something which all of the expressions *must*
1300 /// be coercible to. Use a fresh type variable if needed.
1301 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
1302 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
1303 /// unit. This happens for example if you have a `break` with no expression,
1304 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
1305 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
1306 /// from you so that you don't have to worry your pretty head about it.
1307 /// But if an error is reported, the final type will be `err`.
1308 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
1309 /// previously coerced expressions.
1310 /// - When all done, invoke `complete()`. This will return the LUB of
1311 /// all your expressions.
1312 /// - WARNING: I don't believe this final type is guaranteed to be
1313 /// related to your initial `expected_ty` in any particular way,
1314 /// although it will typically be a subtype, so you should check it.
1315 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
1316 /// previously coerced expressions.
1320 /// ```ignore (illustrative)
1321 /// let mut coerce = CoerceMany::new(expected_ty);
1322 /// for expr in exprs {
1323 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
1324 /// coerce.coerce(fcx, &cause, expr, expr_ty);
1326 /// let final_ty = coerce.complete(fcx);
1328 pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
1329 expected_ty: Ty<'tcx>,
1330 final_ty: Option<Ty<'tcx>>,
1331 expressions: Expressions<'tcx, 'exprs, E>,
1335 /// The type of a `CoerceMany` that is storing up the expressions into
1336 /// a buffer. We use this in `check/mod.rs` for things like `break`.
1337 pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;
1339 enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
1340 Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
1341 UpFront(&'exprs [E]),
1344 impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
1345 /// The usual case; collect the set of expressions dynamically.
1346 /// If the full set of coercion sites is known before hand,
1347 /// consider `with_coercion_sites()` instead to avoid allocation.
1348 pub fn new(expected_ty: Ty<'tcx>) -> Self {
1349 Self::make(expected_ty, Expressions::Dynamic(vec![]))
1352 /// As an optimization, you can create a `CoerceMany` with a
1353 /// pre-existing slice of expressions. In this case, you are
1354 /// expected to pass each element in the slice to `coerce(...)` in
1355 /// order. This is used with arrays in particular to avoid
1356 /// needlessly cloning the slice.
1357 pub fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
1358 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
1361 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
1362 CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
1365 /// Returns the "expected type" with which this coercion was
1366 /// constructed. This represents the "downward propagated" type
1367 /// that was given to us at the start of typing whatever construct
1368 /// we are typing (e.g., the match expression).
1370 /// Typically, this is used as the expected type when
1371 /// type-checking each of the alternative expressions whose types
1372 /// we are trying to merge.
1373 pub fn expected_ty(&self) -> Ty<'tcx> {
1377 /// Returns the current "merged type", representing our best-guess
1378 /// at the LUB of the expressions we've seen so far (if any). This
1379 /// isn't *final* until you call `self.complete()`, which will return
1380 /// the merged type.
1381 pub fn merged_ty(&self) -> Ty<'tcx> {
1382 self.final_ty.unwrap_or(self.expected_ty)
1385 /// Indicates that the value generated by `expression`, which is
1386 /// of type `expression_ty`, is one of the possibilities that we
1387 /// could coerce from. This will record `expression`, and later
1388 /// calls to `coerce` may come back and add adjustments and things
1392 fcx: &FnCtxt<'a, 'tcx>,
1393 cause: &ObligationCause<'tcx>,
1394 expression: &'tcx hir::Expr<'tcx>,
1395 expression_ty: Ty<'tcx>,
1397 self.coerce_inner(fcx, cause, Some(expression), expression_ty, None, false)
1400 /// Indicates that one of the inputs is a "forced unit". This
1401 /// occurs in a case like `if foo { ... };`, where the missing else
1402 /// generates a "forced unit". Another example is a `loop { break;
1403 /// }`, where the `break` has no argument expression. We treat
1404 /// these cases slightly differently for error-reporting
1405 /// purposes. Note that these tend to correspond to cases where
1406 /// the `()` expression is implicit in the source, and hence we do
1407 /// not take an expression argument.
1409 /// The `augment_error` gives you a chance to extend the error
1410 /// message, in case any results (e.g., we use this to suggest
1411 /// removing a `;`).
1412 pub fn coerce_forced_unit<'a>(
1414 fcx: &FnCtxt<'a, 'tcx>,
1415 cause: &ObligationCause<'tcx>,
1416 augment_error: &mut dyn FnMut(&mut Diagnostic),
1417 label_unit_as_expected: bool,
1424 Some(augment_error),
1425 label_unit_as_expected,
1429 /// The inner coercion "engine". If `expression` is `None`, this
1430 /// is a forced-unit case, and hence `expression_ty` must be
1432 #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
1433 pub(crate) fn coerce_inner<'a>(
1435 fcx: &FnCtxt<'a, 'tcx>,
1436 cause: &ObligationCause<'tcx>,
1437 expression: Option<&'tcx hir::Expr<'tcx>>,
1438 mut expression_ty: Ty<'tcx>,
1439 augment_error: Option<&mut dyn FnMut(&mut Diagnostic)>,
1440 label_expression_as_expected: bool,
1442 // Incorporate whatever type inference information we have
1443 // until now; in principle we might also want to process
1444 // pending obligations, but doing so should only improve
1445 // compatibility (hopefully that is true) by helping us
1446 // uncover never types better.
1447 if expression_ty.is_ty_var() {
1448 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1451 // If we see any error types, just propagate that error
1453 if expression_ty.references_error() || self.merged_ty().references_error() {
1454 self.final_ty = Some(fcx.tcx.ty_error());
1458 // Handle the actual type unification etc.
1459 let result = if let Some(expression) = expression {
1460 if self.pushed == 0 {
1461 // Special-case the first expression we are coercing.
1462 // To be honest, I'm not entirely sure why we do this.
1463 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1469 Some(cause.clone()),
1472 match self.expressions {
1473 Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
1480 Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub(
1482 &coercion_sites[0..self.pushed],
1490 // this is a hack for cases where we default to `()` because
1491 // the expression etc has been omitted from the source. An
1492 // example is an `if let` without an else:
1494 // if let Some(x) = ... { }
1496 // we wind up with a second match arm that is like `_ =>
1497 // ()`. That is the case we are considering here. We take
1498 // a different path to get the right "expected, found"
1499 // message and so forth (and because we know that
1500 // `expression_ty` will be unit).
1502 // Another example is `break` with no argument expression.
1503 assert!(expression_ty.is_unit(), "if let hack without unit type");
1504 fcx.at(cause, fcx.param_env)
1505 .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
1507 fcx.register_infer_ok_obligations(infer_ok);
1515 self.final_ty = Some(v);
1516 if let Some(e) = expression {
1517 match self.expressions {
1518 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1519 Expressions::UpFront(coercion_sites) => {
1520 // if the user gave us an array to validate, check that we got
1521 // the next expression in the list, as expected
1523 coercion_sites[self.pushed].as_coercion_site().hir_id,
1531 Err(coercion_error) => {
1532 // Mark that we've failed to coerce the types here to suppress
1533 // any superfluous errors we might encounter while trying to
1534 // emit or provide suggestions on how to fix the initial error.
1535 fcx.set_tainted_by_errors(
1536 fcx.tcx.sess.delay_span_bug(cause.span, "coercion error but no error emitted"),
1538 let (expected, found) = if label_expression_as_expected {
1539 // In the case where this is a "forced unit", like
1540 // `break`, we want to call the `()` "expected"
1541 // since it is implied by the syntax.
1542 // (Note: not all force-units work this way.)"
1543 (expression_ty, self.merged_ty())
1545 // Otherwise, the "expected" type for error
1546 // reporting is the current unification type,
1547 // which is basically the LUB of the expressions
1548 // we've seen so far (combined with the expected
1550 (self.merged_ty(), expression_ty)
1552 let (expected, found) = fcx.resolve_vars_if_possible((expected, found));
1555 let mut unsized_return = false;
1556 let mut visitor = CollectRetsVisitor { ret_exprs: vec![] };
1557 match *cause.code() {
1558 ObligationCauseCode::ReturnNoExpression => {
1559 err = struct_span_err!(
1563 "`return;` in a function whose return type is not `()`"
1565 err.span_label(cause.span, "return type is not `()`");
1567 ObligationCauseCode::BlockTailExpression(blk_id) => {
1568 let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
1569 err = self.report_return_mismatched_types(
1573 coercion_error.clone(),
1579 if !fcx.tcx.features().unsized_locals {
1580 unsized_return = self.is_return_ty_unsized(fcx, blk_id);
1582 if let Some(expression) = expression
1583 && let hir::ExprKind::Loop(loop_blk, ..) = expression.kind {
1584 intravisit::walk_block(& mut visitor, loop_blk);
1587 ObligationCauseCode::ReturnValue(id) => {
1588 err = self.report_return_mismatched_types(
1592 coercion_error.clone(),
1598 if !fcx.tcx.features().unsized_locals {
1599 let id = fcx.tcx.hir().get_parent_node(id);
1600 unsized_return = self.is_return_ty_unsized(fcx, id);
1604 err = fcx.err_ctxt().report_mismatched_types(
1608 coercion_error.clone(),
1613 if let Some(augment_error) = augment_error {
1614 augment_error(&mut err);
1617 let is_insufficiently_polymorphic =
1618 matches!(coercion_error, TypeError::RegionsInsufficientlyPolymorphic(..));
1620 if !is_insufficiently_polymorphic && let Some(expr) = expression {
1621 fcx.emit_coerce_suggestions(
1627 Some(coercion_error),
1631 if visitor.ret_exprs.len() > 0 && let Some(expr) = expression {
1632 self.note_unreachable_loop_return(&mut err, &expr, &visitor.ret_exprs);
1634 let reported = err.emit_unless(unsized_return);
1636 self.final_ty = Some(fcx.tcx.ty_error_with_guaranteed(reported));
1640 fn note_unreachable_loop_return(
1642 err: &mut Diagnostic,
1643 expr: &hir::Expr<'tcx>,
1644 ret_exprs: &Vec<&'tcx hir::Expr<'tcx>>,
1646 let hir::ExprKind::Loop(_, _, _, loop_span) = expr.kind else { return;};
1647 let mut span: MultiSpan = vec![loop_span].into();
1648 span.push_span_label(loop_span, "this might have zero elements to iterate on");
1649 const MAXITER: usize = 3;
1650 let iter = ret_exprs.iter().take(MAXITER);
1651 for ret_expr in iter {
1652 span.push_span_label(
1654 "if the loop doesn't execute, this value would never get returned",
1659 "the function expects a value to always be returned, but loops might run zero times",
1661 if MAXITER < ret_exprs.len() {
1663 "if the loop doesn't execute, {} other values would never get returned",
1664 ret_exprs.len() - MAXITER
1668 "return a value for the case when the loop has zero elements to iterate on, or \
1669 consider changing the return type to account for that possibility",
1673 fn report_return_mismatched_types<'a>(
1675 cause: &ObligationCause<'tcx>,
1678 ty_err: TypeError<'tcx>,
1679 fcx: &FnCtxt<'a, 'tcx>,
1681 expression: Option<&'tcx hir::Expr<'tcx>>,
1682 blk_id: Option<hir::HirId>,
1683 ) -> DiagnosticBuilder<'a, ErrorGuaranteed> {
1684 let mut err = fcx.err_ctxt().report_mismatched_types(cause, expected, found, ty_err);
1686 let mut pointing_at_return_type = false;
1687 let mut fn_output = None;
1689 let parent_id = fcx.tcx.hir().get_parent_node(id);
1690 let parent = fcx.tcx.hir().get(parent_id);
1691 if let Some(expr) = expression
1692 && let hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(&hir::Closure { body, .. }), .. }) = parent
1693 && !matches!(fcx.tcx.hir().body(body).value.kind, hir::ExprKind::Block(..))
1695 fcx.suggest_missing_semicolon(&mut err, expr, expected, true);
1697 // Verify that this is a tail expression of a function, otherwise the
1698 // label pointing out the cause for the type coercion will be wrong
1699 // as prior return coercions would not be relevant (#57664).
1700 let fn_decl = if let (Some(expr), Some(blk_id)) = (expression, blk_id) {
1701 pointing_at_return_type =
1702 fcx.suggest_mismatched_types_on_tail(&mut err, expr, expected, found, blk_id);
1703 if let (Some(cond_expr), true, false) = (
1704 fcx.tcx.hir().get_if_cause(expr.hir_id),
1706 pointing_at_return_type,
1708 // If the block is from an external macro or try (`?`) desugaring, then
1709 // do not suggest adding a semicolon, because there's nowhere to put it.
1710 // See issues #81943 and #87051.
1712 cond_expr.span.desugaring_kind(),
1713 None | Some(DesugaringKind::WhileLoop)
1714 ) && !in_external_macro(fcx.tcx.sess, cond_expr.span)
1717 hir::ExprKind::Match(.., hir::MatchSource::TryDesugar)
1720 err.span_label(cond_expr.span, "expected this to be `()`");
1721 if expr.can_have_side_effects() {
1722 fcx.suggest_semicolon_at_end(cond_expr.span, &mut err);
1725 fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
1727 fcx.get_fn_decl(parent_id)
1730 if let Some((fn_decl, can_suggest)) = fn_decl {
1731 if blk_id.is_none() {
1732 pointing_at_return_type |= fcx.suggest_missing_return_type(
1738 fcx.tcx.hir().get_parent_item(id).into(),
1741 if !pointing_at_return_type {
1742 fn_output = Some(&fn_decl.output); // `impl Trait` return type
1746 let parent_id = fcx.tcx.hir().get_parent_item(id);
1747 let parent_item = fcx.tcx.hir().get_by_def_id(parent_id.def_id);
1749 if let (Some(expr), Some(_), Some((fn_decl, _, _))) =
1750 (expression, blk_id, fcx.get_node_fn_decl(parent_item))
1752 fcx.suggest_missing_break_or_return_expr(
1763 let ret_coercion_span = fcx.ret_coercion_span.get();
1765 if let Some(sp) = ret_coercion_span
1766 // If the closure has an explicit return type annotation, or if
1767 // the closure's return type has been inferred from outside
1768 // requirements (such as an Fn* trait bound), then a type error
1769 // may occur at the first return expression we see in the closure
1770 // (if it conflicts with the declared return type). Skip adding a
1771 // note in this case, since it would be incorrect.
1772 && let Some(fn_sig) = fcx.body_fn_sig()
1773 && fn_sig.output().is_ty_var()
1778 "return type inferred to be `{}` here",
1784 if let (Some(sp), Some(fn_output)) = (ret_coercion_span, fn_output) {
1785 self.add_impl_trait_explanation(&mut err, cause, fcx, expected, sp, fn_output);
1791 fn add_impl_trait_explanation<'a>(
1793 err: &mut Diagnostic,
1794 cause: &ObligationCause<'tcx>,
1795 fcx: &FnCtxt<'a, 'tcx>,
1798 fn_output: &hir::FnRetTy<'_>,
1800 let return_sp = fn_output.span();
1801 err.span_label(return_sp, "expected because this return type...");
1804 format!("...is found to be `{}` here", fcx.resolve_vars_with_obligations(expected)),
1806 let impl_trait_msg = "for information on `impl Trait`, see \
1807 <https://doc.rust-lang.org/book/ch10-02-traits.html\
1808 #returning-types-that-implement-traits>";
1809 let trait_obj_msg = "for information on trait objects, see \
1810 <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
1811 #using-trait-objects-that-allow-for-values-of-different-types>";
1812 err.note("to return `impl Trait`, all returned values must be of the same type");
1813 err.note(impl_trait_msg);
1818 .span_to_snippet(return_sp)
1819 .unwrap_or_else(|_| "dyn Trait".to_string());
1820 let mut snippet_iter = snippet.split_whitespace();
1821 let has_impl = snippet_iter.next().map_or(false, |s| s == "impl");
1822 // Only suggest `Box<dyn Trait>` if `Trait` in `impl Trait` is object safe.
1823 let mut is_object_safe = false;
1824 if let hir::FnRetTy::Return(ty) = fn_output
1825 // Get the return type.
1826 && let hir::TyKind::OpaqueDef(..) = ty.kind
1828 let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty);
1829 // Get the `impl Trait`'s `DefId`.
1830 if let ty::Opaque(def_id, _) = ty.kind()
1831 // Get the `impl Trait`'s `Item` so that we can get its trait bounds and
1832 // get the `Trait`'s `DefId`.
1833 && let hir::ItemKind::OpaqueTy(hir::OpaqueTy { bounds, .. }) =
1834 fcx.tcx.hir().expect_item(def_id.expect_local()).kind
1836 // Are of this `impl Trait`'s traits object safe?
1837 is_object_safe = bounds.iter().all(|bound| {
1840 .and_then(|t| t.trait_def_id())
1841 .map_or(false, |def_id| {
1842 fcx.tcx.object_safety_violations(def_id).is_empty()
1849 err.multipart_suggestion(
1850 "you could change the return type to be a boxed trait object",
1852 (return_sp.with_hi(return_sp.lo() + BytePos(4)), "Box<dyn".to_string()),
1853 (return_sp.shrink_to_hi(), ">".to_string()),
1855 Applicability::MachineApplicable,
1857 let sugg = [sp, cause.span]
1861 (sp.shrink_to_lo(), "Box::new(".to_string()),
1862 (sp.shrink_to_hi(), ")".to_string()),
1866 .collect::<Vec<_>>();
1867 err.multipart_suggestion(
1868 "if you change the return type to expect trait objects, box the returned \
1871 Applicability::MaybeIncorrect,
1875 "if the trait `{}` were object safe, you could return a boxed trait object",
1879 err.note(trait_obj_msg);
1881 err.help("you could instead create a new `enum` with a variant for each returned type");
1884 fn is_return_ty_unsized<'a>(&self, fcx: &FnCtxt<'a, 'tcx>, blk_id: hir::HirId) -> bool {
1885 if let Some((fn_decl, _)) = fcx.get_fn_decl(blk_id)
1886 && let hir::FnRetTy::Return(ty) = fn_decl.output
1887 && let ty = <dyn AstConv<'_>>::ast_ty_to_ty(fcx, ty)
1888 && let ty::Dynamic(..) = ty.kind()
1895 pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
1896 if let Some(final_ty) = self.final_ty {
1899 // If we only had inputs that were of type `!` (or no
1900 // inputs at all), then the final type is `!`.
1901 assert_eq!(self.pushed, 0);
1907 /// Something that can be converted into an expression to which we can
1908 /// apply a coercion.
1909 pub trait AsCoercionSite {
1910 fn as_coercion_site(&self) -> &hir::Expr<'_>;
1913 impl AsCoercionSite for hir::Expr<'_> {
1914 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1919 impl<'a, T> AsCoercionSite for &'a T
1923 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1924 (**self).as_coercion_site()
1928 impl AsCoercionSite for ! {
1929 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1934 impl AsCoercionSite for hir::Arm<'_> {
1935 fn as_coercion_site(&self) -> &hir::Expr<'_> {