3 //! Under certain circumstances we will coerce from one type to another,
4 //! for example by auto-borrowing. This occurs in situations where the
5 //! compiler has a firm 'expected type' that was supplied from the user,
6 //! and where the actual type is similar to that expected type in purpose
7 //! but not in representation (so actual subtyping is inappropriate).
11 //! Note that if we are expecting a reference, we will *reborrow*
12 //! even if the argument provided was already a reference. This is
13 //! useful for freezing mut things (that is, when the expected type is &T
14 //! but you have &mut T) and also for avoiding the linearity
15 //! of mut things (when the expected is &mut T and you have &mut T). See
16 //! the various `src/test/ui/coerce/*.rs` tests for
17 //! examples of where this is useful.
21 //! When inferring the generic arguments of functions, the argument
22 //! order is relevant, which can lead to the following edge case:
24 //! ```ignore (illustrative)
25 //! fn foo<T>(a: T, b: T) {
29 //! foo(&7i32, &mut 7i32);
30 //! // This compiles, as we first infer `T` to be `&i32`,
31 //! // and then coerce `&mut 7i32` to `&7i32`.
33 //! foo(&mut 7i32, &7i32);
34 //! // This does not compile, as we first infer `T` to be `&mut i32`
35 //! // and are then unable to coerce `&7i32` to `&mut i32`.
38 use crate::astconv
::AstConv
;
39 use crate::check
::FnCtxt
;
41 struct_span_err
, Applicability
, Diagnostic
, DiagnosticBuilder
, ErrorGuaranteed
,
44 use rustc_hir
::def_id
::DefId
;
45 use rustc_infer
::infer
::type_variable
::{TypeVariableOrigin, TypeVariableOriginKind}
;
46 use rustc_infer
::infer
::{Coercion, InferOk, InferResult}
;
47 use rustc_infer
::traits
::{Obligation, TraitEngine, TraitEngineExt}
;
48 use rustc_middle
::lint
::in_external_macro
;
49 use rustc_middle
::ty
::adjustment
::{
50 Adjust
, Adjustment
, AllowTwoPhase
, AutoBorrow
, AutoBorrowMutability
, PointerCast
,
52 use rustc_middle
::ty
::error
::TypeError
;
53 use rustc_middle
::ty
::fold
::TypeFoldable
;
54 use rustc_middle
::ty
::relate
::RelateResult
;
55 use rustc_middle
::ty
::subst
::SubstsRef
;
56 use rustc_middle
::ty
::{self, ToPredicate, Ty, TypeAndMut}
;
57 use rustc_session
::parse
::feature_err
;
58 use rustc_span
::symbol
::sym
;
59 use rustc_span
::{self, BytePos, DesugaringKind, Span}
;
60 use rustc_target
::spec
::abi
::Abi
;
61 use rustc_trait_selection
::infer
::InferCtxtExt
as _
;
62 use rustc_trait_selection
::traits
::error_reporting
::InferCtxtExt
as _
;
63 use rustc_trait_selection
::traits
::{self, ObligationCause, ObligationCauseCode}
;
65 use smallvec
::{smallvec, SmallVec}
;
68 struct Coerce
<'a
, 'tcx
> {
69 fcx
: &'a FnCtxt
<'a
, 'tcx
>,
70 cause
: ObligationCause
<'tcx
>,
72 /// Determines whether or not allow_two_phase_borrow is set on any
73 /// autoref adjustments we create while coercing. We don't want to
74 /// allow deref coercions to create two-phase borrows, at least initially,
75 /// but we do need two-phase borrows for function argument reborrows.
76 /// See #47489 and #48598
77 /// See docs on the "AllowTwoPhase" type for a more detailed discussion
78 allow_two_phase
: AllowTwoPhase
,
81 impl<'a
, 'tcx
> Deref
for Coerce
<'a
, 'tcx
> {
82 type Target
= FnCtxt
<'a
, 'tcx
>;
83 fn deref(&self) -> &Self::Target
{
88 type CoerceResult
<'tcx
> = InferResult
<'tcx
, (Vec
<Adjustment
<'tcx
>>, Ty
<'tcx
>)>;
90 /// Coercing a mutable reference to an immutable works, while
91 /// coercing `&T` to `&mut T` should be forbidden.
92 fn coerce_mutbls
<'tcx
>(
93 from_mutbl
: hir
::Mutability
,
94 to_mutbl
: hir
::Mutability
,
95 ) -> RelateResult
<'tcx
, ()> {
96 match (from_mutbl
, to_mutbl
) {
97 (hir
::Mutability
::Mut
, hir
::Mutability
::Mut
| hir
::Mutability
::Not
)
98 | (hir
::Mutability
::Not
, hir
::Mutability
::Not
) => Ok(()),
99 (hir
::Mutability
::Not
, hir
::Mutability
::Mut
) => Err(TypeError
::Mutability
),
103 /// Do not require any adjustments, i.e. coerce `x -> x`.
104 fn identity(_
: Ty
<'_
>) -> Vec
<Adjustment
<'_
>> {
108 fn simple
<'tcx
>(kind
: Adjust
<'tcx
>) -> impl FnOnce(Ty
<'tcx
>) -> Vec
<Adjustment
<'tcx
>> {
109 move |target
| vec
![Adjustment { kind, target }
]
112 /// This always returns `Ok(...)`.
114 adj
: Vec
<Adjustment
<'tcx
>>,
116 obligations
: traits
::PredicateObligations
<'tcx
>,
117 ) -> CoerceResult
<'tcx
> {
118 Ok(InferOk { value: (adj, target), obligations }
)
121 impl<'f
, 'tcx
> Coerce
<'f
, 'tcx
> {
123 fcx
: &'f FnCtxt
<'f
, 'tcx
>,
124 cause
: ObligationCause
<'tcx
>,
125 allow_two_phase
: AllowTwoPhase
,
127 Coerce { fcx, cause, allow_two_phase, use_lub: false }
130 fn unify(&self, a
: Ty
<'tcx
>, b
: Ty
<'tcx
>) -> InferResult
<'tcx
, Ty
<'tcx
>> {
131 debug
!("unify(a: {:?}, b: {:?}, use_lub: {})", a
, b
, self.use_lub
);
132 self.commit_if_ok(|_
| {
134 self.at(&self.cause
, self.fcx
.param_env
).lub(b
, a
)
136 self.at(&self.cause
, self.fcx
.param_env
)
138 .map(|InferOk { value: (), obligations }
| InferOk { value: a, obligations }
)
143 /// Unify two types (using sub or lub) and produce a specific coercion.
144 fn unify_and
<F
>(&self, a
: Ty
<'tcx
>, b
: Ty
<'tcx
>, f
: F
) -> CoerceResult
<'tcx
>
146 F
: FnOnce(Ty
<'tcx
>) -> Vec
<Adjustment
<'tcx
>>,
149 .and_then(|InferOk { value: ty, obligations }
| success(f(ty
), ty
, obligations
))
152 #[instrument(skip(self))]
153 fn coerce(&self, a
: Ty
<'tcx
>, b
: Ty
<'tcx
>) -> CoerceResult
<'tcx
> {
154 // First, remove any resolved type variables (at the top level, at least):
155 let a
= self.shallow_resolve(a
);
156 let b
= self.shallow_resolve(b
);
157 debug
!("Coerce.tys({:?} => {:?})", a
, b
);
159 // Just ignore error types.
160 if a
.references_error() || b
.references_error() {
161 return success(vec
![], self.fcx
.tcx
.ty_error(), vec
![]);
164 // Coercing from `!` to any type is allowed:
166 return success(simple(Adjust
::NeverToAny
)(b
), b
, vec
![]);
169 // Coercing *from* an unresolved inference variable means that
170 // we have no information about the source type. This will always
171 // ultimately fall back to some form of subtyping.
173 return self.coerce_from_inference_variable(a
, b
, identity
);
176 // Consider coercing the subtype to a DST
178 // NOTE: this is wrapped in a `commit_if_ok` because it creates
179 // a "spurious" type variable, and we don't want to have that
180 // type variable in memory if the coercion fails.
181 let unsize
= self.commit_if_ok(|_
| self.coerce_unsized(a
, b
));
184 debug
!("coerce: unsize successful");
187 Err(TypeError
::ObjectUnsafeCoercion(did
)) => {
188 debug
!("coerce: unsize not object safe");
189 return Err(TypeError
::ObjectUnsafeCoercion(did
));
192 debug
!(?error
, "coerce: unsize failed");
196 // Examine the supertype and consider auto-borrowing.
198 ty
::RawPtr(mt_b
) => {
199 return self.coerce_unsafe_ptr(a
, b
, mt_b
.mutbl
);
201 ty
::Ref(r_b
, _
, mutbl_b
) => {
202 return self.coerce_borrowed_pointer(a
, b
, r_b
, mutbl_b
);
209 // Function items are coercible to any closure
210 // type; function pointers are not (that would
211 // require double indirection).
212 // Additionally, we permit coercion of function
213 // items to drop the unsafe qualifier.
214 self.coerce_from_fn_item(a
, b
)
217 // We permit coercion of fn pointers to drop the
219 self.coerce_from_fn_pointer(a
, a_f
, b
)
221 ty
::Closure(closure_def_id_a
, substs_a
) => {
222 // Non-capturing closures are coercible to
223 // function pointers or unsafe function pointers.
224 // It cannot convert closures that require unsafe.
225 self.coerce_closure_to_fn(a
, closure_def_id_a
, substs_a
, b
)
228 // Otherwise, just use unification rules.
229 self.unify_and(a
, b
, identity
)
234 /// Coercing *from* an inference variable. In this case, we have no information
235 /// about the source type, so we can't really do a true coercion and we always
236 /// fall back to subtyping (`unify_and`).
237 fn coerce_from_inference_variable(
241 make_adjustments
: impl FnOnce(Ty
<'tcx
>) -> Vec
<Adjustment
<'tcx
>>,
242 ) -> CoerceResult
<'tcx
> {
243 debug
!("coerce_from_inference_variable(a={:?}, b={:?})", a
, b
);
244 assert
!(a
.is_ty_var() && self.infcx
.shallow_resolve(a
) == a
);
245 assert
!(self.infcx
.shallow_resolve(b
) == b
);
248 // Two unresolved type variables: create a `Coerce` predicate.
249 let target_ty
= if self.use_lub
{
250 self.infcx
.next_ty_var(TypeVariableOrigin
{
251 kind
: TypeVariableOriginKind
::LatticeVariable
,
252 span
: self.cause
.span
,
258 let mut obligations
= Vec
::with_capacity(2);
259 for &source_ty
in &[a
, b
] {
260 if source_ty
!= target_ty
{
261 obligations
.push(Obligation
::new(
264 ty
::Binder
::dummy(ty
::PredicateKind
::Coerce(ty
::CoercePredicate
{
268 .to_predicate(self.tcx()),
274 "coerce_from_inference_variable: two inference variables, target_ty={:?}, obligations={:?}",
275 target_ty
, obligations
277 let adjustments
= make_adjustments(target_ty
);
278 InferResult
::Ok(InferOk { value: (adjustments, target_ty), obligations }
)
280 // One unresolved type variable: just apply subtyping, we may be able
281 // to do something useful.
282 self.unify_and(a
, b
, make_adjustments
)
286 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
287 /// To match `A` with `B`, autoderef will be performed,
288 /// calling `deref`/`deref_mut` where necessary.
289 fn coerce_borrowed_pointer(
293 r_b
: ty
::Region
<'tcx
>,
294 mutbl_b
: hir
::Mutability
,
295 ) -> CoerceResult
<'tcx
> {
296 debug
!("coerce_borrowed_pointer(a={:?}, b={:?})", a
, b
);
298 // If we have a parameter of type `&M T_a` and the value
299 // provided is `expr`, we will be adding an implicit borrow,
300 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
301 // to type check, we will construct the type that `&M*expr` would
304 let (r_a
, mt_a
) = match *a
.kind() {
305 ty
::Ref(r_a
, ty
, mutbl
) => {
306 let mt_a
= ty
::TypeAndMut { ty, mutbl }
;
307 coerce_mutbls(mt_a
.mutbl
, mutbl_b
)?
;
310 _
=> return self.unify_and(a
, b
, identity
),
313 let span
= self.cause
.span
;
315 let mut first_error
= None
;
316 let mut r_borrow_var
= None
;
317 let mut autoderef
= self.autoderef(span
, a
);
318 let mut found
= None
;
320 for (referent_ty
, autoderefs
) in autoderef
.by_ref() {
322 // Don't let this pass, otherwise it would cause
323 // &T to autoref to &&T.
327 // At this point, we have deref'd `a` to `referent_ty`. So
328 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
329 // In the autoderef loop for `&'a mut Vec<T>`, we would get
332 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
333 // - `Vec<T>` -- 1 deref
334 // - `[T]` -- 2 deref
336 // At each point after the first callback, we want to
337 // check to see whether this would match out target type
338 // (`&'b mut [T]`) if we autoref'd it. We can't just
339 // compare the referent types, though, because we still
340 // have to consider the mutability. E.g., in the case
341 // we've been considering, we have an `&mut` reference, so
342 // the `T` in `[T]` needs to be unified with equality.
344 // Therefore, we construct reference types reflecting what
345 // the types will be after we do the final auto-ref and
346 // compare those. Note that this means we use the target
347 // mutability [1], since it may be that we are coercing
348 // from `&mut T` to `&U`.
350 // One fine point concerns the region that we use. We
351 // choose the region such that the region of the final
352 // type that results from `unify` will be the region we
353 // want for the autoref:
355 // - if in sub mode, that means we want to use `'b` (the
356 // region from the target reference) for both
357 // pointers [2]. This is because sub mode (somewhat
358 // arbitrarily) returns the subtype region. In the case
359 // where we are coercing to a target type, we know we
360 // want to use that target type region (`'b`) because --
361 // for the program to type-check -- it must be the
362 // smaller of the two.
363 // - One fine point. It may be surprising that we can
364 // use `'b` without relating `'a` and `'b`. The reason
365 // that this is ok is that what we produce is
366 // effectively a `&'b *x` expression (if you could
367 // annotate the region of a borrow), and regionck has
368 // code that adds edges from the region of a borrow
369 // (`'b`, here) into the regions in the borrowed
370 // expression (`*x`, here). (Search for "link".)
371 // - if in lub mode, things can get fairly complicated. The
372 // easiest thing is just to make a fresh
373 // region variable [4], which effectively means we defer
374 // the decision to region inference (and regionck, which will add
375 // some more edges to this variable). However, this can wind up
376 // creating a crippling number of variables in some cases --
377 // e.g., #32278 -- so we optimize one particular case [3].
378 // Let me try to explain with some examples:
379 // - The "running example" above represents the simple case,
380 // where we have one `&` reference at the outer level and
381 // ownership all the rest of the way down. In this case,
382 // we want `LUB('a, 'b)` as the resulting region.
383 // - However, if there are nested borrows, that region is
384 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
385 // `&'b T`. In this case, `'a` is actually irrelevant.
386 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
387 // we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
388 // (The errors actually show up in borrowck, typically, because
389 // this extra edge causes the region `'a` to be inferred to something
390 // too big, which then results in borrowck errors.)
391 // - We could track the innermost shared reference, but there is already
392 // code in regionck that has the job of creating links between
393 // the region of a borrow and the regions in the thing being
394 // borrowed (here, `'a` and `'x`), and it knows how to handle
395 // all the various cases. So instead we just make a region variable
396 // and let regionck figure it out.
397 let r
= if !self.use_lub
{
399 } else if autoderefs
== 1 {
402 if r_borrow_var
.is_none() {
403 // create var lazily, at most once
404 let coercion
= Coercion(span
);
405 let r
= self.next_region_var(coercion
);
406 r_borrow_var
= Some(r
); // [4] above
408 r_borrow_var
.unwrap()
410 let derefd_ty_a
= self.tcx
.mk_ref(
414 mutbl
: mutbl_b
, // [1] above
417 match self.unify(derefd_ty_a
, b
) {
423 if first_error
.is_none() {
424 first_error
= Some(err
);
430 // Extract type or return an error. We return the first error
431 // we got, which should be from relating the "base" type
432 // (e.g., in example above, the failure from relating `Vec<T>`
433 // to the target type), since that should be the least
435 let Some(InferOk { value: ty, mut obligations }
) = found
else {
436 let err
= first_error
.expect("coerce_borrowed_pointer had no error");
437 debug
!("coerce_borrowed_pointer: failed with err = {:?}", err
);
441 if ty
== a
&& mt_a
.mutbl
== hir
::Mutability
::Not
&& autoderef
.step_count() == 1 {
442 // As a special case, if we would produce `&'a *x`, that's
443 // a total no-op. We end up with the type `&'a T` just as
444 // we started with. In that case, just skip it
445 // altogether. This is just an optimization.
447 // Note that for `&mut`, we DO want to reborrow --
448 // otherwise, this would be a move, which might be an
449 // error. For example `foo(self.x)` where `self` and
450 // `self.x` both have `&mut `type would be a move of
451 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
452 // which is a borrow.
453 assert_eq
!(mutbl_b
, hir
::Mutability
::Not
); // can only coerce &T -> &U
454 return success(vec
![], ty
, obligations
);
457 let InferOk { value: mut adjustments, obligations: o }
=
458 self.adjust_steps_as_infer_ok(&autoderef
);
459 obligations
.extend(o
);
460 obligations
.extend(autoderef
.into_obligations());
462 // Now apply the autoref. We have to extract the region out of
463 // the final ref type we got.
464 let ty
::Ref(r_borrow
, _
, _
) = ty
.kind() else {
465 span_bug
!(span
, "expected a ref type, got {:?}", ty
);
467 let mutbl
= match mutbl_b
{
468 hir
::Mutability
::Not
=> AutoBorrowMutability
::Not
,
469 hir
::Mutability
::Mut
=> {
470 AutoBorrowMutability
::Mut { allow_two_phase_borrow: self.allow_two_phase }
473 adjustments
.push(Adjustment
{
474 kind
: Adjust
::Borrow(AutoBorrow
::Ref(*r_borrow
, mutbl
)),
478 debug
!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty
, adjustments
);
480 success(adjustments
, ty
, obligations
)
483 // &[T; n] or &mut [T; n] -> &[T]
484 // or &mut [T; n] -> &mut [T]
485 // or &Concrete -> &Trait, etc.
486 #[instrument(skip(self), level = "debug")]
487 fn coerce_unsized(&self, mut source
: Ty
<'tcx
>, mut target
: Ty
<'tcx
>) -> CoerceResult
<'tcx
> {
488 source
= self.shallow_resolve(source
);
489 target
= self.shallow_resolve(target
);
490 debug
!(?source
, ?target
);
492 // These 'if' statements require some explanation.
493 // The `CoerceUnsized` trait is special - it is only
494 // possible to write `impl CoerceUnsized<B> for A` where
495 // A and B have 'matching' fields. This rules out the following
496 // two types of blanket impls:
498 // `impl<T> CoerceUnsized<T> for SomeType`
499 // `impl<T> CoerceUnsized<SomeType> for T`
501 // Both of these trigger a special `CoerceUnsized`-related error (E0376)
503 // We can take advantage of this fact to avoid performing unnecessary work.
504 // If either `source` or `target` is a type variable, then any applicable impl
505 // would need to be generic over the self-type (`impl<T> CoerceUnsized<SomeType> for T`)
506 // or generic over the `CoerceUnsized` type parameter (`impl<T> CoerceUnsized<T> for
509 // However, these are exactly the kinds of impls which are forbidden by
510 // the compiler! Therefore, we can be sure that coercion will always fail
511 // when either the source or target type is a type variable. This allows us
512 // to skip performing any trait selection, and immediately bail out.
513 if source
.is_ty_var() {
514 debug
!("coerce_unsized: source is a TyVar, bailing out");
515 return Err(TypeError
::Mismatch
);
517 if target
.is_ty_var() {
518 debug
!("coerce_unsized: target is a TyVar, bailing out");
519 return Err(TypeError
::Mismatch
);
523 (self.tcx
.lang_items().unsize_trait(), self.tcx
.lang_items().coerce_unsized_trait());
524 let (Some(unsize_did
), Some(coerce_unsized_did
)) = traits
else {
525 debug
!("missing Unsize or CoerceUnsized traits");
526 return Err(TypeError
::Mismatch
);
529 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
530 // a DST unless we have to. This currently comes out in the wash since
531 // we can't unify [T] with U. But to properly support DST, we need to allow
532 // that, at which point we will need extra checks on the target here.
534 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
535 let reborrow
= match (source
.kind(), target
.kind()) {
536 (&ty
::Ref(_
, ty_a
, mutbl_a
), &ty
::Ref(_
, _
, mutbl_b
)) => {
537 coerce_mutbls(mutbl_a
, mutbl_b
)?
;
539 let coercion
= Coercion(self.cause
.span
);
540 let r_borrow
= self.next_region_var(coercion
);
541 let mutbl
= match mutbl_b
{
542 hir
::Mutability
::Not
=> AutoBorrowMutability
::Not
,
543 hir
::Mutability
::Mut
=> AutoBorrowMutability
::Mut
{
544 // We don't allow two-phase borrows here, at least for initial
545 // implementation. If it happens that this coercion is a function argument,
546 // the reborrow in coerce_borrowed_ptr will pick it up.
547 allow_two_phase_borrow
: AllowTwoPhase
::No
,
551 Adjustment { kind: Adjust::Deref(None), target: ty_a }
,
553 kind
: Adjust
::Borrow(AutoBorrow
::Ref(r_borrow
, mutbl
)),
556 .mk_ref(r_borrow
, ty
::TypeAndMut { mutbl: mutbl_b, ty: ty_a }
),
560 (&ty
::Ref(_
, ty_a
, mt_a
), &ty
::RawPtr(ty
::TypeAndMut { mutbl: mt_b, .. }
)) => {
561 coerce_mutbls(mt_a
, mt_b
)?
;
564 Adjustment { kind: Adjust::Deref(None), target: ty_a }
,
566 kind
: Adjust
::Borrow(AutoBorrow
::RawPtr(mt_b
)),
567 target
: self.tcx
.mk_ptr(ty
::TypeAndMut { mutbl: mt_b, ty: ty_a }
),
573 let coerce_source
= reborrow
.as_ref().map_or(source
, |&(_
, ref r
)| r
.target
);
575 // Setup either a subtyping or a LUB relationship between
576 // the `CoerceUnsized` target type and the expected type.
577 // We only have the latter, so we use an inference variable
578 // for the former and let type inference do the rest.
579 let origin
= TypeVariableOrigin
{
580 kind
: TypeVariableOriginKind
::MiscVariable
,
581 span
: self.cause
.span
,
583 let coerce_target
= self.next_ty_var(origin
);
584 let mut coercion
= self.unify_and(coerce_target
, target
, |target
| {
585 let unsize
= Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target }
;
587 None
=> vec
![unsize
],
588 Some((ref deref
, ref autoref
)) => vec
![deref
.clone(), autoref
.clone(), unsize
],
592 let mut selcx
= traits
::SelectionContext
::new(self);
594 // Create an obligation for `Source: CoerceUnsized<Target>`.
595 let cause
= ObligationCause
::new(
598 ObligationCauseCode
::Coercion { source, target }
,
601 // Use a FIFO queue for this custom fulfillment procedure.
603 // A Vec (or SmallVec) is not a natural choice for a queue. However,
604 // this code path is hot, and this queue usually has a max length of 1
605 // and almost never more than 3. By using a SmallVec we avoid an
606 // allocation, at the (very small) cost of (occasionally) having to
607 // shift subsequent elements down when removing the front element.
608 let mut queue
: SmallVec
<[_
; 4]> = smallvec
![traits
::predicate_for_trait_def(
615 &[coerce_target
.into()]
618 let mut has_unsized_tuple_coercion
= false;
619 let mut has_trait_upcasting_coercion
= None
;
621 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
622 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
623 // inference might unify those two inner type variables later.
624 let traits
= [coerce_unsized_did
, unsize_did
];
625 while !queue
.is_empty() {
626 let obligation
= queue
.remove(0);
627 debug
!("coerce_unsized resolve step: {:?}", obligation
);
628 let bound_predicate
= obligation
.predicate
.kind();
629 let trait_pred
= match bound_predicate
.skip_binder() {
630 ty
::PredicateKind
::Trait(trait_pred
) if traits
.contains(&trait_pred
.def_id()) => {
631 if unsize_did
== trait_pred
.def_id() {
632 let self_ty
= trait_pred
.self_ty();
633 let unsize_ty
= trait_pred
.trait_ref
.substs
[1].expect_ty();
634 if let (ty
::Dynamic(ref data_a
, ..), ty
::Dynamic(ref data_b
, ..)) =
635 (self_ty
.kind(), unsize_ty
.kind())
636 && data_a
.principal_def_id() != data_b
.principal_def_id()
638 debug
!("coerce_unsized: found trait upcasting coercion");
639 has_trait_upcasting_coercion
= Some((self_ty
, unsize_ty
));
641 if let ty
::Tuple(..) = unsize_ty
.kind() {
642 debug
!("coerce_unsized: found unsized tuple coercion");
643 has_unsized_tuple_coercion
= true;
646 bound_predicate
.rebind(trait_pred
)
649 coercion
.obligations
.push(obligation
);
653 match selcx
.select(&obligation
.with(trait_pred
)) {
654 // Uncertain or unimplemented.
656 if trait_pred
.def_id() == unsize_did
{
657 let trait_pred
= self.resolve_vars_if_possible(trait_pred
);
658 let self_ty
= trait_pred
.skip_binder().self_ty();
659 let unsize_ty
= trait_pred
.skip_binder().trait_ref
.substs
[1].expect_ty();
660 debug
!("coerce_unsized: ambiguous unsize case for {:?}", trait_pred
);
661 match (&self_ty
.kind(), &unsize_ty
.kind()) {
662 (ty
::Infer(ty
::TyVar(v
)), ty
::Dynamic(..))
663 if self.type_var_is_sized(*v
) =>
665 debug
!("coerce_unsized: have sized infer {:?}", v
);
666 coercion
.obligations
.push(obligation
);
667 // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
671 // Some other case for `$0: Unsize<Something>`. Note that we
672 // hit this case even if `Something` is a sized type, so just
673 // don't do the coercion.
674 debug
!("coerce_unsized: ambiguous unsize");
675 return Err(TypeError
::Mismatch
);
679 debug
!("coerce_unsized: early return - ambiguous");
680 return Err(TypeError
::Mismatch
);
683 Err(traits
::Unimplemented
) => {
684 debug
!("coerce_unsized: early return - can't prove obligation");
685 return Err(TypeError
::Mismatch
);
688 // Object safety violations or miscellaneous.
690 self.report_selection_error(obligation
.clone(), &obligation
, &err
, false);
691 // Treat this like an obligation and follow through
692 // with the unsizing - the lack of a coercion should
693 // be silent, as it causes a type mismatch later.
696 Ok(Some(impl_source
)) => queue
.extend(impl_source
.nested_obligations()),
700 if has_unsized_tuple_coercion
&& !self.tcx
.features().unsized_tuple_coercion
{
702 &self.tcx
.sess
.parse_sess
,
703 sym
::unsized_tuple_coercion
,
705 "unsized tuple coercion is not stable enough for use and is subject to change",
710 if let Some((sub
, sup
)) = has_trait_upcasting_coercion
711 && !self.tcx().features().trait_upcasting
713 // Renders better when we erase regions, since they're not really the point here.
714 let (sub
, sup
) = self.tcx
.erase_regions((sub
, sup
));
715 let mut err
= feature_err(
716 &self.tcx
.sess
.parse_sess
,
717 sym
::trait_upcasting
,
719 &format
!("cannot cast `{sub}` to `{sup}`, trait upcasting coercion is experimental"),
721 err
.note(&format
!("required when coercing `{source}` into `{target}`"));
728 fn coerce_from_safe_fn
<F
, G
>(
731 fn_ty_a
: ty
::PolyFnSig
<'tcx
>,
735 ) -> CoerceResult
<'tcx
>
737 F
: FnOnce(Ty
<'tcx
>) -> Vec
<Adjustment
<'tcx
>>,
738 G
: FnOnce(Ty
<'tcx
>) -> Vec
<Adjustment
<'tcx
>>,
740 self.commit_unconditionally(|snapshot
| {
741 let result
= if let ty
::FnPtr(fn_ty_b
) = b
.kind()
742 && let (hir
::Unsafety
::Normal
, hir
::Unsafety
::Unsafe
) =
743 (fn_ty_a
.unsafety(), fn_ty_b
.unsafety())
745 let unsafe_a
= self.tcx
.safe_to_unsafe_fn_ty(fn_ty_a
);
746 self.unify_and(unsafe_a
, b
, to_unsafe
)
748 self.unify_and(a
, b
, normal
)
751 // FIXME(#73154): This is a hack. Currently LUB can generate
752 // unsolvable constraints. Additionally, it returns `a`
753 // unconditionally, even when the "LUB" is `b`. In the future, we
754 // want the coerced type to be the actual supertype of these two,
755 // but for now, we want to just error to ensure we don't lock
756 // ourselves into a specific behavior with NLL.
757 self.leak_check(false, snapshot
)?
;
763 fn coerce_from_fn_pointer(
766 fn_ty_a
: ty
::PolyFnSig
<'tcx
>,
768 ) -> CoerceResult
<'tcx
> {
769 //! Attempts to coerce from the type of a Rust function item
770 //! into a closure or a `proc`.
773 let b
= self.shallow_resolve(b
);
774 debug
!("coerce_from_fn_pointer(a={:?}, b={:?})", a
, b
);
776 self.coerce_from_safe_fn(
780 simple(Adjust
::Pointer(PointerCast
::UnsafeFnPointer
)),
785 fn coerce_from_fn_item(&self, a
: Ty
<'tcx
>, b
: Ty
<'tcx
>) -> CoerceResult
<'tcx
> {
786 //! Attempts to coerce from the type of a Rust function item
787 //! into a closure or a `proc`.
789 let b
= self.shallow_resolve(b
);
790 let InferOk { value: b, mut obligations }
=
791 self.normalize_associated_types_in_as_infer_ok(self.cause
.span
, b
);
792 debug
!("coerce_from_fn_item(a={:?}, b={:?})", a
, b
);
795 ty
::FnPtr(b_sig
) => {
796 let a_sig
= a
.fn_sig(self.tcx
);
797 if let ty
::FnDef(def_id
, _
) = *a
.kind() {
798 // Intrinsics are not coercible to function pointers
799 if self.tcx
.is_intrinsic(def_id
) {
800 return Err(TypeError
::IntrinsicCast
);
803 // Safe `#[target_feature]` functions are not assignable to safe fn pointers (RFC 2396).
805 if b_sig
.unsafety() == hir
::Unsafety
::Normal
806 && !self.tcx
.codegen_fn_attrs(def_id
).target_features
.is_empty()
808 return Err(TypeError
::TargetFeatureCast(def_id
));
812 let InferOk { value: a_sig, obligations: o1 }
=
813 self.normalize_associated_types_in_as_infer_ok(self.cause
.span
, a_sig
);
814 obligations
.extend(o1
);
816 let a_fn_pointer
= self.tcx
.mk_fn_ptr(a_sig
);
817 let InferOk { value, obligations: o2 }
= self.coerce_from_safe_fn(
824 kind
: Adjust
::Pointer(PointerCast
::ReifyFnPointer
),
825 target
: a_fn_pointer
,
828 kind
: Adjust
::Pointer(PointerCast
::UnsafeFnPointer
),
833 simple(Adjust
::Pointer(PointerCast
::ReifyFnPointer
)),
836 obligations
.extend(o2
);
837 Ok(InferOk { value, obligations }
)
839 _
=> self.unify_and(a
, b
, identity
),
843 fn coerce_closure_to_fn(
846 closure_def_id_a
: DefId
,
847 substs_a
: SubstsRef
<'tcx
>,
849 ) -> CoerceResult
<'tcx
> {
850 //! Attempts to coerce from the type of a non-capturing closure
851 //! into a function pointer.
854 let b
= self.shallow_resolve(b
);
857 // At this point we haven't done capture analysis, which means
858 // that the ClosureSubsts just contains an inference variable instead
859 // of tuple of captured types.
861 // All we care here is if any variable is being captured and not the exact paths,
862 // so we check `upvars_mentioned` for root variables being captured.
866 .upvars_mentioned(closure_def_id_a
.expect_local())
867 .map_or(true, |u
| u
.is_empty()) =>
869 // We coerce the closure, which has fn type
870 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
872 // `fn(arg0,arg1,...) -> _`
874 // `unsafe fn(arg0,arg1,...) -> _`
875 let closure_sig
= substs_a
.as_closure().sig();
876 let unsafety
= fn_ty
.unsafety();
878 self.tcx
.mk_fn_ptr(self.tcx
.signature_unclosure(closure_sig
, unsafety
));
879 debug
!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a
, b
, pointer_ty
);
883 simple(Adjust
::Pointer(PointerCast
::ClosureFnPointer(unsafety
))),
886 _
=> self.unify_and(a
, b
, identity
),
890 fn coerce_unsafe_ptr(
894 mutbl_b
: hir
::Mutability
,
895 ) -> CoerceResult
<'tcx
> {
896 debug
!("coerce_unsafe_ptr(a={:?}, b={:?})", a
, b
);
898 let (is_ref
, mt_a
) = match *a
.kind() {
899 ty
::Ref(_
, ty
, mutbl
) => (true, ty
::TypeAndMut { ty, mutbl }
),
900 ty
::RawPtr(mt
) => (false, mt
),
901 _
=> return self.unify_and(a
, b
, identity
),
903 coerce_mutbls(mt_a
.mutbl
, mutbl_b
)?
;
905 // Check that the types which they point at are compatible.
906 let a_unsafe
= self.tcx
.mk_ptr(ty
::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty }
);
907 // Although references and unsafe ptrs have the same
908 // representation, we still register an Adjust::DerefRef so that
909 // regionck knows that the region for `a` must be valid here.
911 self.unify_and(a_unsafe
, b
, |target
| {
913 Adjustment { kind: Adjust::Deref(None), target: mt_a.ty }
,
914 Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target }
,
917 } else if mt_a
.mutbl
!= mutbl_b
{
918 self.unify_and(a_unsafe
, b
, simple(Adjust
::Pointer(PointerCast
::MutToConstPointer
)))
920 self.unify_and(a_unsafe
, b
, identity
)
925 impl<'a
, 'tcx
> FnCtxt
<'a
, 'tcx
> {
926 /// Attempt to coerce an expression to a type, and return the
927 /// adjusted type of the expression, if successful.
928 /// Adjustments are only recorded if the coercion succeeded.
929 /// The expressions *must not* have any pre-existing adjustments.
932 expr
: &hir
::Expr
<'_
>,
935 allow_two_phase
: AllowTwoPhase
,
936 cause
: Option
<ObligationCause
<'tcx
>>,
937 ) -> RelateResult
<'tcx
, Ty
<'tcx
>> {
938 let source
= self.resolve_vars_with_obligations(expr_ty
);
939 debug
!("coercion::try({:?}: {:?} -> {:?})", expr
, source
, target
);
942 cause
.unwrap_or_else(|| self.cause(expr
.span
, ObligationCauseCode
::ExprAssignable
));
943 let coerce
= Coerce
::new(self, cause
, allow_two_phase
);
944 let ok
= self.commit_if_ok(|_
| coerce
.coerce(source
, target
))?
;
946 let (adjustments
, _
) = self.register_infer_ok_obligations(ok
);
947 self.apply_adjustments(expr
, adjustments
);
948 Ok(if expr_ty
.references_error() { self.tcx.ty_error() }
else { target }
)
951 /// Same as `try_coerce()`, but without side-effects.
953 /// Returns false if the coercion creates any obligations that result in
955 pub fn can_coerce(&self, expr_ty
: Ty
<'tcx
>, target
: Ty
<'tcx
>) -> bool
{
956 let source
= self.resolve_vars_with_obligations(expr_ty
);
957 debug
!("coercion::can_with_predicates({:?} -> {:?})", source
, target
);
959 let cause
= self.cause(rustc_span
::DUMMY_SP
, ObligationCauseCode
::ExprAssignable
);
960 // We don't ever need two-phase here since we throw out the result of the coercion
961 let coerce
= Coerce
::new(self, cause
, AllowTwoPhase
::No
);
963 let Ok(ok
) = coerce
.coerce(source
, target
) else {
966 let mut fcx
= traits
::FulfillmentContext
::new_in_snapshot();
967 fcx
.register_predicate_obligations(self, ok
.obligations
);
968 fcx
.select_where_possible(&self).is_empty()
972 /// Given a type and a target type, this function will calculate and return
973 /// how many dereference steps needed to achieve `expr_ty <: target`. If
974 /// it's not possible, return `None`.
975 pub fn deref_steps(&self, expr_ty
: Ty
<'tcx
>, target
: Ty
<'tcx
>) -> Option
<usize> {
976 let cause
= self.cause(rustc_span
::DUMMY_SP
, ObligationCauseCode
::ExprAssignable
);
977 // We don't ever need two-phase here since we throw out the result of the coercion
978 let coerce
= Coerce
::new(self, cause
, AllowTwoPhase
::No
);
980 .autoderef(rustc_span
::DUMMY_SP
, expr_ty
)
981 .find_map(|(ty
, steps
)| self.probe(|_
| coerce
.unify(ty
, target
)).ok().map(|_
| steps
))
984 /// Given a type, this function will calculate and return the type given
985 /// for `<Ty as Deref>::Target` only if `Ty` also implements `DerefMut`.
987 /// This function is for diagnostics only, since it does not register
988 /// trait or region sub-obligations. (presumably we could, but it's not
989 /// particularly important for diagnostics...)
990 pub fn deref_once_mutably_for_diagnostic(&self, expr_ty
: Ty
<'tcx
>) -> Option
<Ty
<'tcx
>> {
991 self.autoderef(rustc_span
::DUMMY_SP
, expr_ty
).nth(1).and_then(|(deref_ty
, _
)| {
993 .type_implements_trait(
994 self.infcx
.tcx
.lang_items().deref_mut_trait()?
,
1004 /// Given some expressions, their known unified type and another expression,
1005 /// tries to unify the types, potentially inserting coercions on any of the
1006 /// provided expressions and returns their LUB (aka "common supertype").
1008 /// This is really an internal helper. From outside the coercion
1009 /// module, you should instantiate a `CoerceMany` instance.
1010 fn try_find_coercion_lub
<E
>(
1012 cause
: &ObligationCause
<'tcx
>,
1015 new
: &hir
::Expr
<'_
>,
1017 ) -> RelateResult
<'tcx
, Ty
<'tcx
>>
1021 let prev_ty
= self.resolve_vars_with_obligations(prev_ty
);
1022 let new_ty
= self.resolve_vars_with_obligations(new_ty
);
1024 "coercion::try_find_coercion_lub({:?}, {:?}, exprs={:?} exprs)",
1030 // The following check fixes #88097, where the compiler erroneously
1031 // attempted to coerce a closure type to itself via a function pointer.
1032 if prev_ty
== new_ty
{
1036 // Special-case that coercion alone cannot handle:
1037 // Function items or non-capturing closures of differing IDs or InternalSubsts.
1038 let (a_sig
, b_sig
) = {
1039 #[allow(rustc::usage_of_ty_tykind)]
1040 let is_capturing_closure
= |ty
: &ty
::TyKind
<'tcx
>| {
1041 if let &ty
::Closure(closure_def_id
, _substs
) = ty
{
1042 self.tcx
.upvars_mentioned(closure_def_id
.expect_local()).is_some()
1047 if is_capturing_closure(prev_ty
.kind()) || is_capturing_closure(new_ty
.kind()) {
1050 match (prev_ty
.kind(), new_ty
.kind()) {
1051 (ty
::FnDef(..), ty
::FnDef(..)) => {
1052 // Don't reify if the function types have a LUB, i.e., they
1053 // are the same function and their parameters have a LUB.
1055 .commit_if_ok(|_
| self.at(cause
, self.param_env
).lub(prev_ty
, new_ty
))
1057 // We have a LUB of prev_ty and new_ty, just return it.
1058 Ok(ok
) => return Ok(self.register_infer_ok_obligations(ok
)),
1060 (Some(prev_ty
.fn_sig(self.tcx
)), Some(new_ty
.fn_sig(self.tcx
)))
1064 (ty
::Closure(_
, substs
), ty
::FnDef(..)) => {
1065 let b_sig
= new_ty
.fn_sig(self.tcx
);
1068 .signature_unclosure(substs
.as_closure().sig(), b_sig
.unsafety());
1069 (Some(a_sig
), Some(b_sig
))
1071 (ty
::FnDef(..), ty
::Closure(_
, substs
)) => {
1072 let a_sig
= prev_ty
.fn_sig(self.tcx
);
1075 .signature_unclosure(substs
.as_closure().sig(), a_sig
.unsafety());
1076 (Some(a_sig
), Some(b_sig
))
1078 (ty
::Closure(_
, substs_a
), ty
::Closure(_
, substs_b
)) => (
1079 Some(self.tcx
.signature_unclosure(
1080 substs_a
.as_closure().sig(),
1081 hir
::Unsafety
::Normal
,
1083 Some(self.tcx
.signature_unclosure(
1084 substs_b
.as_closure().sig(),
1085 hir
::Unsafety
::Normal
,
1092 if let (Some(a_sig
), Some(b_sig
)) = (a_sig
, b_sig
) {
1093 // Intrinsics are not coercible to function pointers.
1094 if a_sig
.abi() == Abi
::RustIntrinsic
1095 || a_sig
.abi() == Abi
::PlatformIntrinsic
1096 || b_sig
.abi() == Abi
::RustIntrinsic
1097 || b_sig
.abi() == Abi
::PlatformIntrinsic
1099 return Err(TypeError
::IntrinsicCast
);
1101 // The signature must match.
1102 let a_sig
= self.normalize_associated_types_in(new
.span
, a_sig
);
1103 let b_sig
= self.normalize_associated_types_in(new
.span
, b_sig
);
1105 .at(cause
, self.param_env
)
1106 .trace(prev_ty
, new_ty
)
1108 .map(|ok
| self.register_infer_ok_obligations(ok
))?
;
1110 // Reify both sides and return the reified fn pointer type.
1111 let fn_ptr
= self.tcx
.mk_fn_ptr(sig
);
1112 let prev_adjustment
= match prev_ty
.kind() {
1113 ty
::Closure(..) => Adjust
::Pointer(PointerCast
::ClosureFnPointer(a_sig
.unsafety())),
1114 ty
::FnDef(..) => Adjust
::Pointer(PointerCast
::ReifyFnPointer
),
1115 _
=> unreachable
!(),
1117 let next_adjustment
= match new_ty
.kind() {
1118 ty
::Closure(..) => Adjust
::Pointer(PointerCast
::ClosureFnPointer(b_sig
.unsafety())),
1119 ty
::FnDef(..) => Adjust
::Pointer(PointerCast
::ReifyFnPointer
),
1120 _
=> unreachable
!(),
1122 for expr
in exprs
.iter().map(|e
| e
.as_coercion_site()) {
1123 self.apply_adjustments(
1125 vec
![Adjustment { kind: prev_adjustment.clone(), target: fn_ptr }
],
1128 self.apply_adjustments(new
, vec
![Adjustment { kind: next_adjustment, target: fn_ptr }
]);
1132 // Configure a Coerce instance to compute the LUB.
1133 // We don't allow two-phase borrows on any autorefs this creates since we
1134 // probably aren't processing function arguments here and even if we were,
1135 // they're going to get autorefed again anyway and we can apply 2-phase borrows
1137 let mut coerce
= Coerce
::new(self, cause
.clone(), AllowTwoPhase
::No
);
1138 coerce
.use_lub
= true;
1140 // First try to coerce the new expression to the type of the previous ones,
1141 // but only if the new expression has no coercion already applied to it.
1142 let mut first_error
= None
;
1143 if !self.typeck_results
.borrow().adjustments().contains_key(new
.hir_id
) {
1144 let result
= self.commit_if_ok(|_
| coerce
.coerce(new_ty
, prev_ty
));
1147 let (adjustments
, target
) = self.register_infer_ok_obligations(ok
);
1148 self.apply_adjustments(new
, adjustments
);
1150 "coercion::try_find_coercion_lub: was able to coerce from new type {:?} to previous type {:?} ({:?})",
1151 new_ty
, prev_ty
, target
1155 Err(e
) => first_error
= Some(e
),
1159 // Then try to coerce the previous expressions to the type of the new one.
1160 // This requires ensuring there are no coercions applied to *any* of the
1161 // previous expressions, other than noop reborrows (ignoring lifetimes).
1163 let expr
= expr
.as_coercion_site();
1164 let noop
= match self.typeck_results
.borrow().expr_adjustments(expr
) {
1166 Adjustment { kind: Adjust::Deref(_), .. }
,
1167 Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. }
,
1169 match *self.node_ty(expr
.hir_id
).kind() {
1170 ty
::Ref(_
, _
, mt_orig
) => {
1171 let mutbl_adj
: hir
::Mutability
= mutbl_adj
.into();
1172 // Reborrow that we can safely ignore, because
1173 // the next adjustment can only be a Deref
1174 // which will be merged into it.
1175 mutbl_adj
== mt_orig
1180 &[Adjustment { kind: Adjust::NeverToAny, .. }
] | &[] => true,
1186 "coercion::try_find_coercion_lub: older expression {:?} had adjustments, requiring LUB",
1191 .commit_if_ok(|_
| self.at(cause
, self.param_env
).lub(prev_ty
, new_ty
))
1192 .map(|ok
| self.register_infer_ok_obligations(ok
));
1196 match self.commit_if_ok(|_
| coerce
.coerce(prev_ty
, new_ty
)) {
1198 // Avoid giving strange errors on failed attempts.
1199 if let Some(e
) = first_error
{
1202 self.commit_if_ok(|_
| self.at(cause
, self.param_env
).lub(prev_ty
, new_ty
))
1203 .map(|ok
| self.register_infer_ok_obligations(ok
))
1207 let (adjustments
, target
) = self.register_infer_ok_obligations(ok
);
1209 let expr
= expr
.as_coercion_site();
1210 self.apply_adjustments(expr
, adjustments
.clone());
1213 "coercion::try_find_coercion_lub: was able to coerce previous type {:?} to new type {:?} ({:?})",
1214 prev_ty
, new_ty
, target
1222 /// CoerceMany encapsulates the pattern you should use when you have
1223 /// many expressions that are all getting coerced to a common
1224 /// type. This arises, for example, when you have a match (the result
1225 /// of each arm is coerced to a common type). It also arises in less
1226 /// obvious places, such as when you have many `break foo` expressions
1227 /// that target the same loop, or the various `return` expressions in
1230 /// The basic protocol is as follows:
1232 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
1233 /// This will also serve as the "starting LUB". The expectation is
1234 /// that this type is something which all of the expressions *must*
1235 /// be coercible to. Use a fresh type variable if needed.
1236 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
1237 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
1238 /// unit. This happens for example if you have a `break` with no expression,
1239 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
1240 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
1241 /// from you so that you don't have to worry your pretty head about it.
1242 /// But if an error is reported, the final type will be `err`.
1243 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
1244 /// previously coerced expressions.
1245 /// - When all done, invoke `complete()`. This will return the LUB of
1246 /// all your expressions.
1247 /// - WARNING: I don't believe this final type is guaranteed to be
1248 /// related to your initial `expected_ty` in any particular way,
1249 /// although it will typically be a subtype, so you should check it.
1250 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
1251 /// previously coerced expressions.
1255 /// ```ignore (illustrative)
1256 /// let mut coerce = CoerceMany::new(expected_ty);
1257 /// for expr in exprs {
1258 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
1259 /// coerce.coerce(fcx, &cause, expr, expr_ty);
1261 /// let final_ty = coerce.complete(fcx);
1263 pub struct CoerceMany
<'tcx
, 'exprs
, E
: AsCoercionSite
> {
1264 expected_ty
: Ty
<'tcx
>,
1265 final_ty
: Option
<Ty
<'tcx
>>,
1266 expressions
: Expressions
<'tcx
, 'exprs
, E
>,
1270 /// The type of a `CoerceMany` that is storing up the expressions into
1271 /// a buffer. We use this in `check/mod.rs` for things like `break`.
1272 pub type DynamicCoerceMany
<'tcx
> = CoerceMany
<'tcx
, 'tcx
, &'tcx hir
::Expr
<'tcx
>>;
1274 enum Expressions
<'tcx
, 'exprs
, E
: AsCoercionSite
> {
1275 Dynamic(Vec
<&'tcx hir
::Expr
<'tcx
>>),
1276 UpFront(&'exprs
[E
]),
1279 impl<'tcx
, 'exprs
, E
: AsCoercionSite
> CoerceMany
<'tcx
, 'exprs
, E
> {
1280 /// The usual case; collect the set of expressions dynamically.
1281 /// If the full set of coercion sites is known before hand,
1282 /// consider `with_coercion_sites()` instead to avoid allocation.
1283 pub fn new(expected_ty
: Ty
<'tcx
>) -> Self {
1284 Self::make(expected_ty
, Expressions
::Dynamic(vec
![]))
1287 /// As an optimization, you can create a `CoerceMany` with a
1288 /// pre-existing slice of expressions. In this case, you are
1289 /// expected to pass each element in the slice to `coerce(...)` in
1290 /// order. This is used with arrays in particular to avoid
1291 /// needlessly cloning the slice.
1292 pub fn with_coercion_sites(expected_ty
: Ty
<'tcx
>, coercion_sites
: &'exprs
[E
]) -> Self {
1293 Self::make(expected_ty
, Expressions
::UpFront(coercion_sites
))
1296 fn make(expected_ty
: Ty
<'tcx
>, expressions
: Expressions
<'tcx
, 'exprs
, E
>) -> Self {
1297 CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
1300 /// Returns the "expected type" with which this coercion was
1301 /// constructed. This represents the "downward propagated" type
1302 /// that was given to us at the start of typing whatever construct
1303 /// we are typing (e.g., the match expression).
1305 /// Typically, this is used as the expected type when
1306 /// type-checking each of the alternative expressions whose types
1307 /// we are trying to merge.
1308 pub fn expected_ty(&self) -> Ty
<'tcx
> {
1312 /// Returns the current "merged type", representing our best-guess
1313 /// at the LUB of the expressions we've seen so far (if any). This
1314 /// isn't *final* until you call `self.complete()`, which will return
1315 /// the merged type.
1316 pub fn merged_ty(&self) -> Ty
<'tcx
> {
1317 self.final_ty
.unwrap_or(self.expected_ty
)
1320 /// Indicates that the value generated by `expression`, which is
1321 /// of type `expression_ty`, is one of the possibilities that we
1322 /// could coerce from. This will record `expression`, and later
1323 /// calls to `coerce` may come back and add adjustments and things
1327 fcx
: &FnCtxt
<'a
, 'tcx
>,
1328 cause
: &ObligationCause
<'tcx
>,
1329 expression
: &'tcx hir
::Expr
<'tcx
>,
1330 expression_ty
: Ty
<'tcx
>,
1332 self.coerce_inner(fcx
, cause
, Some(expression
), expression_ty
, None
, false)
1335 /// Indicates that one of the inputs is a "forced unit". This
1336 /// occurs in a case like `if foo { ... };`, where the missing else
1337 /// generates a "forced unit". Another example is a `loop { break;
1338 /// }`, where the `break` has no argument expression. We treat
1339 /// these cases slightly differently for error-reporting
1340 /// purposes. Note that these tend to correspond to cases where
1341 /// the `()` expression is implicit in the source, and hence we do
1342 /// not take an expression argument.
1344 /// The `augment_error` gives you a chance to extend the error
1345 /// message, in case any results (e.g., we use this to suggest
1346 /// removing a `;`).
1347 pub fn coerce_forced_unit
<'a
>(
1349 fcx
: &FnCtxt
<'a
, 'tcx
>,
1350 cause
: &ObligationCause
<'tcx
>,
1351 augment_error
: &mut dyn FnMut(&mut Diagnostic
),
1352 label_unit_as_expected
: bool
,
1359 Some(augment_error
),
1360 label_unit_as_expected
,
1364 /// The inner coercion "engine". If `expression` is `None`, this
1365 /// is a forced-unit case, and hence `expression_ty` must be
1367 #[instrument(skip(self, fcx, augment_error, label_expression_as_expected), level = "debug")]
1368 pub(crate) fn coerce_inner
<'a
>(
1370 fcx
: &FnCtxt
<'a
, 'tcx
>,
1371 cause
: &ObligationCause
<'tcx
>,
1372 expression
: Option
<&'tcx hir
::Expr
<'tcx
>>,
1373 mut expression_ty
: Ty
<'tcx
>,
1374 augment_error
: Option
<&mut dyn FnMut(&mut Diagnostic
)>,
1375 label_expression_as_expected
: bool
,
1377 // Incorporate whatever type inference information we have
1378 // until now; in principle we might also want to process
1379 // pending obligations, but doing so should only improve
1380 // compatibility (hopefully that is true) by helping us
1381 // uncover never types better.
1382 if expression_ty
.is_ty_var() {
1383 expression_ty
= fcx
.infcx
.shallow_resolve(expression_ty
);
1386 // If we see any error types, just propagate that error
1388 if expression_ty
.references_error() || self.merged_ty().references_error() {
1389 self.final_ty
= Some(fcx
.tcx
.ty_error());
1393 // Handle the actual type unification etc.
1394 let result
= if let Some(expression
) = expression
{
1395 if self.pushed
== 0 {
1396 // Special-case the first expression we are coercing.
1397 // To be honest, I'm not entirely sure why we do this.
1398 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1404 Some(cause
.clone()),
1407 match self.expressions
{
1408 Expressions
::Dynamic(ref exprs
) => fcx
.try_find_coercion_lub(
1415 Expressions
::UpFront(ref coercion_sites
) => fcx
.try_find_coercion_lub(
1417 &coercion_sites
[0..self.pushed
],
1425 // this is a hack for cases where we default to `()` because
1426 // the expression etc has been omitted from the source. An
1427 // example is an `if let` without an else:
1429 // if let Some(x) = ... { }
1431 // we wind up with a second match arm that is like `_ =>
1432 // ()`. That is the case we are considering here. We take
1433 // a different path to get the right "expected, found"
1434 // message and so forth (and because we know that
1435 // `expression_ty` will be unit).
1437 // Another example is `break` with no argument expression.
1438 assert
!(expression_ty
.is_unit(), "if let hack without unit type");
1439 fcx
.at(cause
, fcx
.param_env
)
1440 .eq_exp(label_expression_as_expected
, expression_ty
, self.merged_ty())
1442 fcx
.register_infer_ok_obligations(infer_ok
);
1450 self.final_ty
= Some(v
);
1451 if let Some(e
) = expression
{
1452 match self.expressions
{
1453 Expressions
::Dynamic(ref mut buffer
) => buffer
.push(e
),
1454 Expressions
::UpFront(coercion_sites
) => {
1455 // if the user gave us an array to validate, check that we got
1456 // the next expression in the list, as expected
1458 coercion_sites
[self.pushed
].as_coercion_site().hir_id
,
1466 Err(coercion_error
) => {
1467 let (expected
, found
) = if label_expression_as_expected
{
1468 // In the case where this is a "forced unit", like
1469 // `break`, we want to call the `()` "expected"
1470 // since it is implied by the syntax.
1471 // (Note: not all force-units work this way.)"
1472 (expression_ty
, self.final_ty
.unwrap_or(self.expected_ty
))
1474 // Otherwise, the "expected" type for error
1475 // reporting is the current unification type,
1476 // which is basically the LUB of the expressions
1477 // we've seen so far (combined with the expected
1479 (self.final_ty
.unwrap_or(self.expected_ty
), expression_ty
)
1483 let mut unsized_return
= false;
1484 match *cause
.code() {
1485 ObligationCauseCode
::ReturnNoExpression
=> {
1486 err
= struct_span_err
!(
1490 "`return;` in a function whose return type is not `()`"
1492 err
.span_label(cause
.span
, "return type is not `()`");
1494 ObligationCauseCode
::BlockTailExpression(blk_id
) => {
1495 let parent_id
= fcx
.tcx
.hir().get_parent_node(blk_id
);
1496 err
= self.report_return_mismatched_types(
1500 coercion_error
.clone(),
1506 if !fcx
.tcx
.features().unsized_locals
{
1507 unsized_return
= self.is_return_ty_unsized(fcx
, blk_id
);
1510 ObligationCauseCode
::ReturnValue(id
) => {
1511 err
= self.report_return_mismatched_types(
1515 coercion_error
.clone(),
1521 if !fcx
.tcx
.features().unsized_locals
{
1522 let id
= fcx
.tcx
.hir().get_parent_node(id
);
1523 unsized_return
= self.is_return_ty_unsized(fcx
, id
);
1527 err
= fcx
.report_mismatched_types(
1531 coercion_error
.clone(),
1536 if let Some(augment_error
) = augment_error
{
1537 augment_error(&mut err
);
1540 let is_insufficiently_polymorphic
=
1541 matches
!(coercion_error
, TypeError
::RegionsInsufficientlyPolymorphic(..));
1543 if !is_insufficiently_polymorphic
&& let Some(expr
) = expression
{
1544 fcx
.emit_coerce_suggestions(
1550 Some(coercion_error
),
1554 err
.emit_unless(unsized_return
);
1556 self.final_ty
= Some(fcx
.tcx
.ty_error());
1561 fn report_return_mismatched_types
<'a
>(
1563 cause
: &ObligationCause
<'tcx
>,
1566 ty_err
: TypeError
<'tcx
>,
1567 fcx
: &FnCtxt
<'a
, 'tcx
>,
1569 expression
: Option
<&'tcx hir
::Expr
<'tcx
>>,
1570 blk_id
: Option
<hir
::HirId
>,
1571 ) -> DiagnosticBuilder
<'a
, ErrorGuaranteed
> {
1572 let mut err
= fcx
.report_mismatched_types(cause
, expected
, found
, ty_err
);
1574 let mut pointing_at_return_type
= false;
1575 let mut fn_output
= None
;
1577 let parent_id
= fcx
.tcx
.hir().get_parent_node(id
);
1578 let parent
= fcx
.tcx
.hir().get(parent_id
);
1579 if let Some(expr
) = expression
1580 && let hir
::Node
::Expr(hir
::Expr { kind: hir::ExprKind::Closure { body, .. }
, .. }) = parent
1581 && !matches
!(fcx
.tcx
.hir().body(*body
).value
.kind
, hir
::ExprKind
::Block(..))
1583 fcx
.suggest_missing_semicolon(&mut err
, expr
, expected
, true);
1585 // Verify that this is a tail expression of a function, otherwise the
1586 // label pointing out the cause for the type coercion will be wrong
1587 // as prior return coercions would not be relevant (#57664).
1588 let fn_decl
= if let (Some(expr
), Some(blk_id
)) = (expression
, blk_id
) {
1589 pointing_at_return_type
=
1590 fcx
.suggest_mismatched_types_on_tail(&mut err
, expr
, expected
, found
, blk_id
);
1591 if let (Some(cond_expr
), true, false) = (
1592 fcx
.tcx
.hir().get_if_cause(expr
.hir_id
),
1594 pointing_at_return_type
,
1596 // If the block is from an external macro or try (`?`) desugaring, then
1597 // do not suggest adding a semicolon, because there's nowhere to put it.
1598 // See issues #81943 and #87051.
1600 cond_expr
.span
.desugaring_kind(),
1601 None
| Some(DesugaringKind
::WhileLoop
)
1602 ) && !in_external_macro(fcx
.tcx
.sess
, cond_expr
.span
)
1605 hir
::ExprKind
::Match(.., hir
::MatchSource
::TryDesugar
)
1608 err
.span_label(cond_expr
.span
, "expected this to be `()`");
1609 if expr
.can_have_side_effects() {
1610 fcx
.suggest_semicolon_at_end(cond_expr
.span
, &mut err
);
1613 fcx
.get_node_fn_decl(parent
).map(|(fn_decl
, _
, is_main
)| (fn_decl
, is_main
))
1615 fcx
.get_fn_decl(parent_id
)
1618 if let Some((fn_decl
, can_suggest
)) = fn_decl
{
1619 if blk_id
.is_none() {
1620 pointing_at_return_type
|= fcx
.suggest_missing_return_type(
1626 fcx
.tcx
.hir().local_def_id_to_hir_id(fcx
.tcx
.hir().get_parent_item(id
)),
1629 if !pointing_at_return_type
{
1630 fn_output
= Some(&fn_decl
.output
); // `impl Trait` return type
1634 let parent_id
= fcx
.tcx
.hir().get_parent_item(id
);
1635 let parent_item
= fcx
.tcx
.hir().get_by_def_id(parent_id
);
1637 if let (Some(expr
), Some(_
), Some((fn_decl
, _
, _
))) =
1638 (expression
, blk_id
, fcx
.get_node_fn_decl(parent_item
))
1640 fcx
.suggest_missing_break_or_return_expr(
1647 fcx
.tcx
.hir().local_def_id_to_hir_id(parent_id
),
1651 if let (Some(sp
), Some(fn_output
)) = (fcx
.ret_coercion_span
.get(), fn_output
) {
1652 self.add_impl_trait_explanation(&mut err
, cause
, fcx
, expected
, sp
, fn_output
);
1657 fn add_impl_trait_explanation
<'a
>(
1659 err
: &mut Diagnostic
,
1660 cause
: &ObligationCause
<'tcx
>,
1661 fcx
: &FnCtxt
<'a
, 'tcx
>,
1664 fn_output
: &hir
::FnRetTy
<'_
>,
1666 let return_sp
= fn_output
.span();
1667 err
.span_label(return_sp
, "expected because this return type...");
1670 format
!("...is found to be `{}` here", fcx
.resolve_vars_with_obligations(expected
)),
1672 let impl_trait_msg
= "for information on `impl Trait`, see \
1673 <https://doc.rust-lang.org/book/ch10-02-traits.html\
1674 #returning-types-that-implement-traits>";
1675 let trait_obj_msg
= "for information on trait objects, see \
1676 <https://doc.rust-lang.org/book/ch17-02-trait-objects.html\
1677 #using-trait-objects-that-allow-for-values-of-different-types>";
1678 err
.note("to return `impl Trait`, all returned values must be of the same type");
1679 err
.note(impl_trait_msg
);
1684 .span_to_snippet(return_sp
)
1685 .unwrap_or_else(|_
| "dyn Trait".to_string());
1686 let mut snippet_iter
= snippet
.split_whitespace();
1687 let has_impl
= snippet_iter
.next().map_or(false, |s
| s
== "impl");
1688 // Only suggest `Box<dyn Trait>` if `Trait` in `impl Trait` is object safe.
1689 let mut is_object_safe
= false;
1690 if let hir
::FnRetTy
::Return(ty
) = fn_output
1691 // Get the return type.
1692 && let hir
::TyKind
::OpaqueDef(..) = ty
.kind
1694 let ty
= <dyn AstConv
<'_
>>::ast_ty_to_ty(fcx
, ty
);
1695 // Get the `impl Trait`'s `DefId`.
1696 if let ty
::Opaque(def_id
, _
) = ty
.kind()
1697 // Get the `impl Trait`'s `Item` so that we can get its trait bounds and
1698 // get the `Trait`'s `DefId`.
1699 && let hir
::ItemKind
::OpaqueTy(hir
::OpaqueTy { bounds, .. }
) =
1700 fcx
.tcx
.hir().expect_item(def_id
.expect_local()).kind
1702 // Are of this `impl Trait`'s traits object safe?
1703 is_object_safe
= bounds
.iter().all(|bound
| {
1706 .and_then(|t
| t
.trait_def_id())
1707 .map_or(false, |def_id
| {
1708 fcx
.tcx
.object_safety_violations(def_id
).is_empty()
1715 err
.multipart_suggestion(
1716 "you could change the return type to be a boxed trait object",
1718 (return_sp
.with_hi(return_sp
.lo() + BytePos(4)), "Box<dyn".to_string()),
1719 (return_sp
.shrink_to_hi(), ">".to_string()),
1721 Applicability
::MachineApplicable
,
1723 let sugg
= [sp
, cause
.span
]
1727 (sp
.shrink_to_lo(), "Box::new(".to_string()),
1728 (sp
.shrink_to_hi(), ")".to_string()),
1732 .collect
::<Vec
<_
>>();
1733 err
.multipart_suggestion(
1734 "if you change the return type to expect trait objects, box the returned \
1737 Applicability
::MaybeIncorrect
,
1741 "if the trait `{}` were object safe, you could return a boxed trait object",
1745 err
.note(trait_obj_msg
);
1747 err
.help("you could instead create a new `enum` with a variant for each returned type");
1750 fn is_return_ty_unsized
<'a
>(&self, fcx
: &FnCtxt
<'a
, 'tcx
>, blk_id
: hir
::HirId
) -> bool
{
1751 if let Some((fn_decl
, _
)) = fcx
.get_fn_decl(blk_id
)
1752 && let hir
::FnRetTy
::Return(ty
) = fn_decl
.output
1753 && let ty
= <dyn AstConv
<'_
>>::ast_ty_to_ty(fcx
, ty
)
1754 && let ty
::Dynamic(..) = ty
.kind()
1761 pub fn complete
<'a
>(self, fcx
: &FnCtxt
<'a
, 'tcx
>) -> Ty
<'tcx
> {
1762 if let Some(final_ty
) = self.final_ty
{
1765 // If we only had inputs that were of type `!` (or no
1766 // inputs at all), then the final type is `!`.
1767 assert_eq
!(self.pushed
, 0);
1773 /// Something that can be converted into an expression to which we can
1774 /// apply a coercion.
1775 pub trait AsCoercionSite
{
1776 fn as_coercion_site(&self) -> &hir
::Expr
<'_
>;
1779 impl AsCoercionSite
for hir
::Expr
<'_
> {
1780 fn as_coercion_site(&self) -> &hir
::Expr
<'_
> {
1785 impl<'a
, T
> AsCoercionSite
for &'a T
1789 fn as_coercion_site(&self) -> &hir
::Expr
<'_
> {
1790 (**self).as_coercion_site()
1794 impl AsCoercionSite
for ! {
1795 fn as_coercion_site(&self) -> &hir
::Expr
<'_
> {
1800 impl AsCoercionSite
for hir
::Arm
<'_
> {
1801 fn as_coercion_site(&self) -> &hir
::Expr
<'_
> {