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/const things (that is, when the expected is &T
14 //! but you have &const T or &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-reborrow-*.rs` tests for
17 //! examples of where this is useful.
21 //! When deciding what type coercions to consider, we do not attempt to
22 //! resolve any type variables we may encounter. This is because `b`
23 //! represents the expected type "as the user wrote it", meaning that if
24 //! the user defined a generic function like
26 //! fn foo<A>(a: A, b: A) { ... }
28 //! and then we wrote `foo(&1, @2)`, we will not auto-borrow
29 //! either argument. In older code we went to some lengths to
30 //! resolve the `b` variable, which could mean that we'd
31 //! auto-borrow later arguments but not earlier ones, which
32 //! seems very confusing.
36 //! However, right now, if the user manually specifies the
37 //! values for the type variables, as so:
39 //! foo::<&int>(@1, @2)
41 //! then we *will* auto-borrow, because we can't distinguish this from a
42 //! function that declared `&int`. This is inconsistent but it's easiest
43 //! at the moment. The right thing to do, I think, is to consider the
44 //! *unsubstituted* type when deciding whether to auto-borrow, but the
45 //! *substituted* type when considering the bounds and so forth. But most
46 //! of our methods don't give access to the unsubstituted type, and
47 //! rightly so because they'd be error-prone. So maybe the thing to do is
48 //! to actually determine the kind of coercions that should occur
49 //! separately and pass them in. Or maybe it's ok as is. Anyway, it's
50 //! sort of a minor point so I've opted to leave it for later -- after all,
51 //! we may want to adjust precisely when coercions occur.
53 use crate::check::{FnCtxt, Needs};
54 use errors::DiagnosticBuilder;
56 use rustc::hir::def_id::DefId;
57 use rustc::hir::ptr::P;
58 use rustc::infer::{Coercion, InferResult, InferOk};
59 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
60 use rustc::traits::{self, ObligationCause, ObligationCauseCode};
61 use rustc::ty::adjustment::{
62 Adjustment, Adjust, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast
64 use rustc::ty::{self, TypeAndMut, Ty};
65 use rustc::ty::fold::TypeFoldable;
66 use rustc::ty::error::TypeError;
67 use rustc::ty::relate::RelateResult;
68 use rustc::ty::subst::SubstsRef;
69 use smallvec::{smallvec, SmallVec};
71 use syntax::feature_gate;
72 use syntax::symbol::sym;
74 use rustc_target::spec::abi::Abi;
76 struct Coerce<'a, 'tcx> {
77 fcx: &'a FnCtxt<'a, 'tcx>,
78 cause: ObligationCause<'tcx>,
80 /// Determines whether or not allow_two_phase_borrow is set on any
81 /// autoref adjustments we create while coercing. We don't want to
82 /// allow deref coercions to create two-phase borrows, at least initially,
83 /// but we do need two-phase borrows for function argument reborrows.
84 /// See #47489 and #48598
85 /// See docs on the "AllowTwoPhase" type for a more detailed discussion
86 allow_two_phase: AllowTwoPhase,
89 impl<'a, 'tcx> Deref for Coerce<'a, 'tcx> {
90 type Target = FnCtxt<'a, 'tcx>;
91 fn deref(&self) -> &Self::Target {
96 type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
98 fn coerce_mutbls<'tcx>(from_mutbl: hir::Mutability,
99 to_mutbl: hir::Mutability)
100 -> RelateResult<'tcx, ()> {
101 match (from_mutbl, to_mutbl) {
102 (hir::MutMutable, hir::MutMutable) |
103 (hir::MutImmutable, hir::MutImmutable) |
104 (hir::MutMutable, hir::MutImmutable) => Ok(()),
105 (hir::MutImmutable, hir::MutMutable) => Err(TypeError::Mutability),
109 fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> { vec![] }
111 fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
112 move |target| vec![Adjustment { kind, target }]
115 fn success<'tcx>(adj: Vec<Adjustment<'tcx>>,
117 obligations: traits::PredicateObligations<'tcx>)
118 -> CoerceResult<'tcx> {
120 value: (adj, target),
125 impl<'f, 'tcx> Coerce<'f, 'tcx> {
127 fcx: &'f FnCtxt<'f, 'tcx>,
128 cause: ObligationCause<'tcx>,
129 allow_two_phase: AllowTwoPhase,
139 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
140 self.commit_if_ok(|_| {
142 self.at(&self.cause, self.fcx.param_env).lub(b, a)
144 self.at(&self.cause, self.fcx.param_env)
146 .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
151 /// Unify two types (using sub or lub) and produce a specific coercion.
152 fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F)
153 -> CoerceResult<'tcx>
154 where F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>
156 self.unify(&a, &b).and_then(|InferOk { value: ty, obligations }| {
157 success(f(ty), ty, obligations)
161 fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
162 let a = self.shallow_resolve(a);
163 debug!("Coerce.tys({:?} => {:?})", a, b);
165 // Just ignore error types.
166 if a.references_error() || b.references_error() {
167 return success(vec![], self.fcx.tcx.types.err, vec![]);
171 // Subtle: If we are coercing from `!` to `?T`, where `?T` is an unbound
172 // type variable, we want `?T` to fallback to `!` if not
173 // otherwise constrained. An example where this arises:
175 // let _: Option<?T> = Some({ return; });
177 // here, we would coerce from `!` to `?T`.
178 let b = self.shallow_resolve(b);
179 return if self.shallow_resolve(b).is_ty_var() {
180 // Micro-optimization: no need for this if `b` is
181 // already resolved in some way.
182 let diverging_ty = self.next_diverging_ty_var(
184 kind: TypeVariableOriginKind::AdjustmentType,
185 span: self.cause.span,
188 self.unify_and(&b, &diverging_ty, simple(Adjust::NeverToAny))
190 success(simple(Adjust::NeverToAny)(b), b, vec![])
194 // Consider coercing the subtype to a DST
196 // NOTE: this is wrapped in a `commit_if_ok` because it creates
197 // a "spurious" type variable, and we don't want to have that
198 // type variable in memory if the coercion fails.
199 let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
202 debug!("coerce: unsize successful");
205 Err(TypeError::ObjectUnsafeCoercion(did)) => {
206 debug!("coerce: unsize not object safe");
207 return Err(TypeError::ObjectUnsafeCoercion(did));
211 debug!("coerce: unsize failed");
213 // Examine the supertype and consider auto-borrowing.
215 // Note: does not attempt to resolve type variables we encounter.
216 // See above for details.
218 ty::RawPtr(mt_b) => {
219 return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
222 ty::Ref(r_b, ty, mutbl) => {
223 let mt_b = ty::TypeAndMut { ty, mutbl };
224 return self.coerce_borrowed_pointer(a, b, r_b, mt_b);
232 // Function items are coercible to any closure
233 // type; function pointers are not (that would
234 // require double indirection).
235 // Additionally, we permit coercion of function
236 // items to drop the unsafe qualifier.
237 self.coerce_from_fn_item(a, b)
240 // We permit coercion of fn pointers to drop the
242 self.coerce_from_fn_pointer(a, a_f, b)
244 ty::Closure(def_id_a, substs_a) => {
245 // Non-capturing closures are coercible to
246 // function pointers or unsafe function pointers.
247 // It cannot convert closures that require unsafe.
248 self.coerce_closure_to_fn(a, def_id_a, substs_a, b)
251 // Otherwise, just use unification rules.
252 self.unify_and(a, b, identity)
257 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
258 /// To match `A` with `B`, autoderef will be performed,
259 /// calling `deref`/`deref_mut` where necessary.
260 fn coerce_borrowed_pointer(&self,
263 r_b: ty::Region<'tcx>,
264 mt_b: TypeAndMut<'tcx>)
265 -> CoerceResult<'tcx>
267 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
269 // If we have a parameter of type `&M T_a` and the value
270 // provided is `expr`, we will be adding an implicit borrow,
271 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
272 // to type check, we will construct the type that `&M*expr` would
275 let (r_a, mt_a) = match a.kind {
276 ty::Ref(r_a, ty, mutbl) => {
277 let mt_a = ty::TypeAndMut { ty, mutbl };
278 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
281 _ => return self.unify_and(a, b, identity),
284 let span = self.cause.span;
286 let mut first_error = None;
287 let mut r_borrow_var = None;
288 let mut autoderef = self.autoderef(span, a);
289 let mut found = None;
291 for (referent_ty, autoderefs) in autoderef.by_ref() {
293 // Don't let this pass, otherwise it would cause
294 // &T to autoref to &&T.
298 // At this point, we have deref'd `a` to `referent_ty`. So
299 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
300 // In the autoderef loop for `&'a mut Vec<T>`, we would get
303 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
304 // - `Vec<T>` -- 1 deref
305 // - `[T]` -- 2 deref
307 // At each point after the first callback, we want to
308 // check to see whether this would match out target type
309 // (`&'b mut [T]`) if we autoref'd it. We can't just
310 // compare the referent types, though, because we still
311 // have to consider the mutability. E.g., in the case
312 // we've been considering, we have an `&mut` reference, so
313 // the `T` in `[T]` needs to be unified with equality.
315 // Therefore, we construct reference types reflecting what
316 // the types will be after we do the final auto-ref and
317 // compare those. Note that this means we use the target
318 // mutability [1], since it may be that we are coercing
319 // from `&mut T` to `&U`.
321 // One fine point concerns the region that we use. We
322 // choose the region such that the region of the final
323 // type that results from `unify` will be the region we
324 // want for the autoref:
326 // - if in sub mode, that means we want to use `'b` (the
327 // region from the target reference) for both
328 // pointers [2]. This is because sub mode (somewhat
329 // arbitrarily) returns the subtype region. In the case
330 // where we are coercing to a target type, we know we
331 // want to use that target type region (`'b`) because --
332 // for the program to type-check -- it must be the
333 // smaller of the two.
334 // - One fine point. It may be surprising that we can
335 // use `'b` without relating `'a` and `'b`. The reason
336 // that this is ok is that what we produce is
337 // effectively a `&'b *x` expression (if you could
338 // annotate the region of a borrow), and regionck has
339 // code that adds edges from the region of a borrow
340 // (`'b`, here) into the regions in the borrowed
341 // expression (`*x`, here). (Search for "link".)
342 // - if in lub mode, things can get fairly complicated. The
343 // easiest thing is just to make a fresh
344 // region variable [4], which effectively means we defer
345 // the decision to region inference (and regionck, which will add
346 // some more edges to this variable). However, this can wind up
347 // creating a crippling number of variables in some cases --
348 // e.g., #32278 -- so we optimize one particular case [3].
349 // Let me try to explain with some examples:
350 // - The "running example" above represents the simple case,
351 // where we have one `&` reference at the outer level and
352 // ownership all the rest of the way down. In this case,
353 // we want `LUB('a, 'b)` as the resulting region.
354 // - However, if there are nested borrows, that region is
355 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
356 // `&'b T`. In this case, `'a` is actually irrelevant.
357 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
358 // we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
359 // (The errors actually show up in borrowck, typically, because
360 // this extra edge causes the region `'a` to be inferred to something
361 // too big, which then results in borrowck errors.)
362 // - We could track the innermost shared reference, but there is already
363 // code in regionck that has the job of creating links between
364 // the region of a borrow and the regions in the thing being
365 // borrowed (here, `'a` and `'x`), and it knows how to handle
366 // all the various cases. So instead we just make a region variable
367 // and let regionck figure it out.
368 let r = if !self.use_lub {
370 } else if autoderefs == 1 {
373 if r_borrow_var.is_none() {
374 // create var lazilly, at most once
375 let coercion = Coercion(span);
376 let r = self.next_region_var(coercion);
377 r_borrow_var = Some(r); // [4] above
379 r_borrow_var.unwrap()
381 let derefd_ty_a = self.tcx.mk_ref(r,
384 mutbl: mt_b.mutbl, // [1] above
386 match self.unify(derefd_ty_a, b) {
392 if first_error.is_none() {
393 first_error = Some(err);
399 // Extract type or return an error. We return the first error
400 // we got, which should be from relating the "base" type
401 // (e.g., in example above, the failure from relating `Vec<T>`
402 // to the target type), since that should be the least
404 let InferOk { value: ty, mut obligations } = match found {
407 let err = first_error.expect("coerce_borrowed_pointer had no error");
408 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
413 if ty == a && mt_a.mutbl == hir::MutImmutable && autoderef.step_count() == 1 {
414 // As a special case, if we would produce `&'a *x`, that's
415 // a total no-op. We end up with the type `&'a T` just as
416 // we started with. In that case, just skip it
417 // altogether. This is just an optimization.
419 // Note that for `&mut`, we DO want to reborrow --
420 // otherwise, this would be a move, which might be an
421 // error. For example `foo(self.x)` where `self` and
422 // `self.x` both have `&mut `type would be a move of
423 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
424 // which is a borrow.
425 assert_eq!(mt_b.mutbl, hir::MutImmutable); // can only coerce &T -> &U
426 return success(vec![], ty, obligations);
429 let needs = Needs::maybe_mut_place(mt_b.mutbl);
430 let InferOk { value: mut adjustments, obligations: o }
431 = autoderef.adjust_steps_as_infer_ok(self, needs);
432 obligations.extend(o);
433 obligations.extend(autoderef.into_obligations());
435 // Now apply the autoref. We have to extract the region out of
436 // the final ref type we got.
437 let r_borrow = match ty.kind {
438 ty::Ref(r_borrow, _, _) => r_borrow,
439 _ => span_bug!(span, "expected a ref type, got {:?}", ty),
441 let mutbl = match mt_b.mutbl {
442 hir::MutImmutable => AutoBorrowMutability::Immutable,
443 hir::MutMutable => AutoBorrowMutability::Mutable {
444 allow_two_phase_borrow: self.allow_two_phase,
447 adjustments.push(Adjustment {
448 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
452 debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}",
456 success(adjustments, ty, obligations)
460 // &[T; n] or &mut [T; n] -> &[T]
461 // or &mut [T; n] -> &mut [T]
462 // or &Concrete -> &Trait, etc.
463 fn coerce_unsized(&self, source: Ty<'tcx>, target: Ty<'tcx>) -> CoerceResult<'tcx> {
464 debug!("coerce_unsized(source={:?}, target={:?})", source, target);
466 let traits = (self.tcx.lang_items().unsize_trait(),
467 self.tcx.lang_items().coerce_unsized_trait());
468 let (unsize_did, coerce_unsized_did) = if let (Some(u), Some(cu)) = traits {
471 debug!("missing Unsize or CoerceUnsized traits");
472 return Err(TypeError::Mismatch);
475 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
476 // a DST unless we have to. This currently comes out in the wash since
477 // we can't unify [T] with U. But to properly support DST, we need to allow
478 // that, at which point we will need extra checks on the target here.
480 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
481 let reborrow = match (&source.kind, &target.kind) {
482 (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
483 coerce_mutbls(mutbl_a, mutbl_b)?;
485 let coercion = Coercion(self.cause.span);
486 let r_borrow = self.next_region_var(coercion);
487 let mutbl = match mutbl_b {
488 hir::MutImmutable => AutoBorrowMutability::Immutable,
489 hir::MutMutable => AutoBorrowMutability::Mutable {
490 // We don't allow two-phase borrows here, at least for initial
491 // implementation. If it happens that this coercion is a function argument,
492 // the reborrow in coerce_borrowed_ptr will pick it up.
493 allow_two_phase_borrow: AllowTwoPhase::No,
497 kind: Adjust::Deref(None),
500 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
501 target: self.tcx.mk_ref(r_borrow, ty::TypeAndMut {
507 (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
508 coerce_mutbls(mt_a, mt_b)?;
511 kind: Adjust::Deref(None),
514 kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
515 target: self.tcx.mk_ptr(ty::TypeAndMut {
523 let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
525 // Setup either a subtyping or a LUB relationship between
526 // the `CoerceUnsized` target type and the expected type.
527 // We only have the latter, so we use an inference variable
528 // for the former and let type inference do the rest.
529 let origin = TypeVariableOrigin {
530 kind: TypeVariableOriginKind::MiscVariable,
531 span: self.cause.span,
533 let coerce_target = self.next_ty_var(origin);
534 let mut coercion = self.unify_and(coerce_target, target, |target| {
535 let unsize = Adjustment {
536 kind: Adjust::Pointer(PointerCast::Unsize),
540 None => vec![unsize],
541 Some((ref deref, ref autoref)) => {
542 vec![deref.clone(), autoref.clone(), unsize]
547 let mut selcx = traits::SelectionContext::new(self);
549 // Create an obligation for `Source: CoerceUnsized<Target>`.
550 let cause = ObligationCause::new(
553 ObligationCauseCode::Coercion { source, target },
556 // Use a FIFO queue for this custom fulfillment procedure.
558 // A Vec (or SmallVec) is not a natural choice for a queue. However,
559 // this code path is hot, and this queue usually has a max length of 1
560 // and almost never more than 3. By using a SmallVec we avoid an
561 // allocation, at the (very small) cost of (occasionally) having to
562 // shift subsequent elements down when removing the front element.
563 let mut queue: SmallVec<[_; 4]> =
564 smallvec![self.tcx.predicate_for_trait_def(self.fcx.param_env,
569 &[coerce_target.into()])];
571 let mut has_unsized_tuple_coercion = false;
573 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
574 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
575 // inference might unify those two inner type variables later.
576 let traits = [coerce_unsized_did, unsize_did];
577 while !queue.is_empty() {
578 let obligation = queue.remove(0);
579 debug!("coerce_unsized resolve step: {:?}", obligation);
580 let trait_ref = match obligation.predicate {
581 ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => {
582 if unsize_did == tr.def_id() {
583 let sty = &tr.skip_binder().input_types().nth(1).unwrap().kind;
584 if let ty::Tuple(..) = sty {
585 debug!("coerce_unsized: found unsized tuple coercion");
586 has_unsized_tuple_coercion = true;
592 coercion.obligations.push(obligation);
596 match selcx.select(&obligation.with(trait_ref)) {
597 // Uncertain or unimplemented.
599 if trait_ref.def_id() == unsize_did {
600 let trait_ref = self.resolve_vars_if_possible(&trait_ref);
601 let self_ty = trait_ref.skip_binder().self_ty();
602 let unsize_ty = trait_ref.skip_binder().input_types().nth(1).unwrap();
603 debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_ref);
604 match (&self_ty.kind, &unsize_ty.kind) {
605 (ty::Infer(ty::TyVar(v)),
606 ty::Dynamic(..)) if self.type_var_is_sized(*v) => {
607 debug!("coerce_unsized: have sized infer {:?}", v);
608 coercion.obligations.push(obligation);
609 // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
613 // Some other case for `$0: Unsize<Something>`. Note that we
614 // hit this case even if `Something` is a sized type, so just
615 // don't do the coercion.
616 debug!("coerce_unsized: ambiguous unsize");
617 return Err(TypeError::Mismatch);
621 debug!("coerce_unsized: early return - ambiguous");
622 return Err(TypeError::Mismatch);
625 Err(traits::Unimplemented) => {
626 debug!("coerce_unsized: early return - can't prove obligation");
627 return Err(TypeError::Mismatch);
630 // Object safety violations or miscellaneous.
632 self.report_selection_error(&obligation, &err, false, false);
633 // Treat this like an obligation and follow through
634 // with the unsizing - the lack of a coercion should
635 // be silent, as it causes a type mismatch later.
638 Ok(Some(vtable)) => {
639 queue.extend(vtable.nested_obligations())
644 if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
645 feature_gate::emit_feature_err(&self.tcx.sess.parse_sess,
646 sym::unsized_tuple_coercion,
648 feature_gate::GateIssue::Language,
649 feature_gate::EXPLAIN_UNSIZED_TUPLE_COERCION);
655 fn coerce_from_safe_fn<F, G>(&self,
657 fn_ty_a: ty::PolyFnSig<'tcx>,
661 -> CoerceResult<'tcx>
662 where F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
663 G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>
665 if let ty::FnPtr(fn_ty_b) = b.kind {
666 if let (hir::Unsafety::Normal, hir::Unsafety::Unsafe)
667 = (fn_ty_a.unsafety(), fn_ty_b.unsafety())
669 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
670 return self.unify_and(unsafe_a, b, to_unsafe);
673 self.unify_and(a, b, normal)
676 fn coerce_from_fn_pointer(&self,
678 fn_ty_a: ty::PolyFnSig<'tcx>,
680 -> CoerceResult<'tcx> {
681 //! Attempts to coerce from the type of a Rust function item
682 //! into a closure or a `proc`.
685 let b = self.shallow_resolve(b);
686 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
688 self.coerce_from_safe_fn(a, fn_ty_a, b,
689 simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)), identity)
692 fn coerce_from_fn_item(&self,
695 -> CoerceResult<'tcx> {
696 //! Attempts to coerce from the type of a Rust function item
697 //! into a closure or a `proc`.
699 let b = self.shallow_resolve(b);
700 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
704 let a_sig = a.fn_sig(self.tcx);
705 // Intrinsics are not coercible to function pointers
706 if a_sig.abi() == Abi::RustIntrinsic ||
707 a_sig.abi() == Abi::PlatformIntrinsic {
708 return Err(TypeError::IntrinsicCast);
710 let InferOk { value: a_sig, mut obligations } =
711 self.normalize_associated_types_in_as_infer_ok(self.cause.span, &a_sig);
713 let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
714 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
721 kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
725 kind: Adjust::Pointer(PointerCast::UnsafeFnPointer),
730 simple(Adjust::Pointer(PointerCast::ReifyFnPointer))
733 obligations.extend(o2);
734 Ok(InferOk { value, obligations })
736 _ => self.unify_and(a, b, identity),
740 fn coerce_closure_to_fn(&self,
743 substs_a: SubstsRef<'tcx>,
745 -> CoerceResult<'tcx> {
746 //! Attempts to coerce from the type of a non-capturing closure
747 //! into a function pointer.
750 let b = self.shallow_resolve(b);
753 ty::FnPtr(fn_ty) if self.tcx.upvars(def_id_a).map_or(true, |v| v.is_empty()) => {
754 // We coerce the closure, which has fn type
755 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
757 // `fn(arg0,arg1,...) -> _`
759 // `unsafe fn(arg0,arg1,...) -> _`
760 let sig = self.closure_sig(def_id_a, substs_a);
761 let unsafety = fn_ty.unsafety();
762 let pointer_ty = self.tcx.coerce_closure_fn_ty(sig, unsafety);
763 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})",
765 self.unify_and(pointer_ty, b, simple(
766 Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))
769 _ => self.unify_and(a, b, identity),
773 fn coerce_unsafe_ptr(&self,
776 mutbl_b: hir::Mutability)
777 -> CoerceResult<'tcx> {
778 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
780 let (is_ref, mt_a) = match a.kind {
781 ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
782 ty::RawPtr(mt) => (false, mt),
783 _ => return self.unify_and(a, b, identity)
786 // Check that the types which they point at are compatible.
787 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut {
791 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
792 // Although references and unsafe ptrs have the same
793 // representation, we still register an Adjust::DerefRef so that
794 // regionck knows that the region for `a` must be valid here.
796 self.unify_and(a_unsafe, b, |target| {
798 kind: Adjust::Deref(None),
801 kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)),
805 } else if mt_a.mutbl != mutbl_b {
807 a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer))
810 self.unify_and(a_unsafe, b, identity)
815 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
816 /// Attempt to coerce an expression to a type, and return the
817 /// adjusted type of the expression, if successful.
818 /// Adjustments are only recorded if the coercion succeeded.
819 /// The expressions *must not* have any pre-existing adjustments.
825 allow_two_phase: AllowTwoPhase,
826 ) -> RelateResult<'tcx, Ty<'tcx>> {
827 let source = self.resolve_vars_with_obligations(expr_ty);
828 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
830 let cause = self.cause(expr.span, ObligationCauseCode::ExprAssignable);
831 let coerce = Coerce::new(self, cause, allow_two_phase);
832 let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
834 let (adjustments, _) = self.register_infer_ok_obligations(ok);
835 self.apply_adjustments(expr, adjustments);
836 Ok(if expr_ty.references_error() {
843 /// Same as `try_coerce()`, but without side-effects.
844 pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
845 let source = self.resolve_vars_with_obligations(expr_ty);
846 debug!("coercion::can({:?} -> {:?})", source, target);
848 let cause = self.cause(syntax_pos::DUMMY_SP, ObligationCauseCode::ExprAssignable);
849 // We don't ever need two-phase here since we throw out the result of the coercion
850 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
851 self.probe(|_| coerce.coerce(source, target)).is_ok()
854 /// Given some expressions, their known unified type and another expression,
855 /// tries to unify the types, potentially inserting coercions on any of the
856 /// provided expressions and returns their LUB (aka "common supertype").
858 /// This is really an internal helper. From outside the coercion
859 /// module, you should instantiate a `CoerceMany` instance.
860 fn try_find_coercion_lub<E>(&self,
861 cause: &ObligationCause<'tcx>,
866 -> RelateResult<'tcx, Ty<'tcx>>
867 where E: AsCoercionSite
869 let prev_ty = self.resolve_vars_with_obligations(prev_ty);
870 let new_ty = self.resolve_vars_with_obligations(new_ty);
871 debug!("coercion::try_find_coercion_lub({:?}, {:?})", prev_ty, new_ty);
873 // Special-case that coercion alone cannot handle:
874 // Two function item types of differing IDs or InternalSubsts.
875 if let (&ty::FnDef(..), &ty::FnDef(..)) = (&prev_ty.kind, &new_ty.kind) {
876 // Don't reify if the function types have a LUB, i.e., they
877 // are the same function and their parameters have a LUB.
878 let lub_ty = self.commit_if_ok(|_| {
879 self.at(cause, self.param_env)
880 .lub(prev_ty, new_ty)
881 }).map(|ok| self.register_infer_ok_obligations(ok));
884 // We have a LUB of prev_ty and new_ty, just return it.
888 // The signature must match.
889 let a_sig = prev_ty.fn_sig(self.tcx);
890 let a_sig = self.normalize_associated_types_in(new.span, &a_sig);
891 let b_sig = new_ty.fn_sig(self.tcx);
892 let b_sig = self.normalize_associated_types_in(new.span, &b_sig);
893 let sig = self.at(cause, self.param_env)
894 .trace(prev_ty, new_ty)
896 .map(|ok| self.register_infer_ok_obligations(ok))?;
898 // Reify both sides and return the reified fn pointer type.
899 let fn_ptr = self.tcx.mk_fn_ptr(sig);
900 for expr in exprs.iter().map(|e| e.as_coercion_site()).chain(Some(new)) {
901 // The only adjustment that can produce an fn item is
902 // `NeverToAny`, so this should always be valid.
903 self.apply_adjustments(expr, vec![Adjustment {
904 kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
911 // Configure a Coerce instance to compute the LUB.
912 // We don't allow two-phase borrows on any autorefs this creates since we
913 // probably aren't processing function arguments here and even if we were,
914 // they're going to get autorefed again anyway and we can apply 2-phase borrows
916 let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
917 coerce.use_lub = true;
919 // First try to coerce the new expression to the type of the previous ones,
920 // but only if the new expression has no coercion already applied to it.
921 let mut first_error = None;
922 if !self.tables.borrow().adjustments().contains_key(new.hir_id) {
923 let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
926 let (adjustments, target) = self.register_infer_ok_obligations(ok);
927 self.apply_adjustments(new, adjustments);
930 Err(e) => first_error = Some(e),
934 // Then try to coerce the previous expressions to the type of the new one.
935 // This requires ensuring there are no coercions applied to *any* of the
936 // previous expressions, other than noop reborrows (ignoring lifetimes).
938 let expr = expr.as_coercion_site();
939 let noop = match self.tables.borrow().expr_adjustments(expr) {
941 Adjustment { kind: Adjust::Deref(_), .. },
942 Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. }
944 match self.node_ty(expr.hir_id).kind {
945 ty::Ref(_, _, mt_orig) => {
946 let mutbl_adj: hir::Mutability = mutbl_adj.into();
947 // Reborrow that we can safely ignore, because
948 // the next adjustment can only be a Deref
949 // which will be merged into it.
955 &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
960 return self.commit_if_ok(|_|
961 self.at(cause, self.param_env)
962 .lub(prev_ty, new_ty)
963 ).map(|ok| self.register_infer_ok_obligations(ok));
967 match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
969 // Avoid giving strange errors on failed attempts.
970 if let Some(e) = first_error {
973 self.commit_if_ok(|_|
974 self.at(cause, self.param_env)
975 .lub(prev_ty, new_ty)
976 ).map(|ok| self.register_infer_ok_obligations(ok))
980 let (adjustments, target) = self.register_infer_ok_obligations(ok);
982 let expr = expr.as_coercion_site();
983 self.apply_adjustments(expr, adjustments.clone());
991 /// CoerceMany encapsulates the pattern you should use when you have
992 /// many expressions that are all getting coerced to a common
993 /// type. This arises, for example, when you have a match (the result
994 /// of each arm is coerced to a common type). It also arises in less
995 /// obvious places, such as when you have many `break foo` expressions
996 /// that target the same loop, or the various `return` expressions in
999 /// The basic protocol is as follows:
1001 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
1002 /// This will also serve as the "starting LUB". The expectation is
1003 /// that this type is something which all of the expressions *must*
1004 /// be coercible to. Use a fresh type variable if needed.
1005 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
1006 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
1007 /// unit. This happens for example if you have a `break` with no expression,
1008 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
1009 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
1010 /// from you so that you don't have to worry your pretty head about it.
1011 /// But if an error is reported, the final type will be `err`.
1012 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
1013 /// previously coerced expressions.
1014 /// - When all done, invoke `complete()`. This will return the LUB of
1015 /// all your expressions.
1016 /// - WARNING: I don't believe this final type is guaranteed to be
1017 /// related to your initial `expected_ty` in any particular way,
1018 /// although it will typically be a subtype, so you should check it.
1019 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
1020 /// previously coerced expressions.
1025 /// let mut coerce = CoerceMany::new(expected_ty);
1026 /// for expr in exprs {
1027 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
1028 /// coerce.coerce(fcx, &cause, expr, expr_ty);
1030 /// let final_ty = coerce.complete(fcx);
1032 pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
1033 expected_ty: Ty<'tcx>,
1034 final_ty: Option<Ty<'tcx>>,
1035 expressions: Expressions<'tcx, 'exprs, E>,
1039 /// The type of a `CoerceMany` that is storing up the expressions into
1040 /// a buffer. We use this in `check/mod.rs` for things like `break`.
1041 pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, P<hir::Expr>>;
1043 enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
1044 Dynamic(Vec<&'tcx hir::Expr>),
1045 UpFront(&'exprs [E]),
1048 impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
1049 /// The usual case; collect the set of expressions dynamically.
1050 /// If the full set of coercion sites is known before hand,
1051 /// consider `with_coercion_sites()` instead to avoid allocation.
1052 pub fn new(expected_ty: Ty<'tcx>) -> Self {
1053 Self::make(expected_ty, Expressions::Dynamic(vec![]))
1056 /// As an optimization, you can create a `CoerceMany` with a
1057 /// pre-existing slice of expressions. In this case, you are
1058 /// expected to pass each element in the slice to `coerce(...)` in
1059 /// order. This is used with arrays in particular to avoid
1060 /// needlessly cloning the slice.
1061 pub fn with_coercion_sites(expected_ty: Ty<'tcx>,
1062 coercion_sites: &'exprs [E])
1064 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
1067 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
1076 /// Returns the "expected type" with which this coercion was
1077 /// constructed. This represents the "downward propagated" type
1078 /// that was given to us at the start of typing whatever construct
1079 /// we are typing (e.g., the match expression).
1081 /// Typically, this is used as the expected type when
1082 /// type-checking each of the alternative expressions whose types
1083 /// we are trying to merge.
1084 pub fn expected_ty(&self) -> Ty<'tcx> {
1088 /// Returns the current "merged type", representing our best-guess
1089 /// at the LUB of the expressions we've seen so far (if any). This
1090 /// isn't *final* until you call `self.final()`, which will return
1091 /// the merged type.
1092 pub fn merged_ty(&self) -> Ty<'tcx> {
1093 self.final_ty.unwrap_or(self.expected_ty)
1096 /// Indicates that the value generated by `expression`, which is
1097 /// of type `expression_ty`, is one of the possibilities that we
1098 /// could coerce from. This will record `expression`, and later
1099 /// calls to `coerce` may come back and add adjustments and things
1103 fcx: &FnCtxt<'a, 'tcx>,
1104 cause: &ObligationCause<'tcx>,
1105 expression: &'tcx hir::Expr,
1106 expression_ty: Ty<'tcx>,
1108 self.coerce_inner(fcx,
1115 /// Indicates that one of the inputs is a "forced unit". This
1116 /// occurs in a case like `if foo { ... };`, where the missing else
1117 /// generates a "forced unit". Another example is a `loop { break;
1118 /// }`, where the `break` has no argument expression. We treat
1119 /// these cases slightly differently for error-reporting
1120 /// purposes. Note that these tend to correspond to cases where
1121 /// the `()` expression is implicit in the source, and hence we do
1122 /// not take an expression argument.
1124 /// The `augment_error` gives you a chance to extend the error
1125 /// message, in case any results (e.g., we use this to suggest
1126 /// removing a `;`).
1127 pub fn coerce_forced_unit<'a>(
1129 fcx: &FnCtxt<'a, 'tcx>,
1130 cause: &ObligationCause<'tcx>,
1131 augment_error: &mut dyn FnMut(&mut DiagnosticBuilder<'_>),
1132 label_unit_as_expected: bool,
1134 self.coerce_inner(fcx,
1138 Some(augment_error),
1139 label_unit_as_expected)
1142 /// The inner coercion "engine". If `expression` is `None`, this
1143 /// is a forced-unit case, and hence `expression_ty` must be
1145 fn coerce_inner<'a>(
1147 fcx: &FnCtxt<'a, 'tcx>,
1148 cause: &ObligationCause<'tcx>,
1149 expression: Option<&'tcx hir::Expr>,
1150 mut expression_ty: Ty<'tcx>,
1151 augment_error: Option<&mut dyn FnMut(&mut DiagnosticBuilder<'_>)>,
1152 label_expression_as_expected: bool,
1154 // Incorporate whatever type inference information we have
1155 // until now; in principle we might also want to process
1156 // pending obligations, but doing so should only improve
1157 // compatibility (hopefully that is true) by helping us
1158 // uncover never types better.
1159 if expression_ty.is_ty_var() {
1160 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1163 // If we see any error types, just propagate that error
1165 if expression_ty.references_error() || self.merged_ty().references_error() {
1166 self.final_ty = Some(fcx.tcx.types.err);
1170 // Handle the actual type unification etc.
1171 let result = if let Some(expression) = expression {
1172 if self.pushed == 0 {
1173 // Special-case the first expression we are coercing.
1174 // To be honest, I'm not entirely sure why we do this.
1175 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1176 fcx.try_coerce(expression, expression_ty, self.expected_ty, AllowTwoPhase::No)
1178 match self.expressions {
1179 Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
1186 Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub(
1188 &coercion_sites[0..self.pushed],
1196 // this is a hack for cases where we default to `()` because
1197 // the expression etc has been omitted from the source. An
1198 // example is an `if let` without an else:
1200 // if let Some(x) = ... { }
1202 // we wind up with a second match arm that is like `_ =>
1203 // ()`. That is the case we are considering here. We take
1204 // a different path to get the right "expected, found"
1205 // message and so forth (and because we know that
1206 // `expression_ty` will be unit).
1208 // Another example is `break` with no argument expression.
1209 assert!(expression_ty.is_unit(), "if let hack without unit type");
1210 fcx.at(cause, fcx.param_env)
1211 .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
1213 fcx.register_infer_ok_obligations(infer_ok);
1220 self.final_ty = Some(v);
1221 if let Some(e) = expression {
1222 match self.expressions {
1223 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1224 Expressions::UpFront(coercion_sites) => {
1225 // if the user gave us an array to validate, check that we got
1226 // the next expression in the list, as expected
1227 assert_eq!(coercion_sites[self.pushed].as_coercion_site().hir_id,
1234 Err(coercion_error) => {
1235 let (expected, found) = if label_expression_as_expected {
1236 // In the case where this is a "forced unit", like
1237 // `break`, we want to call the `()` "expected"
1238 // since it is implied by the syntax.
1239 // (Note: not all force-units work this way.)"
1240 (expression_ty, self.final_ty.unwrap_or(self.expected_ty))
1242 // Otherwise, the "expected" type for error
1243 // reporting is the current unification type,
1244 // which is basically the LUB of the expressions
1245 // we've seen so far (combined with the expected
1247 (self.final_ty.unwrap_or(self.expected_ty), expression_ty)
1252 ObligationCauseCode::ReturnNoExpression => {
1253 err = struct_span_err!(
1254 fcx.tcx.sess, cause.span, E0069,
1255 "`return;` in a function whose return type is not `()`");
1256 err.span_label(cause.span, "return type is not `()`");
1258 ObligationCauseCode::BlockTailExpression(blk_id) => {
1259 let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
1260 err = self.report_return_mismatched_types(
1267 expression.map(|expr| (expr, blk_id)),
1270 ObligationCauseCode::ReturnValue(id) => {
1271 err = self.report_return_mismatched_types(
1272 cause, expected, found, coercion_error, fcx, id, None);
1275 err = fcx.report_mismatched_types(cause, expected, found, coercion_error);
1279 if let Some(augment_error) = augment_error {
1280 augment_error(&mut err);
1283 // Error possibly reported in `check_assign` so avoid emitting error again.
1284 err.emit_unless(expression.filter(|e| fcx.is_assign_to_bool(e, expected))
1287 self.final_ty = Some(fcx.tcx.types.err);
1292 fn report_return_mismatched_types<'a>(
1294 cause: &ObligationCause<'tcx>,
1297 ty_err: TypeError<'tcx>,
1298 fcx: &FnCtxt<'a, 'tcx>,
1300 expression: Option<(&'tcx hir::Expr, hir::HirId)>,
1301 ) -> DiagnosticBuilder<'a> {
1302 let mut err = fcx.report_mismatched_types(cause, expected, found, ty_err);
1304 let mut pointing_at_return_type = false;
1305 let mut return_sp = None;
1307 // Verify that this is a tail expression of a function, otherwise the
1308 // label pointing out the cause for the type coercion will be wrong
1309 // as prior return coercions would not be relevant (#57664).
1310 let parent_id = fcx.tcx.hir().get_parent_node(id);
1311 let fn_decl = if let Some((expr, blk_id)) = expression {
1312 pointing_at_return_type = fcx.suggest_mismatched_types_on_tail(
1320 let parent = fcx.tcx.hir().get(parent_id);
1321 if let (Some(match_expr), true, false) = (
1322 fcx.tcx.hir().get_match_if_cause(expr.hir_id),
1324 pointing_at_return_type,
1326 if match_expr.span.desugaring_kind().is_none() {
1327 err.span_label(match_expr.span, "expected this to be `()`");
1328 fcx.suggest_semicolon_at_end(match_expr.span, &mut err);
1331 fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
1333 fcx.get_fn_decl(parent_id)
1336 if let (Some((fn_decl, can_suggest)), _) = (fn_decl, pointing_at_return_type) {
1337 if expression.is_none() {
1338 pointing_at_return_type |= fcx.suggest_missing_return_type(
1339 &mut err, &fn_decl, expected, found, can_suggest);
1341 if !pointing_at_return_type {
1342 return_sp = Some(fn_decl.output.span()); // `impl Trait` return type
1345 if let (Some(sp), Some(return_sp)) = (fcx.ret_coercion_span.borrow().as_ref(), return_sp) {
1346 err.span_label(return_sp, "expected because this return type...");
1347 err.span_label( *sp, format!(
1348 "...is found to be `{}` here",
1349 fcx.resolve_vars_with_obligations(expected),
1355 pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'tcx>) -> Ty<'tcx> {
1356 if let Some(final_ty) = self.final_ty {
1359 // If we only had inputs that were of type `!` (or no
1360 // inputs at all), then the final type is `!`.
1361 assert_eq!(self.pushed, 0);
1367 /// Something that can be converted into an expression to which we can
1368 /// apply a coercion.
1369 pub trait AsCoercionSite {
1370 fn as_coercion_site(&self) -> &hir::Expr;
1373 impl AsCoercionSite for hir::Expr {
1374 fn as_coercion_site(&self) -> &hir::Expr {
1379 impl AsCoercionSite for P<hir::Expr> {
1380 fn as_coercion_site(&self) -> &hir::Expr {
1385 impl<'a, T> AsCoercionSite for &'a T
1386 where T: AsCoercionSite
1388 fn as_coercion_site(&self) -> &hir::Expr {
1389 (**self).as_coercion_site()
1393 impl AsCoercionSite for ! {
1394 fn as_coercion_site(&self) -> &hir::Expr {
1399 impl AsCoercionSite for hir::Arm {
1400 fn as_coercion_site(&self) -> &hir::Expr {