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/run-pass/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::infer::{Coercion, InferResult, InferOk};
58 use rustc::infer::type_variable::TypeVariableOrigin;
59 use rustc::traits::{self, ObligationCause, ObligationCauseCode};
60 use rustc::ty::adjustment::{Adjustment, Adjust, AllowTwoPhase, AutoBorrow, AutoBorrowMutability};
61 use rustc::ty::{self, TypeAndMut, Ty, ClosureSubsts};
62 use rustc::ty::fold::TypeFoldable;
63 use rustc::ty::error::TypeError;
64 use rustc::ty::relate::RelateResult;
65 use smallvec::{smallvec, SmallVec};
67 use syntax::feature_gate;
71 struct Coerce<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
72 fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
73 cause: ObligationCause<'tcx>,
75 /// Determines whether or not allow_two_phase_borrow is set on any
76 /// autoref adjustments we create while coercing. We don't want to
77 /// allow deref coercions to create two-phase borrows, at least initially,
78 /// but we do need two-phase borrows for function argument reborrows.
79 /// See #47489 and #48598
80 /// See docs on the "AllowTwoPhase" type for a more detailed discussion
81 allow_two_phase: AllowTwoPhase,
84 impl<'a, 'gcx, 'tcx> Deref for Coerce<'a, 'gcx, 'tcx> {
85 type Target = FnCtxt<'a, 'gcx, 'tcx>;
86 fn deref(&self) -> &Self::Target {
91 type CoerceResult<'tcx> = InferResult<'tcx, (Vec<Adjustment<'tcx>>, Ty<'tcx>)>;
93 fn coerce_mutbls<'tcx>(from_mutbl: hir::Mutability,
94 to_mutbl: hir::Mutability)
95 -> RelateResult<'tcx, ()> {
96 match (from_mutbl, to_mutbl) {
97 (hir::MutMutable, hir::MutMutable) |
98 (hir::MutImmutable, hir::MutImmutable) |
99 (hir::MutMutable, hir::MutImmutable) => Ok(()),
100 (hir::MutImmutable, hir::MutMutable) => Err(TypeError::Mutability),
104 fn identity(_: Ty) -> Vec<Adjustment> { vec![] }
106 fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
107 move |target| vec![Adjustment { kind, target }]
110 fn success<'tcx>(adj: Vec<Adjustment<'tcx>>,
112 obligations: traits::PredicateObligations<'tcx>)
113 -> CoerceResult<'tcx> {
115 value: (adj, target),
120 impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
121 fn new(fcx: &'f FnCtxt<'f, 'gcx, 'tcx>,
122 cause: ObligationCause<'tcx>,
123 allow_two_phase: AllowTwoPhase) -> Self {
132 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
133 self.commit_if_ok(|_| {
135 self.at(&self.cause, self.fcx.param_env).lub(b, a)
137 self.at(&self.cause, self.fcx.param_env)
139 .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
144 /// Unify two types (using sub or lub) and produce a specific coercion.
145 fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F)
146 -> CoerceResult<'tcx>
147 where F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>
149 self.unify(&a, &b).and_then(|InferOk { value: ty, obligations }| {
150 success(f(ty), ty, obligations)
154 fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
155 let a = self.shallow_resolve(a);
156 debug!("Coerce.tys({:?} => {:?})", a, b);
158 // Just ignore error types.
159 if a.references_error() || b.references_error() {
160 return success(vec![], b, vec![]);
164 // Subtle: If we are coercing from `!` to `?T`, where `?T` is an unbound
165 // type variable, we want `?T` to fallback to `!` if not
166 // otherwise constrained. An example where this arises:
168 // let _: Option<?T> = Some({ return; });
170 // here, we would coerce from `!` to `?T`.
171 let b = self.shallow_resolve(b);
172 return if self.shallow_resolve(b).is_ty_var() {
173 // micro-optimization: no need for this if `b` is
174 // already resolved in some way.
175 let diverging_ty = self.next_diverging_ty_var(
176 TypeVariableOrigin::AdjustmentType(self.cause.span));
177 self.unify_and(&b, &diverging_ty, simple(Adjust::NeverToAny))
179 success(simple(Adjust::NeverToAny)(b), b, vec![])
183 // Consider coercing the subtype to a DST
185 // NOTE: this is wrapped in a `commit_if_ok` because it creates
186 // a "spurious" type variable, and we don't want to have that
187 // type variable in memory if the coercion fails.
188 let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
190 debug!("coerce: unsize successful");
193 debug!("coerce: unsize failed");
195 // Examine the supertype and consider auto-borrowing.
197 // Note: does not attempt to resolve type variables we encounter.
198 // See above for details.
200 ty::RawPtr(mt_b) => {
201 return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
204 ty::Ref(r_b, ty, mutbl) => {
205 let mt_b = ty::TypeAndMut { ty, mutbl };
206 return self.coerce_borrowed_pointer(a, b, r_b, mt_b);
214 // Function items are coercible to any closure
215 // type; function pointers are not (that would
216 // require double indirection).
217 // Additionally, we permit coercion of function
218 // items to drop the unsafe qualifier.
219 self.coerce_from_fn_item(a, b)
222 // We permit coercion of fn pointers to drop the
224 self.coerce_from_fn_pointer(a, a_f, b)
226 ty::Closure(def_id_a, substs_a) => {
227 // Non-capturing closures are coercible to
229 self.coerce_closure_to_fn(a, def_id_a, substs_a, b)
232 // Otherwise, just use unification rules.
233 self.unify_and(a, b, identity)
238 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
239 /// To match `A` with `B`, autoderef will be performed,
240 /// calling `deref`/`deref_mut` where necessary.
241 fn coerce_borrowed_pointer(&self,
244 r_b: ty::Region<'tcx>,
245 mt_b: TypeAndMut<'tcx>)
246 -> CoerceResult<'tcx>
248 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
250 // If we have a parameter of type `&M T_a` and the value
251 // provided is `expr`, we will be adding an implicit borrow,
252 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
253 // to type check, we will construct the type that `&M*expr` would
256 let (r_a, mt_a) = match a.sty {
257 ty::Ref(r_a, ty, mutbl) => {
258 let mt_a = ty::TypeAndMut { ty, mutbl };
259 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
262 _ => return self.unify_and(a, b, identity),
265 let span = self.cause.span;
267 let mut first_error = None;
268 let mut r_borrow_var = None;
269 let mut autoderef = self.autoderef(span, a);
270 let mut found = None;
272 for (referent_ty, autoderefs) in autoderef.by_ref() {
274 // Don't let this pass, otherwise it would cause
275 // &T to autoref to &&T.
279 // At this point, we have deref'd `a` to `referent_ty`. So
280 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
281 // In the autoderef loop for `&'a mut Vec<T>`, we would get
284 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
285 // - `Vec<T>` -- 1 deref
286 // - `[T]` -- 2 deref
288 // At each point after the first callback, we want to
289 // check to see whether this would match out target type
290 // (`&'b mut [T]`) if we autoref'd it. We can't just
291 // compare the referent types, though, because we still
292 // have to consider the mutability. E.g., in the case
293 // we've been considering, we have an `&mut` reference, so
294 // the `T` in `[T]` needs to be unified with equality.
296 // Therefore, we construct reference types reflecting what
297 // the types will be after we do the final auto-ref and
298 // compare those. Note that this means we use the target
299 // mutability [1], since it may be that we are coercing
300 // from `&mut T` to `&U`.
302 // One fine point concerns the region that we use. We
303 // choose the region such that the region of the final
304 // type that results from `unify` will be the region we
305 // want for the autoref:
307 // - if in sub mode, that means we want to use `'b` (the
308 // region from the target reference) for both
309 // pointers [2]. This is because sub mode (somewhat
310 // arbitrarily) returns the subtype region. In the case
311 // where we are coercing to a target type, we know we
312 // want to use that target type region (`'b`) because --
313 // for the program to type-check -- it must be the
314 // smaller of the two.
315 // - One fine point. It may be surprising that we can
316 // use `'b` without relating `'a` and `'b`. The reason
317 // that this is ok is that what we produce is
318 // effectively a `&'b *x` expression (if you could
319 // annotate the region of a borrow), and regionck has
320 // code that adds edges from the region of a borrow
321 // (`'b`, here) into the regions in the borrowed
322 // expression (`*x`, here). (Search for "link".)
323 // - if in lub mode, things can get fairly complicated. The
324 // easiest thing is just to make a fresh
325 // region variable [4], which effectively means we defer
326 // the decision to region inference (and regionck, which will add
327 // some more edges to this variable). However, this can wind up
328 // creating a crippling number of variables in some cases --
329 // e.g., #32278 -- so we optimize one particular case [3].
330 // Let me try to explain with some examples:
331 // - The "running example" above represents the simple case,
332 // where we have one `&` reference at the outer level and
333 // ownership all the rest of the way down. In this case,
334 // we want `LUB('a, 'b)` as the resulting region.
335 // - However, if there are nested borrows, that region is
336 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
337 // `&'b T`. In this case, `'a` is actually irrelevant.
338 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
339 // we get spurious errors (`run-pass/regions-lub-ref-ref-rc.rs`).
340 // (The errors actually show up in borrowck, typically, because
341 // this extra edge causes the region `'a` to be inferred to something
342 // too big, which then results in borrowck errors.)
343 // - We could track the innermost shared reference, but there is already
344 // code in regionck that has the job of creating links between
345 // the region of a borrow and the regions in the thing being
346 // borrowed (here, `'a` and `'x`), and it knows how to handle
347 // all the various cases. So instead we just make a region variable
348 // and let regionck figure it out.
349 let r = if !self.use_lub {
351 } else if autoderefs == 1 {
354 if r_borrow_var.is_none() {
355 // create var lazilly, at most once
356 let coercion = Coercion(span);
357 let r = self.next_region_var(coercion);
358 r_borrow_var = Some(r); // [4] above
360 r_borrow_var.unwrap()
362 let derefd_ty_a = self.tcx.mk_ref(r,
365 mutbl: mt_b.mutbl, // [1] above
367 match self.unify(derefd_ty_a, b) {
373 if first_error.is_none() {
374 first_error = Some(err);
380 // Extract type or return an error. We return the first error
381 // we got, which should be from relating the "base" type
382 // (e.g., in example above, the failure from relating `Vec<T>`
383 // to the target type), since that should be the least
385 let InferOk { value: ty, mut obligations } = match found {
388 let err = first_error.expect("coerce_borrowed_pointer had no error");
389 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
394 if ty == a && mt_a.mutbl == hir::MutImmutable && autoderef.step_count() == 1 {
395 // As a special case, if we would produce `&'a *x`, that's
396 // a total no-op. We end up with the type `&'a T` just as
397 // we started with. In that case, just skip it
398 // altogether. This is just an optimization.
400 // Note that for `&mut`, we DO want to reborrow --
401 // otherwise, this would be a move, which might be an
402 // error. For example `foo(self.x)` where `self` and
403 // `self.x` both have `&mut `type would be a move of
404 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
405 // which is a borrow.
406 assert_eq!(mt_b.mutbl, hir::MutImmutable); // can only coerce &T -> &U
407 return success(vec![], ty, obligations);
410 let needs = Needs::maybe_mut_place(mt_b.mutbl);
411 let InferOk { value: mut adjustments, obligations: o }
412 = autoderef.adjust_steps_as_infer_ok(self, needs);
413 obligations.extend(o);
414 obligations.extend(autoderef.into_obligations());
416 // Now apply the autoref. We have to extract the region out of
417 // the final ref type we got.
418 let r_borrow = match ty.sty {
419 ty::Ref(r_borrow, _, _) => r_borrow,
420 _ => span_bug!(span, "expected a ref type, got {:?}", ty),
422 let mutbl = match mt_b.mutbl {
423 hir::MutImmutable => AutoBorrowMutability::Immutable,
424 hir::MutMutable => AutoBorrowMutability::Mutable {
425 allow_two_phase_borrow: self.allow_two_phase,
428 adjustments.push(Adjustment {
429 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
433 debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}",
437 success(adjustments, ty, obligations)
441 // &[T; n] or &mut [T; n] -> &[T]
442 // or &mut [T; n] -> &mut [T]
443 // or &Concrete -> &Trait, etc.
444 fn coerce_unsized(&self, source: Ty<'tcx>, target: Ty<'tcx>) -> CoerceResult<'tcx> {
445 debug!("coerce_unsized(source={:?}, target={:?})", source, target);
447 let traits = (self.tcx.lang_items().unsize_trait(),
448 self.tcx.lang_items().coerce_unsized_trait());
449 let (unsize_did, coerce_unsized_did) = if let (Some(u), Some(cu)) = traits {
452 debug!("Missing Unsize or CoerceUnsized traits");
453 return Err(TypeError::Mismatch);
456 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
457 // a DST unless we have to. This currently comes out in the wash since
458 // we can't unify [T] with U. But to properly support DST, we need to allow
459 // that, at which point we will need extra checks on the target here.
461 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
462 let reborrow = match (&source.sty, &target.sty) {
463 (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
464 coerce_mutbls(mutbl_a, mutbl_b)?;
466 let coercion = Coercion(self.cause.span);
467 let r_borrow = self.next_region_var(coercion);
468 let mutbl = match mutbl_b {
469 hir::MutImmutable => AutoBorrowMutability::Immutable,
470 hir::MutMutable => AutoBorrowMutability::Mutable {
471 // We don't allow two-phase borrows here, at least for initial
472 // implementation. If it happens that this coercion is a function argument,
473 // the reborrow in coerce_borrowed_ptr will pick it up.
474 allow_two_phase_borrow: AllowTwoPhase::No,
478 kind: Adjust::Deref(None),
481 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
482 target: self.tcx.mk_ref(r_borrow, ty::TypeAndMut {
488 (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
489 coerce_mutbls(mt_a, mt_b)?;
492 kind: Adjust::Deref(None),
495 kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
496 target: self.tcx.mk_ptr(ty::TypeAndMut {
504 let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
506 // Setup either a subtyping or a LUB relationship between
507 // the `CoerceUnsized` target type and the expected type.
508 // We only have the latter, so we use an inference variable
509 // for the former and let type inference do the rest.
510 let origin = TypeVariableOrigin::MiscVariable(self.cause.span);
511 let coerce_target = self.next_ty_var(origin);
512 let mut coercion = self.unify_and(coerce_target, target, |target| {
513 let unsize = Adjustment {
514 kind: Adjust::Unsize,
518 None => vec![unsize],
519 Some((ref deref, ref autoref)) => {
520 vec![deref.clone(), autoref.clone(), unsize]
525 let mut selcx = traits::SelectionContext::new(self);
527 // Create an obligation for `Source: CoerceUnsized<Target>`.
528 let cause = ObligationCause::misc(self.cause.span, self.body_id);
530 // Use a FIFO queue for this custom fulfillment procedure.
532 // A Vec (or SmallVec) is not a natural choice for a queue. However,
533 // this code path is hot, and this queue usually has a max length of 1
534 // and almost never more than 3. By using a SmallVec we avoid an
535 // allocation, at the (very small) cost of (occasionally) having to
536 // shift subsequent elements down when removing the front element.
537 let mut queue: SmallVec<[_; 4]> =
538 smallvec![self.tcx.predicate_for_trait_def(self.fcx.param_env,
543 &[coerce_target.into()])];
545 let mut has_unsized_tuple_coercion = false;
547 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
548 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
549 // inference might unify those two inner type variables later.
550 let traits = [coerce_unsized_did, unsize_did];
551 while !queue.is_empty() {
552 let obligation = queue.remove(0);
553 debug!("coerce_unsized resolve step: {:?}", obligation);
554 let trait_ref = match obligation.predicate {
555 ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => {
556 if unsize_did == tr.def_id() {
557 let sty = &tr.skip_binder().input_types().nth(1).unwrap().sty;
558 if let ty::Tuple(..) = sty {
559 debug!("coerce_unsized: found unsized tuple coercion");
560 has_unsized_tuple_coercion = true;
566 coercion.obligations.push(obligation);
570 match selcx.select(&obligation.with(trait_ref)) {
571 // Uncertain or unimplemented.
573 if trait_ref.def_id() == unsize_did {
574 let trait_ref = self.resolve_type_vars_if_possible(&trait_ref);
575 let self_ty = trait_ref.skip_binder().self_ty();
576 let unsize_ty = trait_ref.skip_binder().input_types().nth(1).unwrap();
577 debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_ref);
578 match (&self_ty.sty, &unsize_ty.sty) {
579 (ty::Infer(ty::TyVar(v)),
580 ty::Dynamic(..)) if self.type_var_is_sized(*v) => {
581 debug!("coerce_unsized: have sized infer {:?}", v);
582 coercion.obligations.push(obligation);
583 // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
587 // Some other case for `$0: Unsize<Something>`. Note that we
588 // hit this case even if `Something` is a sized type, so just
589 // don't do the coercion.
590 debug!("coerce_unsized: ambiguous unsize");
591 return Err(TypeError::Mismatch);
595 debug!("coerce_unsized: early return - ambiguous");
596 return Err(TypeError::Mismatch);
599 Err(traits::Unimplemented) => {
600 debug!("coerce_unsized: early return - can't prove obligation");
601 return Err(TypeError::Mismatch);
604 // Object safety violations or miscellaneous.
606 self.report_selection_error(&obligation, &err, false);
607 // Treat this like an obligation and follow through
608 // with the unsizing - the lack of a coercion should
609 // be silent, as it causes a type mismatch later.
612 Ok(Some(vtable)) => {
613 queue.extend(vtable.nested_obligations())
618 if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
619 feature_gate::emit_feature_err(&self.tcx.sess.parse_sess,
620 "unsized_tuple_coercion",
622 feature_gate::GateIssue::Language,
623 feature_gate::EXPLAIN_UNSIZED_TUPLE_COERCION);
629 fn coerce_from_safe_fn<F, G>(&self,
631 fn_ty_a: ty::PolyFnSig<'tcx>,
635 -> CoerceResult<'tcx>
636 where F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
637 G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>
639 if let ty::FnPtr(fn_ty_b) = b.sty {
640 if let (hir::Unsafety::Normal, hir::Unsafety::Unsafe)
641 = (fn_ty_a.unsafety(), fn_ty_b.unsafety())
643 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
644 return self.unify_and(unsafe_a, b, to_unsafe);
647 self.unify_and(a, b, normal)
650 fn coerce_from_fn_pointer(&self,
652 fn_ty_a: ty::PolyFnSig<'tcx>,
654 -> CoerceResult<'tcx> {
655 //! Attempts to coerce from the type of a Rust function item
656 //! into a closure or a `proc`.
659 let b = self.shallow_resolve(b);
660 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
662 self.coerce_from_safe_fn(a, fn_ty_a, b,
663 simple(Adjust::UnsafeFnPointer), identity)
666 fn coerce_from_fn_item(&self,
669 -> CoerceResult<'tcx> {
670 //! Attempts to coerce from the type of a Rust function item
671 //! into a closure or a `proc`.
673 let b = self.shallow_resolve(b);
674 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
678 let a_sig = a.fn_sig(self.tcx);
679 let InferOk { value: a_sig, mut obligations } =
680 self.normalize_associated_types_in_as_infer_ok(self.cause.span, &a_sig);
682 let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
683 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
689 Adjustment { kind: Adjust::ReifyFnPointer, target: a_fn_pointer },
690 Adjustment { kind: Adjust::UnsafeFnPointer, target: unsafe_ty },
693 simple(Adjust::ReifyFnPointer)
696 obligations.extend(o2);
697 Ok(InferOk { value, obligations })
699 _ => self.unify_and(a, b, identity),
703 fn coerce_closure_to_fn(&self,
706 substs_a: ClosureSubsts<'tcx>,
708 -> CoerceResult<'tcx> {
709 //! Attempts to coerce from the type of a non-capturing closure
710 //! into a function pointer.
713 let b = self.shallow_resolve(b);
715 let node_id_a = self.tcx.hir().as_local_node_id(def_id_a).unwrap();
717 ty::FnPtr(_) if self.tcx.with_freevars(node_id_a, |v| v.is_empty()) => {
718 // We coerce the closure, which has fn type
719 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
721 // `fn(arg0,arg1,...) -> _`
722 let sig = self.closure_sig(def_id_a, substs_a);
723 let pointer_ty = self.tcx.coerce_closure_fn_ty(sig);
724 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})",
726 self.unify_and(pointer_ty, b, simple(Adjust::ClosureFnPointer))
728 _ => self.unify_and(a, b, identity),
732 fn coerce_unsafe_ptr(&self,
735 mutbl_b: hir::Mutability)
736 -> CoerceResult<'tcx> {
737 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
739 let (is_ref, mt_a) = match a.sty {
740 ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
741 ty::RawPtr(mt) => (false, mt),
742 _ => return self.unify_and(a, b, identity)
745 // Check that the types which they point at are compatible.
746 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut {
750 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
751 // Although references and unsafe ptrs have the same
752 // representation, we still register an Adjust::DerefRef so that
753 // regionck knows that the region for `a` must be valid here.
755 self.unify_and(a_unsafe, b, |target| {
757 kind: Adjust::Deref(None),
760 kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)),
764 } else if mt_a.mutbl != mutbl_b {
765 self.unify_and(a_unsafe, b, simple(Adjust::MutToConstPointer))
767 self.unify_and(a_unsafe, b, identity)
772 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
773 /// Attempt to coerce an expression to a type, and return the
774 /// adjusted type of the expression, if successful.
775 /// Adjustments are only recorded if the coercion succeeded.
776 /// The expressions *must not* have any pre-existing adjustments.
777 pub fn try_coerce(&self,
781 allow_two_phase: AllowTwoPhase)
782 -> RelateResult<'tcx, Ty<'tcx>> {
783 let source = self.resolve_type_vars_with_obligations(expr_ty);
784 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
786 let cause = self.cause(expr.span, ObligationCauseCode::ExprAssignable);
787 let coerce = Coerce::new(self, cause, allow_two_phase);
788 let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
790 let (adjustments, _) = self.register_infer_ok_obligations(ok);
791 self.apply_adjustments(expr, adjustments);
795 /// Same as `try_coerce()`, but without side-effects.
796 pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
797 let source = self.resolve_type_vars_with_obligations(expr_ty);
798 debug!("coercion::can({:?} -> {:?})", source, target);
800 let cause = self.cause(syntax_pos::DUMMY_SP, ObligationCauseCode::ExprAssignable);
801 // We don't ever need two-phase here since we throw out the result of the coercion
802 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
803 self.probe(|_| coerce.coerce(source, target)).is_ok()
806 /// Given some expressions, their known unified type and another expression,
807 /// tries to unify the types, potentially inserting coercions on any of the
808 /// provided expressions and returns their LUB (aka "common supertype").
810 /// This is really an internal helper. From outside the coercion
811 /// module, you should instantiate a `CoerceMany` instance.
812 fn try_find_coercion_lub<E>(&self,
813 cause: &ObligationCause<'tcx>,
818 -> RelateResult<'tcx, Ty<'tcx>>
819 where E: AsCoercionSite
821 let prev_ty = self.resolve_type_vars_with_obligations(prev_ty);
822 let new_ty = self.resolve_type_vars_with_obligations(new_ty);
823 debug!("coercion::try_find_coercion_lub({:?}, {:?})", prev_ty, new_ty);
825 // Special-case that coercion alone cannot handle:
826 // Two function item types of differing IDs or Substs.
827 if let (&ty::FnDef(..), &ty::FnDef(..)) = (&prev_ty.sty, &new_ty.sty) {
828 // Don't reify if the function types have a LUB, i.e., they
829 // are the same function and their parameters have a LUB.
830 let lub_ty = self.commit_if_ok(|_| {
831 self.at(cause, self.param_env)
832 .lub(prev_ty, new_ty)
833 }).map(|ok| self.register_infer_ok_obligations(ok));
836 // We have a LUB of prev_ty and new_ty, just return it.
840 // The signature must match.
841 let a_sig = prev_ty.fn_sig(self.tcx);
842 let a_sig = self.normalize_associated_types_in(new.span, &a_sig);
843 let b_sig = new_ty.fn_sig(self.tcx);
844 let b_sig = self.normalize_associated_types_in(new.span, &b_sig);
845 let sig = self.at(cause, self.param_env)
846 .trace(prev_ty, new_ty)
848 .map(|ok| self.register_infer_ok_obligations(ok))?;
850 // Reify both sides and return the reified fn pointer type.
851 let fn_ptr = self.tcx.mk_fn_ptr(sig);
852 for expr in exprs.iter().map(|e| e.as_coercion_site()).chain(Some(new)) {
853 // The only adjustment that can produce an fn item is
854 // `NeverToAny`, so this should always be valid.
855 self.apply_adjustments(expr, vec![Adjustment {
856 kind: Adjust::ReifyFnPointer,
863 // Configure a Coerce instance to compute the LUB.
864 // We don't allow two-phase borrows on any autorefs this creates since we
865 // probably aren't processing function arguments here and even if we were,
866 // they're going to get autorefed again anyway and we can apply 2-phase borrows
868 let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
869 coerce.use_lub = true;
871 // First try to coerce the new expression to the type of the previous ones,
872 // but only if the new expression has no coercion already applied to it.
873 let mut first_error = None;
874 if !self.tables.borrow().adjustments().contains_key(new.hir_id) {
875 let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
878 let (adjustments, target) = self.register_infer_ok_obligations(ok);
879 self.apply_adjustments(new, adjustments);
882 Err(e) => first_error = Some(e),
886 // Then try to coerce the previous expressions to the type of the new one.
887 // This requires ensuring there are no coercions applied to *any* of the
888 // previous expressions, other than noop reborrows (ignoring lifetimes).
890 let expr = expr.as_coercion_site();
891 let noop = match self.tables.borrow().expr_adjustments(expr) {
893 Adjustment { kind: Adjust::Deref(_), .. },
894 Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. }
896 match self.node_ty(expr.hir_id).sty {
897 ty::Ref(_, _, mt_orig) => {
898 let mutbl_adj: hir::Mutability = mutbl_adj.into();
899 // Reborrow that we can safely ignore, because
900 // the next adjustment can only be a Deref
901 // which will be merged into it.
907 &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
912 return self.commit_if_ok(|_|
913 self.at(cause, self.param_env)
914 .lub(prev_ty, new_ty)
915 ).map(|ok| self.register_infer_ok_obligations(ok));
919 match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
921 // Avoid giving strange errors on failed attempts.
922 if let Some(e) = first_error {
925 self.commit_if_ok(|_|
926 self.at(cause, self.param_env)
927 .lub(prev_ty, new_ty)
928 ).map(|ok| self.register_infer_ok_obligations(ok))
932 let (adjustments, target) = self.register_infer_ok_obligations(ok);
934 let expr = expr.as_coercion_site();
935 self.apply_adjustments(expr, adjustments.clone());
943 /// CoerceMany encapsulates the pattern you should use when you have
944 /// many expressions that are all getting coerced to a common
945 /// type. This arises, for example, when you have a match (the result
946 /// of each arm is coerced to a common type). It also arises in less
947 /// obvious places, such as when you have many `break foo` expressions
948 /// that target the same loop, or the various `return` expressions in
951 /// The basic protocol is as follows:
953 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
954 /// This will also serve as the "starting LUB". The expectation is
955 /// that this type is something which all of the expressions *must*
956 /// be coercible to. Use a fresh type variable if needed.
957 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
958 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
959 /// unit. This happens for example if you have a `break` with no expression,
960 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
961 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
962 /// from you so that you don't have to worry your pretty head about it.
963 /// But if an error is reported, the final type will be `err`.
964 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
965 /// previously coerced expressions.
966 /// - When all done, invoke `complete()`. This will return the LUB of
967 /// all your expressions.
968 /// - WARNING: I don't believe this final type is guaranteed to be
969 /// related to your initial `expected_ty` in any particular way,
970 /// although it will typically be a subtype, so you should check it.
971 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
972 /// previously coerced expressions.
977 /// let mut coerce = CoerceMany::new(expected_ty);
978 /// for expr in exprs {
979 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
980 /// coerce.coerce(fcx, &cause, expr, expr_ty);
982 /// let final_ty = coerce.complete(fcx);
984 pub struct CoerceMany<'gcx, 'tcx, 'exprs, E>
985 where 'gcx: 'tcx, E: 'exprs + AsCoercionSite,
987 expected_ty: Ty<'tcx>,
988 final_ty: Option<Ty<'tcx>>,
989 expressions: Expressions<'gcx, 'exprs, E>,
993 /// The type of a `CoerceMany` that is storing up the expressions into
994 /// a buffer. We use this in `check/mod.rs` for things like `break`.
995 pub type DynamicCoerceMany<'gcx, 'tcx> = CoerceMany<'gcx, 'tcx, 'gcx, P<hir::Expr>>;
997 enum Expressions<'gcx, 'exprs, E>
998 where E: 'exprs + AsCoercionSite,
1000 Dynamic(Vec<&'gcx hir::Expr>),
1001 UpFront(&'exprs [E]),
1004 impl<'gcx, 'tcx, 'exprs, E> CoerceMany<'gcx, 'tcx, 'exprs, E>
1005 where 'gcx: 'tcx, E: 'exprs + AsCoercionSite,
1007 /// The usual case; collect the set of expressions dynamically.
1008 /// If the full set of coercion sites is known before hand,
1009 /// consider `with_coercion_sites()` instead to avoid allocation.
1010 pub fn new(expected_ty: Ty<'tcx>) -> Self {
1011 Self::make(expected_ty, Expressions::Dynamic(vec![]))
1014 /// As an optimization, you can create a `CoerceMany` with a
1015 /// pre-existing slice of expressions. In this case, you are
1016 /// expected to pass each element in the slice to `coerce(...)` in
1017 /// order. This is used with arrays in particular to avoid
1018 /// needlessly cloning the slice.
1019 pub fn with_coercion_sites(expected_ty: Ty<'tcx>,
1020 coercion_sites: &'exprs [E])
1022 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
1025 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'gcx, 'exprs, E>) -> Self {
1034 /// Returns the "expected type" with which this coercion was
1035 /// constructed. This represents the "downward propagated" type
1036 /// that was given to us at the start of typing whatever construct
1037 /// we are typing (e.g., the match expression).
1039 /// Typically, this is used as the expected type when
1040 /// type-checking each of the alternative expressions whose types
1041 /// we are trying to merge.
1042 pub fn expected_ty(&self) -> Ty<'tcx> {
1046 /// Returns the current "merged type", representing our best-guess
1047 /// at the LUB of the expressions we've seen so far (if any). This
1048 /// isn't *final* until you call `self.final()`, which will return
1049 /// the merged type.
1050 pub fn merged_ty(&self) -> Ty<'tcx> {
1051 self.final_ty.unwrap_or(self.expected_ty)
1054 /// Indicates that the value generated by `expression`, which is
1055 /// of type `expression_ty`, is one of the possibilities that we
1056 /// could coerce from. This will record `expression`, and later
1057 /// calls to `coerce` may come back and add adjustments and things
1059 pub fn coerce<'a>(&mut self,
1060 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1061 cause: &ObligationCause<'tcx>,
1062 expression: &'gcx hir::Expr,
1063 expression_ty: Ty<'tcx>)
1065 self.coerce_inner(fcx,
1072 /// Indicates that one of the inputs is a "forced unit". This
1073 /// occurs in a case like `if foo { ... };`, where the missing else
1074 /// generates a "forced unit". Another example is a `loop { break;
1075 /// }`, where the `break` has no argument expression. We treat
1076 /// these cases slightly differently for error-reporting
1077 /// purposes. Note that these tend to correspond to cases where
1078 /// the `()` expression is implicit in the source, and hence we do
1079 /// not take an expression argument.
1081 /// The `augment_error` gives you a chance to extend the error
1082 /// message, in case any results (e.g., we use this to suggest
1083 /// removing a `;`).
1084 pub fn coerce_forced_unit<'a>(&mut self,
1085 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1086 cause: &ObligationCause<'tcx>,
1087 augment_error: &mut dyn FnMut(&mut DiagnosticBuilder),
1088 label_unit_as_expected: bool)
1090 self.coerce_inner(fcx,
1094 Some(augment_error),
1095 label_unit_as_expected)
1098 /// The inner coercion "engine". If `expression` is `None`, this
1099 /// is a forced-unit case, and hence `expression_ty` must be
1101 fn coerce_inner<'a>(&mut self,
1102 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1103 cause: &ObligationCause<'tcx>,
1104 expression: Option<&'gcx hir::Expr>,
1105 mut expression_ty: Ty<'tcx>,
1106 augment_error: Option<&mut dyn FnMut(&mut DiagnosticBuilder)>,
1107 label_expression_as_expected: bool)
1109 // Incorporate whatever type inference information we have
1110 // until now; in principle we might also want to process
1111 // pending obligations, but doing so should only improve
1112 // compatibility (hopefully that is true) by helping us
1113 // uncover never types better.
1114 if expression_ty.is_ty_var() {
1115 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1118 // If we see any error types, just propagate that error
1120 if expression_ty.references_error() || self.merged_ty().references_error() {
1121 self.final_ty = Some(fcx.tcx.types.err);
1125 // Handle the actual type unification etc.
1126 let result = if let Some(expression) = expression {
1127 if self.pushed == 0 {
1128 // Special-case the first expression we are coercing.
1129 // To be honest, I'm not entirely sure why we do this.
1130 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1131 fcx.try_coerce(expression, expression_ty, self.expected_ty, AllowTwoPhase::No)
1133 match self.expressions {
1134 Expressions::Dynamic(ref exprs) =>
1135 fcx.try_find_coercion_lub(cause,
1140 Expressions::UpFront(ref coercion_sites) =>
1141 fcx.try_find_coercion_lub(cause,
1142 &coercion_sites[0..self.pushed],
1149 // this is a hack for cases where we default to `()` because
1150 // the expression etc has been omitted from the source. An
1151 // example is an `if let` without an else:
1153 // if let Some(x) = ... { }
1155 // we wind up with a second match arm that is like `_ =>
1156 // ()`. That is the case we are considering here. We take
1157 // a different path to get the right "expected, found"
1158 // message and so forth (and because we know that
1159 // `expression_ty` will be unit).
1161 // Another example is `break` with no argument expression.
1162 assert!(expression_ty.is_unit(), "if let hack without unit type");
1163 fcx.at(cause, fcx.param_env)
1164 .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
1166 fcx.register_infer_ok_obligations(infer_ok);
1173 self.final_ty = Some(v);
1174 if let Some(e) = expression {
1175 match self.expressions {
1176 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1177 Expressions::UpFront(coercion_sites) => {
1178 // if the user gave us an array to validate, check that we got
1179 // the next expression in the list, as expected
1180 assert_eq!(coercion_sites[self.pushed].as_coercion_site().id, e.id);
1187 let (expected, found) = if label_expression_as_expected {
1188 // In the case where this is a "forced unit", like
1189 // `break`, we want to call the `()` "expected"
1190 // since it is implied by the syntax.
1191 // (Note: not all force-units work this way.)"
1192 (expression_ty, self.final_ty.unwrap_or(self.expected_ty))
1194 // Otherwise, the "expected" type for error
1195 // reporting is the current unification type,
1196 // which is basically the LUB of the expressions
1197 // we've seen so far (combined with the expected
1199 (self.final_ty.unwrap_or(self.expected_ty), expression_ty)
1204 ObligationCauseCode::ReturnNoExpression => {
1205 db = struct_span_err!(
1206 fcx.tcx.sess, cause.span, E0069,
1207 "`return;` in a function whose return type is not `()`");
1208 db.span_label(cause.span, "return type is not `()`");
1210 ObligationCauseCode::BlockTailExpression(blk_id) => {
1211 let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
1212 db = self.report_return_mismatched_types(
1219 expression.map(|expr| (expr, blk_id)),
1222 ObligationCauseCode::ReturnType(id) => {
1223 db = self.report_return_mismatched_types(
1224 cause, expected, found, err, fcx, id, None);
1227 db = fcx.report_mismatched_types(cause, expected, found, err);
1231 if let Some(augment_error) = augment_error {
1232 augment_error(&mut db);
1237 self.final_ty = Some(fcx.tcx.types.err);
1242 fn report_return_mismatched_types<'a>(
1244 cause: &ObligationCause<'tcx>,
1247 err: TypeError<'tcx>,
1248 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1249 id: syntax::ast::NodeId,
1250 expression: Option<(&'gcx hir::Expr, syntax::ast::NodeId)>,
1251 ) -> DiagnosticBuilder<'a> {
1252 let mut db = fcx.report_mismatched_types(cause, expected, found, err);
1254 let mut pointing_at_return_type = false;
1255 let mut return_sp = None;
1257 // Verify that this is a tail expression of a function, otherwise the
1258 // label pointing out the cause for the type coercion will be wrong
1259 // as prior return coercions would not be relevant (#57664).
1260 let parent_id = fcx.tcx.hir().get_parent_node(id);
1261 let fn_decl = if let Some((expr, blk_id)) = expression {
1262 pointing_at_return_type = fcx.suggest_mismatched_types_on_tail(
1270 let parent = fcx.tcx.hir().get(parent_id);
1271 fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
1273 fcx.get_fn_decl(parent_id)
1276 if let (Some((fn_decl, can_suggest)), _) = (fn_decl, pointing_at_return_type) {
1277 if expression.is_none() {
1278 pointing_at_return_type |= fcx.suggest_missing_return_type(
1279 &mut db, &fn_decl, expected, found, can_suggest);
1281 if !pointing_at_return_type {
1282 return_sp = Some(fn_decl.output.span()); // `impl Trait` return type
1285 if let (Some(sp), Some(return_sp)) = (fcx.ret_coercion_span.borrow().as_ref(), return_sp) {
1286 db.span_label(return_sp, "expected because this return type...");
1287 db.span_label( *sp, format!(
1288 "...is found to be `{}` here",
1289 fcx.resolve_type_vars_with_obligations(expected),
1295 pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) -> Ty<'tcx> {
1296 if let Some(final_ty) = self.final_ty {
1299 // If we only had inputs that were of type `!` (or no
1300 // inputs at all), then the final type is `!`.
1301 assert_eq!(self.pushed, 0);
1307 /// Something that can be converted into an expression to which we can
1308 /// apply a coercion.
1309 pub trait AsCoercionSite {
1310 fn as_coercion_site(&self) -> &hir::Expr;
1313 impl AsCoercionSite for hir::Expr {
1314 fn as_coercion_site(&self) -> &hir::Expr {
1319 impl AsCoercionSite for P<hir::Expr> {
1320 fn as_coercion_site(&self) -> &hir::Expr {
1325 impl<'a, T> AsCoercionSite for &'a T
1326 where T: AsCoercionSite
1328 fn as_coercion_site(&self) -> &hir::Expr {
1329 (**self).as_coercion_site()
1333 impl AsCoercionSite for ! {
1334 fn as_coercion_site(&self) -> &hir::Expr {
1339 impl AsCoercionSite for hir::Arm {
1340 fn as_coercion_site(&self) -> &hir::Expr {