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 rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
55 use rustc::infer::{Coercion, InferOk, InferResult};
56 use rustc::session::parse::feature_err;
57 use rustc::traits::{self, ObligationCause, ObligationCauseCode};
58 use rustc::ty::adjustment::{
59 Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability, PointerCast,
61 use rustc::ty::error::TypeError;
62 use rustc::ty::fold::TypeFoldable;
63 use rustc::ty::relate::RelateResult;
64 use rustc::ty::subst::SubstsRef;
65 use rustc::ty::{self, Ty, TypeAndMut};
66 use rustc_error_codes::*;
67 use rustc_errors::{struct_span_err, DiagnosticBuilder};
69 use rustc_hir::def_id::DefId;
71 use rustc_span::symbol::sym;
72 use rustc_target::spec::abi::Abi;
73 use smallvec::{smallvec, SmallVec};
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>(
99 from_mutbl: hir::Mutability,
100 to_mutbl: hir::Mutability,
101 ) -> RelateResult<'tcx, ()> {
102 match (from_mutbl, to_mutbl) {
103 (hir::Mutability::Mut, hir::Mutability::Mut)
104 | (hir::Mutability::Not, hir::Mutability::Not)
105 | (hir::Mutability::Mut, hir::Mutability::Not) => Ok(()),
106 (hir::Mutability::Not, hir::Mutability::Mut) => Err(TypeError::Mutability),
110 fn identity(_: Ty<'_>) -> Vec<Adjustment<'_>> {
114 fn simple<'tcx>(kind: Adjust<'tcx>) -> impl FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>> {
115 move |target| vec![Adjustment { kind, target }]
119 adj: Vec<Adjustment<'tcx>>,
121 obligations: traits::PredicateObligations<'tcx>,
122 ) -> CoerceResult<'tcx> {
123 Ok(InferOk { value: (adj, target), obligations })
126 impl<'f, 'tcx> Coerce<'f, 'tcx> {
128 fcx: &'f FnCtxt<'f, 'tcx>,
129 cause: ObligationCause<'tcx>,
130 allow_two_phase: AllowTwoPhase,
132 Coerce { fcx, cause, allow_two_phase, use_lub: false }
135 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
136 self.commit_if_ok(|_| {
138 self.at(&self.cause, self.fcx.param_env).lub(b, a)
140 self.at(&self.cause, self.fcx.param_env)
142 .map(|InferOk { value: (), obligations }| InferOk { value: a, obligations })
147 /// Unify two types (using sub or lub) and produce a specific coercion.
148 fn unify_and<F>(&self, a: Ty<'tcx>, b: Ty<'tcx>, f: F) -> CoerceResult<'tcx>
150 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
153 .and_then(|InferOk { value: ty, obligations }| success(f(ty), ty, obligations))
156 fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
157 let a = self.shallow_resolve(a);
158 debug!("Coerce.tys({:?} => {:?})", a, b);
160 // Just ignore error types.
161 if a.references_error() || b.references_error() {
162 return success(vec![], self.fcx.tcx.types.err, vec![]);
166 // Subtle: If we are coercing from `!` to `?T`, where `?T` is an unbound
167 // type variable, we want `?T` to fallback to `!` if not
168 // otherwise constrained. An example where this arises:
170 // let _: Option<?T> = Some({ return; });
172 // here, we would coerce from `!` to `?T`.
173 let b = self.shallow_resolve(b);
174 return if self.shallow_resolve(b).is_ty_var() {
175 // Micro-optimization: no need for this if `b` is
176 // already resolved in some way.
177 let diverging_ty = self.next_diverging_ty_var(TypeVariableOrigin {
178 kind: TypeVariableOriginKind::AdjustmentType,
179 span: self.cause.span,
181 self.unify_and(&b, &diverging_ty, simple(Adjust::NeverToAny))
183 success(simple(Adjust::NeverToAny)(b), b, vec![])
187 // Consider coercing the subtype to a DST
189 // NOTE: this is wrapped in a `commit_if_ok` because it creates
190 // a "spurious" type variable, and we don't want to have that
191 // type variable in memory if the coercion fails.
192 let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
195 debug!("coerce: unsize successful");
198 Err(TypeError::ObjectUnsafeCoercion(did)) => {
199 debug!("coerce: unsize not object safe");
200 return Err(TypeError::ObjectUnsafeCoercion(did));
204 debug!("coerce: unsize failed");
206 // Examine the supertype and consider auto-borrowing.
208 // Note: does not attempt to resolve type variables we encounter.
209 // See above for details.
211 ty::RawPtr(mt_b) => {
212 return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
215 ty::Ref(r_b, ty, mutbl) => {
216 let mt_b = ty::TypeAndMut { ty, mutbl };
217 return self.coerce_borrowed_pointer(a, b, r_b, mt_b);
225 // Function items are coercible to any closure
226 // type; function pointers are not (that would
227 // require double indirection).
228 // Additionally, we permit coercion of function
229 // items to drop the unsafe qualifier.
230 self.coerce_from_fn_item(a, b)
233 // We permit coercion of fn pointers to drop the
235 self.coerce_from_fn_pointer(a, a_f, b)
237 ty::Closure(def_id_a, substs_a) => {
238 // Non-capturing closures are coercible to
239 // function pointers or unsafe function pointers.
240 // It cannot convert closures that require unsafe.
241 self.coerce_closure_to_fn(a, def_id_a, substs_a, b)
244 // Otherwise, just use unification rules.
245 self.unify_and(a, b, identity)
250 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
251 /// To match `A` with `B`, autoderef will be performed,
252 /// calling `deref`/`deref_mut` where necessary.
253 fn coerce_borrowed_pointer(
257 r_b: ty::Region<'tcx>,
258 mt_b: TypeAndMut<'tcx>,
259 ) -> CoerceResult<'tcx> {
260 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
262 // If we have a parameter of type `&M T_a` and the value
263 // provided is `expr`, we will be adding an implicit borrow,
264 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
265 // to type check, we will construct the type that `&M*expr` would
268 let (r_a, mt_a) = match a.kind {
269 ty::Ref(r_a, ty, mutbl) => {
270 let mt_a = ty::TypeAndMut { ty, mutbl };
271 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
274 _ => return self.unify_and(a, b, identity),
277 let span = self.cause.span;
279 let mut first_error = None;
280 let mut r_borrow_var = None;
281 let mut autoderef = self.autoderef(span, a);
282 let mut found = None;
284 for (referent_ty, autoderefs) in autoderef.by_ref() {
286 // Don't let this pass, otherwise it would cause
287 // &T to autoref to &&T.
291 // At this point, we have deref'd `a` to `referent_ty`. So
292 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
293 // In the autoderef loop for `&'a mut Vec<T>`, we would get
296 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
297 // - `Vec<T>` -- 1 deref
298 // - `[T]` -- 2 deref
300 // At each point after the first callback, we want to
301 // check to see whether this would match out target type
302 // (`&'b mut [T]`) if we autoref'd it. We can't just
303 // compare the referent types, though, because we still
304 // have to consider the mutability. E.g., in the case
305 // we've been considering, we have an `&mut` reference, so
306 // the `T` in `[T]` needs to be unified with equality.
308 // Therefore, we construct reference types reflecting what
309 // the types will be after we do the final auto-ref and
310 // compare those. Note that this means we use the target
311 // mutability [1], since it may be that we are coercing
312 // from `&mut T` to `&U`.
314 // One fine point concerns the region that we use. We
315 // choose the region such that the region of the final
316 // type that results from `unify` will be the region we
317 // want for the autoref:
319 // - if in sub mode, that means we want to use `'b` (the
320 // region from the target reference) for both
321 // pointers [2]. This is because sub mode (somewhat
322 // arbitrarily) returns the subtype region. In the case
323 // where we are coercing to a target type, we know we
324 // want to use that target type region (`'b`) because --
325 // for the program to type-check -- it must be the
326 // smaller of the two.
327 // - One fine point. It may be surprising that we can
328 // use `'b` without relating `'a` and `'b`. The reason
329 // that this is ok is that what we produce is
330 // effectively a `&'b *x` expression (if you could
331 // annotate the region of a borrow), and regionck has
332 // code that adds edges from the region of a borrow
333 // (`'b`, here) into the regions in the borrowed
334 // expression (`*x`, here). (Search for "link".)
335 // - if in lub mode, things can get fairly complicated. The
336 // easiest thing is just to make a fresh
337 // region variable [4], which effectively means we defer
338 // the decision to region inference (and regionck, which will add
339 // some more edges to this variable). However, this can wind up
340 // creating a crippling number of variables in some cases --
341 // e.g., #32278 -- so we optimize one particular case [3].
342 // Let me try to explain with some examples:
343 // - The "running example" above represents the simple case,
344 // where we have one `&` reference at the outer level and
345 // ownership all the rest of the way down. In this case,
346 // we want `LUB('a, 'b)` as the resulting region.
347 // - However, if there are nested borrows, that region is
348 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
349 // `&'b T`. In this case, `'a` is actually irrelevant.
350 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
351 // we get spurious errors (`ui/regions-lub-ref-ref-rc.rs`).
352 // (The errors actually show up in borrowck, typically, because
353 // this extra edge causes the region `'a` to be inferred to something
354 // too big, which then results in borrowck errors.)
355 // - We could track the innermost shared reference, but there is already
356 // code in regionck that has the job of creating links between
357 // the region of a borrow and the regions in the thing being
358 // borrowed (here, `'a` and `'x`), and it knows how to handle
359 // all the various cases. So instead we just make a region variable
360 // and let regionck figure it out.
361 let r = if !self.use_lub {
363 } else if autoderefs == 1 {
366 if r_borrow_var.is_none() {
367 // create var lazilly, at most once
368 let coercion = Coercion(span);
369 let r = self.next_region_var(coercion);
370 r_borrow_var = Some(r); // [4] above
372 r_borrow_var.unwrap()
374 let derefd_ty_a = self.tcx.mk_ref(
378 mutbl: mt_b.mutbl, // [1] above
381 match self.unify(derefd_ty_a, b) {
387 if first_error.is_none() {
388 first_error = Some(err);
394 // Extract type or return an error. We return the first error
395 // we got, which should be from relating the "base" type
396 // (e.g., in example above, the failure from relating `Vec<T>`
397 // to the target type), since that should be the least
399 let InferOk { value: ty, mut obligations } = match found {
402 let err = first_error.expect("coerce_borrowed_pointer had no error");
403 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
408 if ty == a && mt_a.mutbl == hir::Mutability::Not && autoderef.step_count() == 1 {
409 // As a special case, if we would produce `&'a *x`, that's
410 // a total no-op. We end up with the type `&'a T` just as
411 // we started with. In that case, just skip it
412 // altogether. This is just an optimization.
414 // Note that for `&mut`, we DO want to reborrow --
415 // otherwise, this would be a move, which might be an
416 // error. For example `foo(self.x)` where `self` and
417 // `self.x` both have `&mut `type would be a move of
418 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
419 // which is a borrow.
420 assert_eq!(mt_b.mutbl, hir::Mutability::Not); // can only coerce &T -> &U
421 return success(vec![], ty, obligations);
424 let needs = Needs::maybe_mut_place(mt_b.mutbl);
425 let InferOk { value: mut adjustments, obligations: o } =
426 autoderef.adjust_steps_as_infer_ok(self, needs);
427 obligations.extend(o);
428 obligations.extend(autoderef.into_obligations());
430 // Now apply the autoref. We have to extract the region out of
431 // the final ref type we got.
432 let r_borrow = match ty.kind {
433 ty::Ref(r_borrow, _, _) => r_borrow,
434 _ => span_bug!(span, "expected a ref type, got {:?}", ty),
436 let mutbl = match mt_b.mutbl {
437 hir::Mutability::Not => AutoBorrowMutability::Not,
438 hir::Mutability::Mut => {
439 AutoBorrowMutability::Mut { allow_two_phase_borrow: self.allow_two_phase }
442 adjustments.push(Adjustment {
443 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
447 debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}", ty, adjustments);
449 success(adjustments, ty, obligations)
452 // &[T; n] or &mut [T; n] -> &[T]
453 // or &mut [T; n] -> &mut [T]
454 // or &Concrete -> &Trait, etc.
455 fn coerce_unsized(&self, source: Ty<'tcx>, target: Ty<'tcx>) -> CoerceResult<'tcx> {
456 debug!("coerce_unsized(source={:?}, target={:?})", source, target);
459 (self.tcx.lang_items().unsize_trait(), self.tcx.lang_items().coerce_unsized_trait());
460 let (unsize_did, coerce_unsized_did) = if let (Some(u), Some(cu)) = traits {
463 debug!("missing Unsize or CoerceUnsized traits");
464 return Err(TypeError::Mismatch);
467 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
468 // a DST unless we have to. This currently comes out in the wash since
469 // we can't unify [T] with U. But to properly support DST, we need to allow
470 // that, at which point we will need extra checks on the target here.
472 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
473 let reborrow = match (&source.kind, &target.kind) {
474 (&ty::Ref(_, ty_a, mutbl_a), &ty::Ref(_, _, mutbl_b)) => {
475 coerce_mutbls(mutbl_a, mutbl_b)?;
477 let coercion = Coercion(self.cause.span);
478 let r_borrow = self.next_region_var(coercion);
479 let mutbl = match mutbl_b {
480 hir::Mutability::Not => AutoBorrowMutability::Not,
481 hir::Mutability::Mut => AutoBorrowMutability::Mut {
482 // We don't allow two-phase borrows here, at least for initial
483 // implementation. If it happens that this coercion is a function argument,
484 // the reborrow in coerce_borrowed_ptr will pick it up.
485 allow_two_phase_borrow: AllowTwoPhase::No,
489 Adjustment { kind: Adjust::Deref(None), target: ty_a },
491 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
494 .mk_ref(r_borrow, ty::TypeAndMut { mutbl: mutbl_b, ty: ty_a }),
498 (&ty::Ref(_, ty_a, mt_a), &ty::RawPtr(ty::TypeAndMut { mutbl: mt_b, .. })) => {
499 coerce_mutbls(mt_a, mt_b)?;
502 Adjustment { kind: Adjust::Deref(None), target: ty_a },
504 kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b)),
505 target: self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mt_b, ty: ty_a }),
511 let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
513 // Setup either a subtyping or a LUB relationship between
514 // the `CoerceUnsized` target type and the expected type.
515 // We only have the latter, so we use an inference variable
516 // for the former and let type inference do the rest.
517 let origin = TypeVariableOrigin {
518 kind: TypeVariableOriginKind::MiscVariable,
519 span: self.cause.span,
521 let coerce_target = self.next_ty_var(origin);
522 let mut coercion = self.unify_and(coerce_target, target, |target| {
523 let unsize = Adjustment { kind: Adjust::Pointer(PointerCast::Unsize), target };
525 None => vec![unsize],
526 Some((ref deref, ref autoref)) => vec![deref.clone(), autoref.clone(), unsize],
530 let mut selcx = traits::SelectionContext::new(self);
532 // Create an obligation for `Source: CoerceUnsized<Target>`.
533 let cause = ObligationCause::new(
536 ObligationCauseCode::Coercion { source, target },
539 // Use a FIFO queue for this custom fulfillment procedure.
541 // A Vec (or SmallVec) is not a natural choice for a queue. However,
542 // this code path is hot, and this queue usually has a max length of 1
543 // and almost never more than 3. By using a SmallVec we avoid an
544 // allocation, at the (very small) cost of (occasionally) having to
545 // shift subsequent elements down when removing the front element.
546 let mut queue: SmallVec<[_; 4]> = smallvec![traits::predicate_for_trait_def(
553 &[coerce_target.into()]
556 let mut has_unsized_tuple_coercion = false;
558 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
559 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
560 // inference might unify those two inner type variables later.
561 let traits = [coerce_unsized_did, unsize_did];
562 while !queue.is_empty() {
563 let obligation = queue.remove(0);
564 debug!("coerce_unsized resolve step: {:?}", obligation);
565 let trait_ref = match obligation.predicate {
566 ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => {
567 if unsize_did == tr.def_id() {
568 let sty = &tr.skip_binder().input_types().nth(1).unwrap().kind;
569 if let ty::Tuple(..) = sty {
570 debug!("coerce_unsized: found unsized tuple coercion");
571 has_unsized_tuple_coercion = true;
577 coercion.obligations.push(obligation);
581 match selcx.select(&obligation.with(trait_ref)) {
582 // Uncertain or unimplemented.
584 if trait_ref.def_id() == unsize_did {
585 let trait_ref = self.resolve_vars_if_possible(&trait_ref);
586 let self_ty = trait_ref.skip_binder().self_ty();
587 let unsize_ty = trait_ref.skip_binder().input_types().nth(1).unwrap();
588 debug!("coerce_unsized: ambiguous unsize case for {:?}", trait_ref);
589 match (&self_ty.kind, &unsize_ty.kind) {
590 (ty::Infer(ty::TyVar(v)), ty::Dynamic(..))
591 if self.type_var_is_sized(*v) =>
593 debug!("coerce_unsized: have sized infer {:?}", v);
594 coercion.obligations.push(obligation);
595 // `$0: Unsize<dyn Trait>` where we know that `$0: Sized`, try going
599 // Some other case for `$0: Unsize<Something>`. Note that we
600 // hit this case even if `Something` is a sized type, so just
601 // don't do the coercion.
602 debug!("coerce_unsized: ambiguous unsize");
603 return Err(TypeError::Mismatch);
607 debug!("coerce_unsized: early return - ambiguous");
608 return Err(TypeError::Mismatch);
611 Err(traits::Unimplemented) => {
612 debug!("coerce_unsized: early return - can't prove obligation");
613 return Err(TypeError::Mismatch);
616 // Object safety violations or miscellaneous.
618 self.report_selection_error(&obligation, &err, false, false);
619 // Treat this like an obligation and follow through
620 // with the unsizing - the lack of a coercion should
621 // be silent, as it causes a type mismatch later.
624 Ok(Some(vtable)) => queue.extend(vtable.nested_obligations()),
628 if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
630 &self.tcx.sess.parse_sess,
631 sym::unsized_tuple_coercion,
633 "unsized tuple coercion is not stable enough for use and is subject to change",
641 fn coerce_from_safe_fn<F, G>(
644 fn_ty_a: ty::PolyFnSig<'tcx>,
648 ) -> CoerceResult<'tcx>
650 F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
651 G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
653 if let ty::FnPtr(fn_ty_b) = b.kind {
654 if let (hir::Unsafety::Normal, hir::Unsafety::Unsafe) =
655 (fn_ty_a.unsafety(), fn_ty_b.unsafety())
657 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
658 return self.unify_and(unsafe_a, b, to_unsafe);
661 self.unify_and(a, b, normal)
664 fn coerce_from_fn_pointer(
667 fn_ty_a: ty::PolyFnSig<'tcx>,
669 ) -> CoerceResult<'tcx> {
670 //! Attempts to coerce from the type of a Rust function item
671 //! into a closure or a `proc`.
674 let b = self.shallow_resolve(b);
675 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
677 self.coerce_from_safe_fn(
681 simple(Adjust::Pointer(PointerCast::UnsafeFnPointer)),
686 fn coerce_from_fn_item(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
687 //! Attempts to coerce from the type of a Rust function item
688 //! into a closure or a `proc`.
690 let b = self.shallow_resolve(b);
691 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
695 let a_sig = a.fn_sig(self.tcx);
696 // Intrinsics are not coercible to function pointers
697 if a_sig.abi() == Abi::RustIntrinsic || a_sig.abi() == Abi::PlatformIntrinsic {
698 return Err(TypeError::IntrinsicCast);
700 let InferOk { value: a_sig, mut obligations } =
701 self.normalize_associated_types_in_as_infer_ok(self.cause.span, &a_sig);
703 let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
704 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
711 kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
712 target: a_fn_pointer,
715 kind: Adjust::Pointer(PointerCast::UnsafeFnPointer),
720 simple(Adjust::Pointer(PointerCast::ReifyFnPointer)),
723 obligations.extend(o2);
724 Ok(InferOk { value, obligations })
726 _ => self.unify_and(a, b, identity),
730 fn coerce_closure_to_fn(
734 substs_a: SubstsRef<'tcx>,
736 ) -> CoerceResult<'tcx> {
737 //! Attempts to coerce from the type of a non-capturing closure
738 //! into a function pointer.
741 let b = self.shallow_resolve(b);
744 ty::FnPtr(fn_ty) if self.tcx.upvars(def_id_a).map_or(true, |v| v.is_empty()) => {
745 // We coerce the closure, which has fn type
746 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
748 // `fn(arg0,arg1,...) -> _`
750 // `unsafe fn(arg0,arg1,...) -> _`
751 let sig = self.closure_sig(def_id_a, substs_a);
752 let unsafety = fn_ty.unsafety();
753 let pointer_ty = self.tcx.coerce_closure_fn_ty(sig, unsafety);
754 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})", a, b, pointer_ty);
758 simple(Adjust::Pointer(PointerCast::ClosureFnPointer(unsafety))),
761 _ => self.unify_and(a, b, identity),
765 fn coerce_unsafe_ptr(
769 mutbl_b: hir::Mutability,
770 ) -> CoerceResult<'tcx> {
771 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
773 let (is_ref, mt_a) = match a.kind {
774 ty::Ref(_, ty, mutbl) => (true, ty::TypeAndMut { ty, mutbl }),
775 ty::RawPtr(mt) => (false, mt),
776 _ => return self.unify_and(a, b, identity),
779 // Check that the types which they point at are compatible.
780 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut { mutbl: mutbl_b, ty: mt_a.ty });
781 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
782 // Although references and unsafe ptrs have the same
783 // representation, we still register an Adjust::DerefRef so that
784 // regionck knows that the region for `a` must be valid here.
786 self.unify_and(a_unsafe, b, |target| {
788 Adjustment { kind: Adjust::Deref(None), target: mt_a.ty },
789 Adjustment { kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)), target },
792 } else if mt_a.mutbl != mutbl_b {
793 self.unify_and(a_unsafe, b, simple(Adjust::Pointer(PointerCast::MutToConstPointer)))
795 self.unify_and(a_unsafe, b, identity)
800 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
801 /// Attempt to coerce an expression to a type, and return the
802 /// adjusted type of the expression, if successful.
803 /// Adjustments are only recorded if the coercion succeeded.
804 /// The expressions *must not* have any pre-existing adjustments.
807 expr: &hir::Expr<'_>,
810 allow_two_phase: AllowTwoPhase,
811 ) -> RelateResult<'tcx, Ty<'tcx>> {
812 let source = self.resolve_vars_with_obligations(expr_ty);
813 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
815 let cause = self.cause(expr.span, ObligationCauseCode::ExprAssignable);
816 let coerce = Coerce::new(self, cause, allow_two_phase);
817 let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
819 let (adjustments, _) = self.register_infer_ok_obligations(ok);
820 self.apply_adjustments(expr, adjustments);
821 Ok(if expr_ty.references_error() { self.tcx.types.err } else { target })
824 /// Same as `try_coerce()`, but without side-effects.
825 pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
826 let source = self.resolve_vars_with_obligations(expr_ty);
827 debug!("coercion::can({:?} -> {:?})", source, target);
829 let cause = self.cause(rustc_span::DUMMY_SP, ObligationCauseCode::ExprAssignable);
830 // We don't ever need two-phase here since we throw out the result of the coercion
831 let coerce = Coerce::new(self, cause, AllowTwoPhase::No);
832 self.probe(|_| coerce.coerce(source, target)).is_ok()
835 /// Given some expressions, their known unified type and another expression,
836 /// tries to unify the types, potentially inserting coercions on any of the
837 /// provided expressions and returns their LUB (aka "common supertype").
839 /// This is really an internal helper. From outside the coercion
840 /// module, you should instantiate a `CoerceMany` instance.
841 fn try_find_coercion_lub<E>(
843 cause: &ObligationCause<'tcx>,
848 ) -> RelateResult<'tcx, Ty<'tcx>>
852 let prev_ty = self.resolve_vars_with_obligations(prev_ty);
853 let new_ty = self.resolve_vars_with_obligations(new_ty);
854 debug!("coercion::try_find_coercion_lub({:?}, {:?})", prev_ty, new_ty);
856 // Special-case that coercion alone cannot handle:
857 // Two function item types of differing IDs or InternalSubsts.
858 if let (&ty::FnDef(..), &ty::FnDef(..)) = (&prev_ty.kind, &new_ty.kind) {
859 // Don't reify if the function types have a LUB, i.e., they
860 // are the same function and their parameters have a LUB.
862 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
863 .map(|ok| self.register_infer_ok_obligations(ok));
866 // We have a LUB of prev_ty and new_ty, just return it.
870 // The signature must match.
871 let a_sig = prev_ty.fn_sig(self.tcx);
872 let a_sig = self.normalize_associated_types_in(new.span, &a_sig);
873 let b_sig = new_ty.fn_sig(self.tcx);
874 let b_sig = self.normalize_associated_types_in(new.span, &b_sig);
876 .at(cause, self.param_env)
877 .trace(prev_ty, new_ty)
879 .map(|ok| self.register_infer_ok_obligations(ok))?;
881 // Reify both sides and return the reified fn pointer type.
882 let fn_ptr = self.tcx.mk_fn_ptr(sig);
883 for expr in exprs.iter().map(|e| e.as_coercion_site()).chain(Some(new)) {
884 // The only adjustment that can produce an fn item is
885 // `NeverToAny`, so this should always be valid.
886 self.apply_adjustments(
889 kind: Adjust::Pointer(PointerCast::ReifyFnPointer),
897 // Configure a Coerce instance to compute the LUB.
898 // We don't allow two-phase borrows on any autorefs this creates since we
899 // probably aren't processing function arguments here and even if we were,
900 // they're going to get autorefed again anyway and we can apply 2-phase borrows
902 let mut coerce = Coerce::new(self, cause.clone(), AllowTwoPhase::No);
903 coerce.use_lub = true;
905 // First try to coerce the new expression to the type of the previous ones,
906 // but only if the new expression has no coercion already applied to it.
907 let mut first_error = None;
908 if !self.tables.borrow().adjustments().contains_key(new.hir_id) {
909 let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
912 let (adjustments, target) = self.register_infer_ok_obligations(ok);
913 self.apply_adjustments(new, adjustments);
916 Err(e) => first_error = Some(e),
920 // Then try to coerce the previous expressions to the type of the new one.
921 // This requires ensuring there are no coercions applied to *any* of the
922 // previous expressions, other than noop reborrows (ignoring lifetimes).
924 let expr = expr.as_coercion_site();
925 let noop = match self.tables.borrow().expr_adjustments(expr) {
926 &[Adjustment { kind: Adjust::Deref(_), .. }, Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. }] =>
928 match self.node_ty(expr.hir_id).kind {
929 ty::Ref(_, _, mt_orig) => {
930 let mutbl_adj: hir::Mutability = mutbl_adj.into();
931 // Reborrow that we can safely ignore, because
932 // the next adjustment can only be a Deref
933 // which will be merged into it.
939 &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
945 .commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
946 .map(|ok| self.register_infer_ok_obligations(ok));
950 match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
952 // Avoid giving strange errors on failed attempts.
953 if let Some(e) = first_error {
956 self.commit_if_ok(|_| self.at(cause, self.param_env).lub(prev_ty, new_ty))
957 .map(|ok| self.register_infer_ok_obligations(ok))
961 let (adjustments, target) = self.register_infer_ok_obligations(ok);
963 let expr = expr.as_coercion_site();
964 self.apply_adjustments(expr, adjustments.clone());
972 /// CoerceMany encapsulates the pattern you should use when you have
973 /// many expressions that are all getting coerced to a common
974 /// type. This arises, for example, when you have a match (the result
975 /// of each arm is coerced to a common type). It also arises in less
976 /// obvious places, such as when you have many `break foo` expressions
977 /// that target the same loop, or the various `return` expressions in
980 /// The basic protocol is as follows:
982 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
983 /// This will also serve as the "starting LUB". The expectation is
984 /// that this type is something which all of the expressions *must*
985 /// be coercible to. Use a fresh type variable if needed.
986 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
987 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
988 /// unit. This happens for example if you have a `break` with no expression,
989 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
990 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
991 /// from you so that you don't have to worry your pretty head about it.
992 /// But if an error is reported, the final type will be `err`.
993 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
994 /// previously coerced expressions.
995 /// - When all done, invoke `complete()`. This will return the LUB of
996 /// all your expressions.
997 /// - WARNING: I don't believe this final type is guaranteed to be
998 /// related to your initial `expected_ty` in any particular way,
999 /// although it will typically be a subtype, so you should check it.
1000 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
1001 /// previously coerced expressions.
1006 /// let mut coerce = CoerceMany::new(expected_ty);
1007 /// for expr in exprs {
1008 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
1009 /// coerce.coerce(fcx, &cause, expr, expr_ty);
1011 /// let final_ty = coerce.complete(fcx);
1013 pub struct CoerceMany<'tcx, 'exprs, E: AsCoercionSite> {
1014 expected_ty: Ty<'tcx>,
1015 final_ty: Option<Ty<'tcx>>,
1016 expressions: Expressions<'tcx, 'exprs, E>,
1020 /// The type of a `CoerceMany` that is storing up the expressions into
1021 /// a buffer. We use this in `check/mod.rs` for things like `break`.
1022 pub type DynamicCoerceMany<'tcx> = CoerceMany<'tcx, 'tcx, &'tcx hir::Expr<'tcx>>;
1024 enum Expressions<'tcx, 'exprs, E: AsCoercionSite> {
1025 Dynamic(Vec<&'tcx hir::Expr<'tcx>>),
1026 UpFront(&'exprs [E]),
1029 impl<'tcx, 'exprs, E: AsCoercionSite> CoerceMany<'tcx, 'exprs, E> {
1030 /// The usual case; collect the set of expressions dynamically.
1031 /// If the full set of coercion sites is known before hand,
1032 /// consider `with_coercion_sites()` instead to avoid allocation.
1033 pub fn new(expected_ty: Ty<'tcx>) -> Self {
1034 Self::make(expected_ty, Expressions::Dynamic(vec![]))
1037 /// As an optimization, you can create a `CoerceMany` with a
1038 /// pre-existing slice of expressions. In this case, you are
1039 /// expected to pass each element in the slice to `coerce(...)` in
1040 /// order. This is used with arrays in particular to avoid
1041 /// needlessly cloning the slice.
1042 pub fn with_coercion_sites(expected_ty: Ty<'tcx>, coercion_sites: &'exprs [E]) -> Self {
1043 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
1046 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'tcx, 'exprs, E>) -> Self {
1047 CoerceMany { expected_ty, final_ty: None, expressions, pushed: 0 }
1050 /// Returns the "expected type" with which this coercion was
1051 /// constructed. This represents the "downward propagated" type
1052 /// that was given to us at the start of typing whatever construct
1053 /// we are typing (e.g., the match expression).
1055 /// Typically, this is used as the expected type when
1056 /// type-checking each of the alternative expressions whose types
1057 /// we are trying to merge.
1058 pub fn expected_ty(&self) -> Ty<'tcx> {
1062 /// Returns the current "merged type", representing our best-guess
1063 /// at the LUB of the expressions we've seen so far (if any). This
1064 /// isn't *final* until you call `self.final()`, which will return
1065 /// the merged type.
1066 pub fn merged_ty(&self) -> Ty<'tcx> {
1067 self.final_ty.unwrap_or(self.expected_ty)
1070 /// Indicates that the value generated by `expression`, which is
1071 /// of type `expression_ty`, is one of the possibilities that we
1072 /// could coerce from. This will record `expression`, and later
1073 /// calls to `coerce` may come back and add adjustments and things
1077 fcx: &FnCtxt<'a, 'tcx>,
1078 cause: &ObligationCause<'tcx>,
1079 expression: &'tcx hir::Expr<'tcx>,
1080 expression_ty: Ty<'tcx>,
1082 self.coerce_inner(fcx, cause, Some(expression), expression_ty, None, false)
1085 /// Indicates that one of the inputs is a "forced unit". This
1086 /// occurs in a case like `if foo { ... };`, where the missing else
1087 /// generates a "forced unit". Another example is a `loop { break;
1088 /// }`, where the `break` has no argument expression. We treat
1089 /// these cases slightly differently for error-reporting
1090 /// purposes. Note that these tend to correspond to cases where
1091 /// the `()` expression is implicit in the source, and hence we do
1092 /// not take an expression argument.
1094 /// The `augment_error` gives you a chance to extend the error
1095 /// message, in case any results (e.g., we use this to suggest
1096 /// removing a `;`).
1097 pub fn coerce_forced_unit<'a>(
1099 fcx: &FnCtxt<'a, 'tcx>,
1100 cause: &ObligationCause<'tcx>,
1101 augment_error: &mut dyn FnMut(&mut DiagnosticBuilder<'_>),
1102 label_unit_as_expected: bool,
1109 Some(augment_error),
1110 label_unit_as_expected,
1114 /// The inner coercion "engine". If `expression` is `None`, this
1115 /// is a forced-unit case, and hence `expression_ty` must be
1117 fn coerce_inner<'a>(
1119 fcx: &FnCtxt<'a, 'tcx>,
1120 cause: &ObligationCause<'tcx>,
1121 expression: Option<&'tcx hir::Expr<'tcx>>,
1122 mut expression_ty: Ty<'tcx>,
1123 augment_error: Option<&mut dyn FnMut(&mut DiagnosticBuilder<'_>)>,
1124 label_expression_as_expected: bool,
1126 // Incorporate whatever type inference information we have
1127 // until now; in principle we might also want to process
1128 // pending obligations, but doing so should only improve
1129 // compatibility (hopefully that is true) by helping us
1130 // uncover never types better.
1131 if expression_ty.is_ty_var() {
1132 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1135 // If we see any error types, just propagate that error
1137 if expression_ty.references_error() || self.merged_ty().references_error() {
1138 self.final_ty = Some(fcx.tcx.types.err);
1142 // Handle the actual type unification etc.
1143 let result = if let Some(expression) = expression {
1144 if self.pushed == 0 {
1145 // Special-case the first expression we are coercing.
1146 // To be honest, I'm not entirely sure why we do this.
1147 // We don't allow two-phase borrows, see comment in try_find_coercion_lub for why
1148 fcx.try_coerce(expression, expression_ty, self.expected_ty, AllowTwoPhase::No)
1150 match self.expressions {
1151 Expressions::Dynamic(ref exprs) => fcx.try_find_coercion_lub(
1158 Expressions::UpFront(ref coercion_sites) => fcx.try_find_coercion_lub(
1160 &coercion_sites[0..self.pushed],
1168 // this is a hack for cases where we default to `()` because
1169 // the expression etc has been omitted from the source. An
1170 // example is an `if let` without an else:
1172 // if let Some(x) = ... { }
1174 // we wind up with a second match arm that is like `_ =>
1175 // ()`. That is the case we are considering here. We take
1176 // a different path to get the right "expected, found"
1177 // message and so forth (and because we know that
1178 // `expression_ty` will be unit).
1180 // Another example is `break` with no argument expression.
1181 assert!(expression_ty.is_unit(), "if let hack without unit type");
1182 fcx.at(cause, fcx.param_env)
1183 .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
1185 fcx.register_infer_ok_obligations(infer_ok);
1192 self.final_ty = Some(v);
1193 if let Some(e) = expression {
1194 match self.expressions {
1195 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1196 Expressions::UpFront(coercion_sites) => {
1197 // if the user gave us an array to validate, check that we got
1198 // the next expression in the list, as expected
1200 coercion_sites[self.pushed].as_coercion_site().hir_id,
1208 Err(coercion_error) => {
1209 let (expected, found) = if label_expression_as_expected {
1210 // In the case where this is a "forced unit", like
1211 // `break`, we want to call the `()` "expected"
1212 // since it is implied by the syntax.
1213 // (Note: not all force-units work this way.)"
1214 (expression_ty, self.final_ty.unwrap_or(self.expected_ty))
1216 // Otherwise, the "expected" type for error
1217 // reporting is the current unification type,
1218 // which is basically the LUB of the expressions
1219 // we've seen so far (combined with the expected
1221 (self.final_ty.unwrap_or(self.expected_ty), expression_ty)
1226 ObligationCauseCode::ReturnNoExpression => {
1227 err = struct_span_err!(
1231 "`return;` in a function whose return type is not `()`"
1233 err.span_label(cause.span, "return type is not `()`");
1235 ObligationCauseCode::BlockTailExpression(blk_id) => {
1236 let parent_id = fcx.tcx.hir().get_parent_node(blk_id);
1237 err = self.report_return_mismatched_types(
1244 expression.map(|expr| (expr, blk_id)),
1247 ObligationCauseCode::ReturnValue(id) => {
1248 err = self.report_return_mismatched_types(
1259 err = fcx.report_mismatched_types(cause, expected, found, coercion_error);
1263 if let Some(augment_error) = augment_error {
1264 augment_error(&mut err);
1267 if let Some(expr) = expression {
1268 fcx.emit_coerce_suggestions(&mut err, expr, found, expected);
1271 // Error possibly reported in `check_assign` so avoid emitting error again.
1272 let assign_to_bool = expression
1273 // #67273: Use initial expected type as opposed to `expected`.
1274 // Otherwise we end up using prior coercions in e.g. a `match` expression:
1277 // 0 => true, // Because of this...
1278 // 1 => i = 1, // ...`expected == bool` now, but not when checking `i = 1`.
1282 .filter(|e| fcx.is_assign_to_bool(e, self.expected_ty()))
1285 err.emit_unless(assign_to_bool);
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<'tcx>, 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(
1313 &mut err, expr, expected, found, cause.span, blk_id,
1315 let parent = fcx.tcx.hir().get(parent_id);
1316 if let (Some(match_expr), true, false) = (
1317 fcx.tcx.hir().get_match_if_cause(expr.hir_id),
1319 pointing_at_return_type,
1321 if match_expr.span.desugaring_kind().is_none() {
1322 err.span_label(match_expr.span, "expected this to be `()`");
1323 fcx.suggest_semicolon_at_end(match_expr.span, &mut err);
1326 fcx.get_node_fn_decl(parent).map(|(fn_decl, _, is_main)| (fn_decl, is_main))
1328 fcx.get_fn_decl(parent_id)
1331 if let (Some((fn_decl, can_suggest)), _) = (fn_decl, pointing_at_return_type) {
1332 if expression.is_none() {
1333 pointing_at_return_type |= fcx.suggest_missing_return_type(
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<'a, T> AsCoercionSite for &'a T
1383 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1384 (**self).as_coercion_site()
1388 impl AsCoercionSite for ! {
1389 fn as_coercion_site(&self) -> &hir::Expr<'_> {
1394 impl AsCoercionSite for hir::Arm<'_> {
1395 fn as_coercion_site(&self) -> &hir::Expr<'_> {