1 // Copyright 2012 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
13 //! Under certain circumstances we will coerce from one type to another,
14 //! for example by auto-borrowing. This occurs in situations where the
15 //! compiler has a firm 'expected type' that was supplied from the user,
16 //! and where the actual type is similar to that expected type in purpose
17 //! but not in representation (so actual subtyping is inappropriate).
21 //! Note that if we are expecting a reference, we will *reborrow*
22 //! even if the argument provided was already a reference. This is
23 //! useful for freezing mut/const things (that is, when the expected is &T
24 //! but you have &const T or &mut T) and also for avoiding the linearity
25 //! of mut things (when the expected is &mut T and you have &mut T). See
26 //! the various `src/test/run-pass/coerce-reborrow-*.rs` tests for
27 //! examples of where this is useful.
31 //! When deciding what type coercions to consider, we do not attempt to
32 //! resolve any type variables we may encounter. This is because `b`
33 //! represents the expected type "as the user wrote it", meaning that if
34 //! the user defined a generic function like
36 //! fn foo<A>(a: A, b: A) { ... }
38 //! and then we wrote `foo(&1, @2)`, we will not auto-borrow
39 //! either argument. In older code we went to some lengths to
40 //! resolve the `b` variable, which could mean that we'd
41 //! auto-borrow later arguments but not earlier ones, which
42 //! seems very confusing.
46 //! However, right now, if the user manually specifies the
47 //! values for the type variables, as so:
49 //! foo::<&int>(@1, @2)
51 //! then we *will* auto-borrow, because we can't distinguish this from a
52 //! function that declared `&int`. This is inconsistent but it's easiest
53 //! at the moment. The right thing to do, I think, is to consider the
54 //! *unsubstituted* type when deciding whether to auto-borrow, but the
55 //! *substituted* type when considering the bounds and so forth. But most
56 //! of our methods don't give access to the unsubstituted type, and
57 //! rightly so because they'd be error-prone. So maybe the thing to do is
58 //! to actually determine the kind of coercions that should occur
59 //! separately and pass them in. Or maybe it's ok as is. Anyway, it's
60 //! sort of a minor point so I've opted to leave it for later---after all
61 //! we may want to adjust precisely when coercions occur.
63 use check::{Diverges, FnCtxt, Needs};
66 use rustc::hir::def_id::DefId;
67 use rustc::infer::{Coercion, InferResult, InferOk};
68 use rustc::infer::type_variable::TypeVariableOrigin;
69 use rustc::traits::{self, ObligationCause, ObligationCauseCode};
70 use rustc::ty::adjustment::{Adjustment, Adjust, AutoBorrow, AutoBorrowMutability};
71 use rustc::ty::{self, TypeAndMut, Ty, ClosureSubsts};
72 use rustc::ty::fold::TypeFoldable;
73 use rustc::ty::error::TypeError;
74 use rustc::ty::relate::RelateResult;
75 use errors::DiagnosticBuilder;
76 use syntax::feature_gate;
80 use std::collections::VecDeque;
83 struct Coerce<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
84 fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
85 cause: ObligationCause<'tcx>,
89 impl<'a, 'gcx, 'tcx> Deref for Coerce<'a, 'gcx, 'tcx> {
90 type Target = FnCtxt<'a, 'gcx, '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, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
126 fn new(fcx: &'f FnCtxt<'f, 'gcx, 'tcx>, cause: ObligationCause<'tcx>) -> Self {
134 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
135 self.commit_if_ok(|_| {
137 self.at(&self.cause, self.fcx.param_env)
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)
149 -> CoerceResult<'tcx>
150 where F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>
152 self.unify(&a, &b).and_then(|InferOk { value: ty, obligations }| {
153 success(f(ty), ty, obligations)
157 fn coerce(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> CoerceResult<'tcx> {
158 let a = self.shallow_resolve(a);
159 debug!("Coerce.tys({:?} => {:?})", a, b);
161 // Just ignore error types.
162 if a.references_error() || b.references_error() {
163 return success(vec![], b, vec![]);
167 // Subtle: If we are coercing from `!` to `?T`, where `?T` is an unbound
168 // type variable, we want `?T` to fallback to `!` if not
169 // otherwise constrained. An example where this arises:
171 // let _: Option<?T> = Some({ return; });
173 // here, we would coerce from `!` to `?T`.
174 let b = self.shallow_resolve(b);
175 return if self.shallow_resolve(b).is_ty_var() {
176 // micro-optimization: no need for this if `b` is
177 // already resolved in some way.
178 let diverging_ty = self.next_diverging_ty_var(
179 TypeVariableOrigin::AdjustmentType(self.cause.span));
180 self.unify_and(&b, &diverging_ty, simple(Adjust::NeverToAny))
182 success(simple(Adjust::NeverToAny)(b), b, vec![])
186 // Consider coercing the subtype to a DST
188 // NOTE: this is wrapped in a `commit_if_ok` because it creates
189 // a "spurious" type variable, and we don't want to have that
190 // type variable in memory if the coercion fails.
191 let unsize = self.commit_if_ok(|_| self.coerce_unsized(a, b));
193 debug!("coerce: unsize successful");
196 debug!("coerce: unsize failed");
198 // Examine the supertype and consider auto-borrowing.
200 // Note: does not attempt to resolve type variables we encounter.
201 // See above for details.
203 ty::TyRawPtr(mt_b) => {
204 return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
207 ty::TyRef(r_b, mt_b) => {
208 return self.coerce_borrowed_pointer(a, b, r_b, mt_b);
216 // Function items are coercible to any closure
217 // type; function pointers are not (that would
218 // require double indirection).
219 // Additionally, we permit coercion of function
220 // items to drop the unsafe qualifier.
221 self.coerce_from_fn_item(a, b)
223 ty::TyFnPtr(a_f) => {
224 // We permit coercion of fn pointers to drop the
226 self.coerce_from_fn_pointer(a, a_f, b)
228 ty::TyClosure(def_id_a, substs_a) => {
229 // Non-capturing closures are coercible to
231 self.coerce_closure_to_fn(a, def_id_a, substs_a, b)
234 // Otherwise, just use unification rules.
235 self.unify_and(a, b, identity)
240 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
241 /// To match `A` with `B`, autoderef will be performed,
242 /// calling `deref`/`deref_mut` where necessary.
243 fn coerce_borrowed_pointer(&self,
246 r_b: ty::Region<'tcx>,
247 mt_b: TypeAndMut<'tcx>)
248 -> CoerceResult<'tcx> {
250 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
252 // If we have a parameter of type `&M T_a` and the value
253 // provided is `expr`, we will be adding an implicit borrow,
254 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
255 // to type check, we will construct the type that `&M*expr` would
258 let (r_a, mt_a) = match a.sty {
259 ty::TyRef(r_a, mt_a) => {
260 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
263 _ => return self.unify_and(a, b, identity),
266 let span = self.cause.span;
268 let mut first_error = None;
269 let mut r_borrow_var = None;
270 let mut autoderef = self.autoderef(span, a);
271 let mut found = None;
273 for (referent_ty, autoderefs) in autoderef.by_ref() {
275 // Don't let this pass, otherwise it would cause
276 // &T to autoref to &&T.
280 // At this point, we have deref'd `a` to `referent_ty`. So
281 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
282 // In the autoderef loop for `&'a mut Vec<T>`, we would get
285 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
286 // - `Vec<T>` -- 1 deref
287 // - `[T]` -- 2 deref
289 // At each point after the first callback, we want to
290 // check to see whether this would match out target type
291 // (`&'b mut [T]`) if we autoref'd it. We can't just
292 // compare the referent types, though, because we still
293 // have to consider the mutability. E.g., in the case
294 // we've been considering, we have an `&mut` reference, so
295 // the `T` in `[T]` needs to be unified with equality.
297 // Therefore, we construct reference types reflecting what
298 // the types will be after we do the final auto-ref and
299 // compare those. Note that this means we use the target
300 // mutability [1], since it may be that we are coercing
301 // from `&mut T` to `&U`.
303 // One fine point concerns the region that we use. We
304 // choose the region such that the region of the final
305 // type that results from `unify` will be the region we
306 // want for the autoref:
308 // - if in sub mode, that means we want to use `'b` (the
309 // region from the target reference) for both
310 // pointers [2]. This is because sub mode (somewhat
311 // arbitrarily) returns the subtype region. In the case
312 // where we are coercing to a target type, we know we
313 // want to use that target type region (`'b`) because --
314 // for the program to type-check -- it must be the
315 // smaller of the two.
316 // - One fine point. It may be surprising that we can
317 // use `'b` without relating `'a` and `'b`. The reason
318 // that this is ok is that what we produce is
319 // effectively a `&'b *x` expression (if you could
320 // annotate the region of a borrow), and regionck has
321 // code that adds edges from the region of a borrow
322 // (`'b`, here) into the regions in the borrowed
323 // expression (`*x`, here). (Search for "link".)
324 // - if in lub mode, things can get fairly complicated. The
325 // easiest thing is just to make a fresh
326 // region variable [4], which effectively means we defer
327 // the decision to region inference (and regionck, which will add
328 // some more edges to this variable). However, this can wind up
329 // creating a crippling number of variables in some cases --
330 // e.g. #32278 -- so we optimize one particular case [3].
331 // Let me try to explain with some examples:
332 // - The "running example" above represents the simple case,
333 // where we have one `&` reference at the outer level and
334 // ownership all the rest of the way down. In this case,
335 // we want `LUB('a, 'b)` as the resulting region.
336 // - However, if there are nested borrows, that region is
337 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
338 // `&'b T`. In this case, `'a` is actually irrelevant.
339 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
340 // we get spurious errors (`run-pass/regions-lub-ref-ref-rc.rs`).
341 // (The errors actually show up in borrowck, typically, because
342 // this extra edge causes the region `'a` to be inferred to something
343 // too big, which then results in borrowck errors.)
344 // - We could track the innermost shared reference, but there is already
345 // code in regionck that has the job of creating links between
346 // the region of a borrow and the regions in the thing being
347 // borrowed (here, `'a` and `'x`), and it knows how to handle
348 // all the various cases. So instead we just make a region variable
349 // and let regionck figure it out.
350 let r = if !self.use_lub {
352 } else if autoderefs == 1 {
355 if r_borrow_var.is_none() {
356 // create var lazilly, at most once
357 let coercion = Coercion(span);
358 let r = self.next_region_var(coercion);
359 r_borrow_var = Some(r); // [4] above
361 r_borrow_var.unwrap()
363 let derefd_ty_a = self.tcx.mk_ref(r,
366 mutbl: mt_b.mutbl, // [1] above
368 match self.unify(derefd_ty_a, b) {
374 if first_error.is_none() {
375 first_error = Some(err);
381 // Extract type or return an error. We return the first error
382 // we got, which should be from relating the "base" type
383 // (e.g., in example above, the failure from relating `Vec<T>`
384 // to the target type), since that should be the least
386 let InferOk { value: ty, mut obligations } = match found {
389 let err = first_error.expect("coerce_borrowed_pointer had no error");
390 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
395 if ty == a && mt_a.mutbl == hir::MutImmutable && autoderef.step_count() == 1 {
396 // As a special case, if we would produce `&'a *x`, that's
397 // a total no-op. We end up with the type `&'a T` just as
398 // we started with. In that case, just skip it
399 // altogether. This is just an optimization.
401 // Note that for `&mut`, we DO want to reborrow --
402 // otherwise, this would be a move, which might be an
403 // error. For example `foo(self.x)` where `self` and
404 // `self.x` both have `&mut `type would be a move of
405 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
406 // which is a borrow.
407 assert_eq!(mt_b.mutbl, hir::MutImmutable); // can only coerce &T -> &U
408 return success(vec![], ty, obligations);
411 let needs = Needs::maybe_mut_place(mt_b.mutbl);
412 let InferOk { value: mut adjustments, obligations: o }
413 = autoderef.adjust_steps_as_infer_ok(needs);
414 obligations.extend(o);
415 obligations.extend(autoderef.into_obligations());
417 // Now apply the autoref. We have to extract the region out of
418 // the final ref type we got.
419 let r_borrow = match ty.sty {
420 ty::TyRef(r_borrow, _) => r_borrow,
421 _ => span_bug!(span, "expected a ref type, got {:?}", ty),
423 let mutbl = match mt_b.mutbl {
424 hir::MutImmutable => AutoBorrowMutability::Immutable,
425 hir::MutMutable => AutoBorrowMutability::Mutable {
426 // Deref-coercion is a case where we deliberately
427 // disallow two-phase borrows in its initial
428 // deployment; see discussion on PR #47489.
429 allow_two_phase_borrow: false,
432 adjustments.push(Adjustment {
433 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
437 debug!("coerce_borrowed_pointer: succeeded ty={:?} adjustments={:?}",
441 success(adjustments, ty, obligations)
445 // &[T; n] or &mut [T; n] -> &[T]
446 // or &mut [T; n] -> &mut [T]
447 // or &Concrete -> &Trait, etc.
448 fn coerce_unsized(&self, source: Ty<'tcx>, target: Ty<'tcx>) -> CoerceResult<'tcx> {
449 debug!("coerce_unsized(source={:?}, target={:?})", source, target);
451 let traits = (self.tcx.lang_items().unsize_trait(),
452 self.tcx.lang_items().coerce_unsized_trait());
453 let (unsize_did, coerce_unsized_did) = if let (Some(u), Some(cu)) = traits {
456 debug!("Missing Unsize or CoerceUnsized traits");
457 return Err(TypeError::Mismatch);
460 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
461 // a DST unless we have to. This currently comes out in the wash since
462 // we can't unify [T] with U. But to properly support DST, we need to allow
463 // that, at which point we will need extra checks on the target here.
465 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
466 let reborrow = match (&source.sty, &target.sty) {
467 (&ty::TyRef(_, mt_a), &ty::TyRef(_, mt_b)) => {
468 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
470 let coercion = Coercion(self.cause.span);
471 let r_borrow = self.next_region_var(coercion);
472 let mutbl = match mt_b.mutbl {
473 hir::MutImmutable => AutoBorrowMutability::Immutable,
474 hir::MutMutable => AutoBorrowMutability::Mutable {
475 allow_two_phase_borrow: false,
479 kind: Adjust::Deref(None),
482 kind: Adjust::Borrow(AutoBorrow::Ref(r_borrow, mutbl)),
483 target: self.tcx.mk_ref(r_borrow, ty::TypeAndMut {
489 (&ty::TyRef(_, mt_a), &ty::TyRawPtr(mt_b)) => {
490 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
493 kind: Adjust::Deref(None),
496 kind: Adjust::Borrow(AutoBorrow::RawPtr(mt_b.mutbl)),
497 target: self.tcx.mk_ptr(ty::TypeAndMut {
505 let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
507 // Setup either a subtyping or a LUB relationship between
508 // the `CoerceUnsized` target type and the expected type.
509 // We only have the latter, so we use an inference variable
510 // for the former and let type inference do the rest.
511 let origin = TypeVariableOrigin::MiscVariable(self.cause.span);
512 let coerce_target = self.next_ty_var(origin);
513 let mut coercion = self.unify_and(coerce_target, target, |target| {
514 let unsize = Adjustment {
515 kind: Adjust::Unsize,
519 None => vec![unsize],
520 Some((ref deref, ref autoref)) => {
521 vec![deref.clone(), autoref.clone(), unsize]
526 let mut selcx = traits::SelectionContext::new(self);
528 // Use a FIFO queue for this custom fulfillment procedure.
529 let mut queue = VecDeque::new();
531 // Create an obligation for `Source: CoerceUnsized<Target>`.
532 let cause = ObligationCause::misc(self.cause.span, self.body_id);
533 queue.push_back(self.tcx.predicate_for_trait_def(self.fcx.param_env,
540 let mut has_unsized_tuple_coercion = false;
542 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
543 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
544 // inference might unify those two inner type variables later.
545 let traits = [coerce_unsized_did, unsize_did];
546 while let Some(obligation) = queue.pop_front() {
547 debug!("coerce_unsized resolve step: {:?}", obligation);
548 let trait_ref = match obligation.predicate {
549 ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => {
550 if unsize_did == tr.def_id() {
551 if let ty::TyTuple(..) = tr.0.input_types().nth(1).unwrap().sty {
552 debug!("coerce_unsized: found unsized tuple coercion");
553 has_unsized_tuple_coercion = true;
559 coercion.obligations.push(obligation);
563 match selcx.select(&obligation.with(trait_ref)) {
564 // Uncertain or unimplemented.
566 Err(traits::Unimplemented) => {
567 debug!("coerce_unsized: early return - can't prove obligation");
568 return Err(TypeError::Mismatch);
571 // Object safety violations or miscellaneous.
573 self.report_selection_error(&obligation, &err, false);
574 // Treat this like an obligation and follow through
575 // with the unsizing - the lack of a coercion should
576 // be silent, as it causes a type mismatch later.
579 Ok(Some(vtable)) => {
580 for obligation in vtable.nested_obligations() {
581 queue.push_back(obligation);
587 if has_unsized_tuple_coercion && !self.tcx.features().unsized_tuple_coercion {
588 feature_gate::emit_feature_err(&self.tcx.sess.parse_sess,
589 "unsized_tuple_coercion",
591 feature_gate::GateIssue::Language,
592 feature_gate::EXPLAIN_UNSIZED_TUPLE_COERCION);
598 fn coerce_from_safe_fn<F, G>(&self,
600 fn_ty_a: ty::PolyFnSig<'tcx>,
604 -> CoerceResult<'tcx>
605 where F: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>,
606 G: FnOnce(Ty<'tcx>) -> Vec<Adjustment<'tcx>>
608 if let ty::TyFnPtr(fn_ty_b) = b.sty {
609 match (fn_ty_a.unsafety(), fn_ty_b.unsafety()) {
610 (hir::Unsafety::Normal, hir::Unsafety::Unsafe) => {
611 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
612 return self.unify_and(unsafe_a, b, to_unsafe);
617 self.unify_and(a, b, normal)
620 fn coerce_from_fn_pointer(&self,
622 fn_ty_a: ty::PolyFnSig<'tcx>,
624 -> CoerceResult<'tcx> {
625 //! Attempts to coerce from the type of a Rust function item
626 //! into a closure or a `proc`.
629 let b = self.shallow_resolve(b);
630 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
632 self.coerce_from_safe_fn(a, fn_ty_a, b,
633 simple(Adjust::UnsafeFnPointer), identity)
636 fn coerce_from_fn_item(&self,
639 -> CoerceResult<'tcx> {
640 //! Attempts to coerce from the type of a Rust function item
641 //! into a closure or a `proc`.
644 let b = self.shallow_resolve(b);
645 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
649 let a_sig = a.fn_sig(self.tcx);
650 let InferOk { value: a_sig, mut obligations } =
651 self.normalize_associated_types_in_as_infer_ok(self.cause.span, &a_sig);
653 let a_fn_pointer = self.tcx.mk_fn_ptr(a_sig);
654 let InferOk { value, obligations: o2 } = self.coerce_from_safe_fn(
660 Adjustment { kind: Adjust::ReifyFnPointer, target: a_fn_pointer },
661 Adjustment { kind: Adjust::UnsafeFnPointer, target: unsafe_ty },
664 simple(Adjust::ReifyFnPointer)
667 obligations.extend(o2);
668 Ok(InferOk { value, obligations })
670 _ => self.unify_and(a, b, identity),
674 fn coerce_closure_to_fn(&self,
677 substs_a: ClosureSubsts<'tcx>,
679 -> CoerceResult<'tcx> {
680 //! Attempts to coerce from the type of a non-capturing closure
681 //! into a function pointer.
684 let b = self.shallow_resolve(b);
686 let node_id_a = self.tcx.hir.as_local_node_id(def_id_a).unwrap();
688 ty::TyFnPtr(_) if self.tcx.with_freevars(node_id_a, |v| v.is_empty()) => {
689 // We coerce the closure, which has fn type
690 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
692 // `fn(arg0,arg1,...) -> _`
693 let sig = self.closure_sig(def_id_a, substs_a);
694 let pointer_ty = self.tcx.coerce_closure_fn_ty(sig);
695 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})",
697 self.unify_and(pointer_ty, b, simple(Adjust::ClosureFnPointer))
699 _ => self.unify_and(a, b, identity),
703 fn coerce_unsafe_ptr(&self,
706 mutbl_b: hir::Mutability)
707 -> CoerceResult<'tcx> {
708 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
710 let (is_ref, mt_a) = match a.sty {
711 ty::TyRef(_, mt) => (true, mt),
712 ty::TyRawPtr(mt) => (false, mt),
714 return self.unify_and(a, b, identity);
718 // Check that the types which they point at are compatible.
719 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut {
723 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
724 // Although references and unsafe ptrs have the same
725 // representation, we still register an Adjust::DerefRef so that
726 // regionck knows that the region for `a` must be valid here.
728 self.unify_and(a_unsafe, b, |target| {
730 kind: Adjust::Deref(None),
733 kind: Adjust::Borrow(AutoBorrow::RawPtr(mutbl_b)),
737 } else if mt_a.mutbl != mutbl_b {
738 self.unify_and(a_unsafe, b, simple(Adjust::MutToConstPointer))
740 self.unify_and(a_unsafe, b, identity)
745 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
746 /// Attempt to coerce an expression to a type, and return the
747 /// adjusted type of the expression, if successful.
748 /// Adjustments are only recorded if the coercion succeeded.
749 /// The expressions *must not* have any pre-existing adjustments.
750 pub fn try_coerce(&self,
754 -> RelateResult<'tcx, Ty<'tcx>> {
755 let source = self.resolve_type_vars_with_obligations(expr_ty);
756 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
758 let cause = self.cause(expr.span, ObligationCauseCode::ExprAssignable);
759 let coerce = Coerce::new(self, cause);
760 let ok = self.commit_if_ok(|_| coerce.coerce(source, target))?;
762 let (adjustments, _) = self.register_infer_ok_obligations(ok);
763 self.apply_adjustments(expr, adjustments);
767 /// Same as `try_coerce()`, but without side-effects.
768 pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
769 let source = self.resolve_type_vars_with_obligations(expr_ty);
770 debug!("coercion::can({:?} -> {:?})", source, target);
772 let cause = self.cause(syntax_pos::DUMMY_SP, ObligationCauseCode::ExprAssignable);
773 let coerce = Coerce::new(self, cause);
774 self.probe(|_| coerce.coerce(source, target)).is_ok()
777 /// Given some expressions, their known unified type and another expression,
778 /// tries to unify the types, potentially inserting coercions on any of the
779 /// provided expressions and returns their LUB (aka "common supertype").
781 /// This is really an internal helper. From outside the coercion
782 /// module, you should instantiate a `CoerceMany` instance.
783 fn try_find_coercion_lub<E>(&self,
784 cause: &ObligationCause<'tcx>,
789 new_diverges: Diverges)
790 -> RelateResult<'tcx, Ty<'tcx>>
791 where E: AsCoercionSite
793 let prev_ty = self.resolve_type_vars_with_obligations(prev_ty);
794 let new_ty = self.resolve_type_vars_with_obligations(new_ty);
795 debug!("coercion::try_find_coercion_lub({:?}, {:?})", prev_ty, new_ty);
797 // Special-ish case: we can coerce any type `T` into the `!`
798 // type, but only if the source expression diverges.
799 if prev_ty.is_never() && new_diverges.always() {
800 debug!("permit coercion to `!` because expr diverges");
804 // Special-case that coercion alone cannot handle:
805 // Two function item types of differing IDs or Substs.
806 if let (&ty::TyFnDef(..), &ty::TyFnDef(..)) = (&prev_ty.sty, &new_ty.sty) {
807 // Don't reify if the function types have a LUB, i.e. they
808 // are the same function and their parameters have a LUB.
809 let lub_ty = self.commit_if_ok(|_| {
810 self.at(cause, self.param_env)
811 .lub(prev_ty, new_ty)
812 }).map(|ok| self.register_infer_ok_obligations(ok));
815 // We have a LUB of prev_ty and new_ty, just return it.
819 // The signature must match.
820 let a_sig = prev_ty.fn_sig(self.tcx);
821 let a_sig = self.normalize_associated_types_in(new.span, &a_sig);
822 let b_sig = new_ty.fn_sig(self.tcx);
823 let b_sig = self.normalize_associated_types_in(new.span, &b_sig);
824 let sig = self.at(cause, self.param_env)
825 .trace(prev_ty, new_ty)
827 .map(|ok| self.register_infer_ok_obligations(ok))?;
829 // Reify both sides and return the reified fn pointer type.
830 let fn_ptr = self.tcx.mk_fn_ptr(sig);
831 for expr in exprs.iter().map(|e| e.as_coercion_site()).chain(Some(new)) {
832 // The only adjustment that can produce an fn item is
833 // `NeverToAny`, so this should always be valid.
834 self.apply_adjustments(expr, vec![Adjustment {
835 kind: Adjust::ReifyFnPointer,
842 let mut coerce = Coerce::new(self, cause.clone());
843 coerce.use_lub = true;
845 // First try to coerce the new expression to the type of the previous ones,
846 // but only if the new expression has no coercion already applied to it.
847 let mut first_error = None;
848 if !self.tables.borrow().adjustments().contains_key(new.hir_id) {
849 let result = self.commit_if_ok(|_| coerce.coerce(new_ty, prev_ty));
852 let (adjustments, target) = self.register_infer_ok_obligations(ok);
853 self.apply_adjustments(new, adjustments);
856 Err(e) => first_error = Some(e),
860 // Then try to coerce the previous expressions to the type of the new one.
861 // This requires ensuring there are no coercions applied to *any* of the
862 // previous expressions, other than noop reborrows (ignoring lifetimes).
864 let expr = expr.as_coercion_site();
865 let noop = match self.tables.borrow().expr_adjustments(expr) {
867 Adjustment { kind: Adjust::Deref(_), .. },
868 Adjustment { kind: Adjust::Borrow(AutoBorrow::Ref(_, mutbl_adj)), .. }
870 match self.node_ty(expr.hir_id).sty {
871 ty::TyRef(_, mt_orig) => {
872 let mutbl_adj: hir::Mutability = mutbl_adj.into();
873 // Reborrow that we can safely ignore, because
874 // the next adjustment can only be a Deref
875 // which will be merged into it.
876 mutbl_adj == mt_orig.mutbl
881 &[Adjustment { kind: Adjust::NeverToAny, .. }] | &[] => true,
886 return self.commit_if_ok(|_| {
887 self.at(cause, self.param_env)
888 .lub(prev_ty, new_ty)
889 }).map(|ok| self.register_infer_ok_obligations(ok));
893 match self.commit_if_ok(|_| coerce.coerce(prev_ty, new_ty)) {
895 // Avoid giving strange errors on failed attempts.
896 if let Some(e) = first_error {
899 self.commit_if_ok(|_| {
900 self.at(cause, self.param_env)
901 .lub(prev_ty, new_ty)
902 }).map(|ok| self.register_infer_ok_obligations(ok))
906 let (adjustments, target) = self.register_infer_ok_obligations(ok);
908 let expr = expr.as_coercion_site();
909 self.apply_adjustments(expr, adjustments.clone());
917 /// CoerceMany encapsulates the pattern you should use when you have
918 /// many expressions that are all getting coerced to a common
919 /// type. This arises, for example, when you have a match (the result
920 /// of each arm is coerced to a common type). It also arises in less
921 /// obvious places, such as when you have many `break foo` expressions
922 /// that target the same loop, or the various `return` expressions in
925 /// The basic protocol is as follows:
927 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
928 /// This will also serve as the "starting LUB". The expectation is
929 /// that this type is something which all of the expressions *must*
930 /// be coercible to. Use a fresh type variable if needed.
931 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
932 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
933 /// unit. This happens for example if you have a `break` with no expression,
934 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
935 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
936 /// from you so that you don't have to worry your pretty head about it.
937 /// But if an error is reported, the final type will be `err`.
938 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
939 /// previously coerced expressions.
940 /// - When all done, invoke `complete()`. This will return the LUB of
941 /// all your expressions.
942 /// - WARNING: I don't believe this final type is guaranteed to be
943 /// related to your initial `expected_ty` in any particular way,
944 /// although it will typically be a subtype, so you should check it.
945 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
946 /// previously coerced expressions.
951 /// let mut coerce = CoerceMany::new(expected_ty);
952 /// for expr in exprs {
953 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
954 /// coerce.coerce(fcx, &cause, expr, expr_ty);
956 /// let final_ty = coerce.complete(fcx);
958 pub struct CoerceMany<'gcx, 'tcx, 'exprs, E>
959 where 'gcx: 'tcx, E: 'exprs + AsCoercionSite,
961 expected_ty: Ty<'tcx>,
962 final_ty: Option<Ty<'tcx>>,
963 expressions: Expressions<'gcx, 'exprs, E>,
967 /// The type of a `CoerceMany` that is storing up the expressions into
968 /// a buffer. We use this in `check/mod.rs` for things like `break`.
969 pub type DynamicCoerceMany<'gcx, 'tcx> = CoerceMany<'gcx, 'tcx, 'gcx, P<hir::Expr>>;
971 enum Expressions<'gcx, 'exprs, E>
972 where E: 'exprs + AsCoercionSite,
974 Dynamic(Vec<&'gcx hir::Expr>),
975 UpFront(&'exprs [E]),
978 impl<'gcx, 'tcx, 'exprs, E> CoerceMany<'gcx, 'tcx, 'exprs, E>
979 where 'gcx: 'tcx, E: 'exprs + AsCoercionSite,
981 /// The usual case; collect the set of expressions dynamically.
982 /// If the full set of coercion sites is known before hand,
983 /// consider `with_coercion_sites()` instead to avoid allocation.
984 pub fn new(expected_ty: Ty<'tcx>) -> Self {
985 Self::make(expected_ty, Expressions::Dynamic(vec![]))
988 /// As an optimization, you can create a `CoerceMany` with a
989 /// pre-existing slice of expressions. In this case, you are
990 /// expected to pass each element in the slice to `coerce(...)` in
991 /// order. This is used with arrays in particular to avoid
992 /// needlessly cloning the slice.
993 pub fn with_coercion_sites(expected_ty: Ty<'tcx>,
994 coercion_sites: &'exprs [E])
996 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
999 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'gcx, 'exprs, E>) -> Self {
1008 /// Return the "expected type" with which this coercion was
1009 /// constructed. This represents the "downward propagated" type
1010 /// that was given to us at the start of typing whatever construct
1011 /// we are typing (e.g., the match expression).
1013 /// Typically, this is used as the expected type when
1014 /// type-checking each of the alternative expressions whose types
1015 /// we are trying to merge.
1016 pub fn expected_ty(&self) -> Ty<'tcx> {
1020 /// Returns the current "merged type", representing our best-guess
1021 /// at the LUB of the expressions we've seen so far (if any). This
1022 /// isn't *final* until you call `self.final()`, which will return
1023 /// the merged type.
1024 pub fn merged_ty(&self) -> Ty<'tcx> {
1025 self.final_ty.unwrap_or(self.expected_ty)
1028 /// Indicates that the value generated by `expression`, which is
1029 /// of type `expression_ty`, is one of the possibility that we
1030 /// could coerce from. This will record `expression` and later
1031 /// calls to `coerce` may come back and add adjustments and things
1033 pub fn coerce<'a>(&mut self,
1034 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1035 cause: &ObligationCause<'tcx>,
1036 expression: &'gcx hir::Expr,
1037 expression_ty: Ty<'tcx>,
1038 expression_diverges: Diverges)
1040 self.coerce_inner(fcx,
1044 expression_diverges,
1048 /// Indicates that one of the inputs is a "forced unit". This
1049 /// occurs in a case like `if foo { ... };`, where the missing else
1050 /// generates a "forced unit". Another example is a `loop { break;
1051 /// }`, where the `break` has no argument expression. We treat
1052 /// these cases slightly differently for error-reporting
1053 /// purposes. Note that these tend to correspond to cases where
1054 /// the `()` expression is implicit in the source, and hence we do
1055 /// not take an expression argument.
1057 /// The `augment_error` gives you a chance to extend the error
1058 /// message, in case any results (e.g., we use this to suggest
1059 /// removing a `;`).
1060 pub fn coerce_forced_unit<'a>(&mut self,
1061 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1062 cause: &ObligationCause<'tcx>,
1063 augment_error: &mut FnMut(&mut DiagnosticBuilder),
1064 label_unit_as_expected: bool)
1066 self.coerce_inner(fcx,
1071 Some(augment_error),
1072 label_unit_as_expected)
1075 /// The inner coercion "engine". If `expression` is `None`, this
1076 /// is a forced-unit case, and hence `expression_ty` must be
1078 fn coerce_inner<'a>(&mut self,
1079 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1080 cause: &ObligationCause<'tcx>,
1081 expression: Option<&'gcx hir::Expr>,
1082 mut expression_ty: Ty<'tcx>,
1083 expression_diverges: Diverges,
1084 augment_error: Option<&mut FnMut(&mut DiagnosticBuilder)>,
1085 label_expression_as_expected: bool)
1087 // Incorporate whatever type inference information we have
1088 // until now; in principle we might also want to process
1089 // pending obligations, but doing so should only improve
1090 // compatibility (hopefully that is true) by helping us
1091 // uncover never types better.
1092 if expression_ty.is_ty_var() {
1093 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1096 // If we see any error types, just propagate that error
1098 if expression_ty.references_error() || self.merged_ty().references_error() {
1099 self.final_ty = Some(fcx.tcx.types.err);
1103 // Handle the actual type unification etc.
1104 let result = if let Some(expression) = expression {
1105 if self.pushed == 0 {
1106 // Special-case the first expression we are coercing.
1107 // To be honest, I'm not entirely sure why we do this.
1108 fcx.try_coerce(expression, expression_ty, self.expected_ty)
1110 match self.expressions {
1111 Expressions::Dynamic(ref exprs) =>
1112 fcx.try_find_coercion_lub(cause,
1117 expression_diverges),
1118 Expressions::UpFront(ref coercion_sites) =>
1119 fcx.try_find_coercion_lub(cause,
1120 &coercion_sites[0..self.pushed],
1124 expression_diverges),
1128 // this is a hack for cases where we default to `()` because
1129 // the expression etc has been omitted from the source. An
1130 // example is an `if let` without an else:
1132 // if let Some(x) = ... { }
1134 // we wind up with a second match arm that is like `_ =>
1135 // ()`. That is the case we are considering here. We take
1136 // a different path to get the right "expected, found"
1137 // message and so forth (and because we know that
1138 // `expression_ty` will be unit).
1140 // Another example is `break` with no argument expression.
1141 assert!(expression_ty.is_nil());
1142 assert!(expression_ty.is_nil(), "if let hack without unit type");
1143 fcx.at(cause, fcx.param_env)
1144 .eq_exp(label_expression_as_expected, expression_ty, self.merged_ty())
1146 fcx.register_infer_ok_obligations(infer_ok);
1153 self.final_ty = Some(v);
1154 if let Some(e) = expression {
1155 match self.expressions {
1156 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1157 Expressions::UpFront(coercion_sites) => {
1158 // if the user gave us an array to validate, check that we got
1159 // the next expression in the list, as expected
1160 assert_eq!(coercion_sites[self.pushed].as_coercion_site().id, e.id);
1167 let (expected, found) = if label_expression_as_expected {
1168 // In the case where this is a "forced unit", like
1169 // `break`, we want to call the `()` "expected"
1170 // since it is implied by the syntax.
1171 // (Note: not all force-units work this way.)"
1172 (expression_ty, self.final_ty.unwrap_or(self.expected_ty))
1174 // Otherwise, the "expected" type for error
1175 // reporting is the current unification type,
1176 // which is basically the LUB of the expressions
1177 // we've seen so far (combined with the expected
1179 (self.final_ty.unwrap_or(self.expected_ty), expression_ty)
1184 ObligationCauseCode::ReturnNoExpression => {
1185 db = struct_span_err!(
1186 fcx.tcx.sess, cause.span, E0069,
1187 "`return;` in a function whose return type is not `()`");
1188 db.span_label(cause.span, "return type is not ()");
1190 ObligationCauseCode::BlockTailExpression(blk_id) => {
1191 db = fcx.report_mismatched_types(cause, expected, found, err);
1193 let expr = expression.unwrap_or_else(|| {
1194 span_bug!(cause.span,
1195 "supposed to be part of a block tail expression, but the \
1196 expression is empty");
1198 fcx.suggest_mismatched_types_on_tail(&mut db, expr,
1200 cause.span, blk_id);
1203 db = fcx.report_mismatched_types(cause, expected, found, err);
1207 if let Some(augment_error) = augment_error {
1208 augment_error(&mut db);
1213 self.final_ty = Some(fcx.tcx.types.err);
1218 pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) -> Ty<'tcx> {
1219 if let Some(final_ty) = self.final_ty {
1222 // If we only had inputs that were of type `!` (or no
1223 // inputs at all), then the final type is `!`.
1224 assert_eq!(self.pushed, 0);
1230 /// Something that can be converted into an expression to which we can
1231 /// apply a coercion.
1232 pub trait AsCoercionSite {
1233 fn as_coercion_site(&self) -> &hir::Expr;
1236 impl AsCoercionSite for hir::Expr {
1237 fn as_coercion_site(&self) -> &hir::Expr {
1242 impl AsCoercionSite for P<hir::Expr> {
1243 fn as_coercion_site(&self) -> &hir::Expr {
1248 impl<'a, T> AsCoercionSite for &'a T
1249 where T: AsCoercionSite
1251 fn as_coercion_site(&self) -> &hir::Expr {
1252 (**self).as_coercion_site()
1256 impl AsCoercionSite for ! {
1257 fn as_coercion_site(&self) -> &hir::Expr {
1262 impl AsCoercionSite for hir::Arm {
1263 fn as_coercion_site(&self) -> &hir::Expr {