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};
66 use rustc::hir::def_id::DefId;
67 use rustc::infer::{Coercion, InferResult, InferOk, TypeTrace};
68 use rustc::infer::type_variable::TypeVariableOrigin;
69 use rustc::traits::{self, ObligationCause, ObligationCauseCode};
70 use rustc::ty::adjustment::{Adjustment, Adjust, AutoBorrow};
71 use rustc::ty::{self, LvaluePreference, TypeAndMut,
73 use rustc::ty::fold::TypeFoldable;
74 use rustc::ty::error::TypeError;
75 use rustc::ty::relate::RelateResult;
76 use rustc::ty::subst::Subst;
77 use errors::DiagnosticBuilder;
79 use syntax::feature_gate;
82 use std::collections::VecDeque;
85 struct Coerce<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
86 fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
87 cause: ObligationCause<'tcx>,
91 impl<'a, 'gcx, 'tcx> Deref for Coerce<'a, 'gcx, 'tcx> {
92 type Target = FnCtxt<'a, 'gcx, 'tcx>;
93 fn deref(&self) -> &Self::Target {
98 type CoerceResult<'tcx> = InferResult<'tcx, Adjustment<'tcx>>;
100 fn coerce_mutbls<'tcx>(from_mutbl: hir::Mutability,
101 to_mutbl: hir::Mutability)
102 -> RelateResult<'tcx, ()> {
103 match (from_mutbl, to_mutbl) {
104 (hir::MutMutable, hir::MutMutable) |
105 (hir::MutImmutable, hir::MutImmutable) |
106 (hir::MutMutable, hir::MutImmutable) => Ok(()),
107 (hir::MutImmutable, hir::MutMutable) => Err(TypeError::Mutability),
111 fn identity<'tcx>() -> Adjust<'tcx> {
119 fn success<'tcx>(kind: Adjust<'tcx>,
121 obligations: traits::PredicateObligations<'tcx>)
122 -> CoerceResult<'tcx> {
132 impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
133 fn new(fcx: &'f FnCtxt<'f, 'gcx, 'tcx>, cause: ObligationCause<'tcx>) -> Self {
141 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
142 self.commit_if_ok(|_| {
143 let trace = TypeTrace::types(&self.cause, false, a, b);
145 self.lub(false, trace, &a, &b)
147 self.sub(false, trace, &a, &b)
152 /// Unify two types (using sub or lub) and produce a specific coercion.
153 fn unify_and(&self, a: Ty<'tcx>, b: Ty<'tcx>, kind: Adjust<'tcx>)
154 -> CoerceResult<'tcx> {
155 self.unify(&a, &b).and_then(|InferOk { value: ty, obligations }| {
156 success(kind, ty, obligations)
164 -> CoerceResult<'tcx>
165 where E: AsCoercionSite
167 let a = self.shallow_resolve(a);
168 debug!("Coerce.tys({:?} => {:?})", a, b);
170 // Just ignore error types.
171 if a.references_error() || b.references_error() {
172 return success(identity(), b, vec![]);
176 // Subtle: If we are coercing from `!` to `?T`, where `?T` is an unbound
177 // type variable, we want `?T` to fallback to `!` if not
178 // otherwise constrained. An example where this arises:
180 // let _: Option<?T> = Some({ return; });
182 // here, we would coerce from `!` to `?T`.
183 let b = self.shallow_resolve(b);
184 return if self.shallow_resolve(b).is_ty_var() {
185 // micro-optimization: no need for this if `b` is
186 // already resolved in some way.
187 let diverging_ty = self.next_diverging_ty_var(
188 TypeVariableOrigin::AdjustmentType(self.cause.span));
189 self.unify_and(&b, &diverging_ty, Adjust::NeverToAny)
191 success(Adjust::NeverToAny, b, vec![])
195 // Consider coercing the subtype to a DST
196 let unsize = self.coerce_unsized(a, b);
198 debug!("coerce: unsize successful");
201 debug!("coerce: unsize failed");
203 // Examine the supertype and consider auto-borrowing.
205 // Note: does not attempt to resolve type variables we encounter.
206 // See above for details.
208 ty::TyRawPtr(mt_b) => {
209 return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
212 ty::TyRef(r_b, mt_b) => {
213 return self.coerce_borrowed_pointer(exprs, a, b, r_b, mt_b);
220 ty::TyFnDef(.., a_f) => {
221 // Function items are coercible to any closure
222 // type; function pointers are not (that would
223 // require double indirection).
224 // Additionally, we permit coercion of function
225 // items to drop the unsafe qualifier.
226 self.coerce_from_fn_item(a, a_f, b)
228 ty::TyFnPtr(a_f) => {
229 // We permit coercion of fn pointers to drop the
231 self.coerce_from_fn_pointer(a, a_f, b)
233 ty::TyClosure(def_id_a, substs_a) => {
234 // Non-capturing closures are coercible to
236 self.coerce_closure_to_fn(a, def_id_a, substs_a, b)
239 // Otherwise, just use unification rules.
240 self.unify_and(a, b, identity())
245 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
246 /// To match `A` with `B`, autoderef will be performed,
247 /// calling `deref`/`deref_mut` where necessary.
248 fn coerce_borrowed_pointer<E>(&self,
252 r_b: &'tcx ty::Region,
253 mt_b: TypeAndMut<'tcx>)
254 -> CoerceResult<'tcx>
255 where E: AsCoercionSite
258 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
260 // If we have a parameter of type `&M T_a` and the value
261 // provided is `expr`, we will be adding an implicit borrow,
262 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
263 // to type check, we will construct the type that `&M*expr` would
266 let (r_a, mt_a) = match a.sty {
267 ty::TyRef(r_a, mt_a) => {
268 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
271 _ => return self.unify_and(a, b, identity()),
274 let span = self.cause.span;
276 let mut first_error = None;
277 let mut r_borrow_var = None;
278 let mut autoderef = self.autoderef(span, a);
279 let mut found = None;
281 for (referent_ty, autoderefs) in autoderef.by_ref() {
283 // Don't let this pass, otherwise it would cause
284 // &T to autoref to &&T.
288 // At this point, we have deref'd `a` to `referent_ty`. So
289 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
290 // In the autoderef loop for `&'a mut Vec<T>`, we would get
293 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
294 // - `Vec<T>` -- 1 deref
295 // - `[T]` -- 2 deref
297 // At each point after the first callback, we want to
298 // check to see whether this would match out target type
299 // (`&'b mut [T]`) if we autoref'd it. We can't just
300 // compare the referent types, though, because we still
301 // have to consider the mutability. E.g., in the case
302 // we've been considering, we have an `&mut` reference, so
303 // the `T` in `[T]` needs to be unified with equality.
305 // Therefore, we construct reference types reflecting what
306 // the types will be after we do the final auto-ref and
307 // compare those. Note that this means we use the target
308 // mutability [1], since it may be that we are coercing
309 // from `&mut T` to `&U`.
311 // One fine point concerns the region that we use. We
312 // choose the region such that the region of the final
313 // type that results from `unify` will be the region we
314 // want for the autoref:
316 // - if in sub mode, that means we want to use `'b` (the
317 // region from the target reference) for both
318 // pointers [2]. This is because sub mode (somewhat
319 // arbitrarily) returns the subtype region. In the case
320 // where we are coercing to a target type, we know we
321 // want to use that target type region (`'b`) because --
322 // for the program to type-check -- it must be the
323 // smaller of the two.
324 // - One fine point. It may be surprising that we can
325 // use `'b` without relating `'a` and `'b`. The reason
326 // that this is ok is that what we produce is
327 // effectively a `&'b *x` expression (if you could
328 // annotate the region of a borrow), and regionck has
329 // code that adds edges from the region of a borrow
330 // (`'b`, here) into the regions in the borrowed
331 // expression (`*x`, here). (Search for "link".)
332 // - if in lub mode, things can get fairly complicated. The
333 // easiest thing is just to make a fresh
334 // region variable [4], which effectively means we defer
335 // the decision to region inference (and regionck, which will add
336 // some more edges to this variable). However, this can wind up
337 // creating a crippling number of variables in some cases --
338 // e.g. #32278 -- so we optimize one particular case [3].
339 // Let me try to explain with some examples:
340 // - The "running example" above represents the simple case,
341 // where we have one `&` reference at the outer level and
342 // ownership all the rest of the way down. In this case,
343 // we want `LUB('a, 'b)` as the resulting region.
344 // - However, if there are nested borrows, that region is
345 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
346 // `&'b T`. In this case, `'a` is actually irrelevant.
347 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
348 // we get spurious errors (`run-pass/regions-lub-ref-ref-rc.rs`).
349 // (The errors actually show up in borrowck, typically, because
350 // this extra edge causes the region `'a` to be inferred to something
351 // too big, which then results in borrowck errors.)
352 // - We could track the innermost shared reference, but there is already
353 // code in regionck that has the job of creating links between
354 // the region of a borrow and the regions in the thing being
355 // borrowed (here, `'a` and `'x`), and it knows how to handle
356 // all the various cases. So instead we just make a region variable
357 // and let regionck figure it out.
358 let r = if !self.use_lub {
360 } else if autoderefs == 1 {
363 if r_borrow_var.is_none() {
364 // create var lazilly, at most once
365 let coercion = Coercion(span);
366 let r = self.next_region_var(coercion);
367 r_borrow_var = Some(r); // [4] above
369 r_borrow_var.unwrap()
371 let derefd_ty_a = self.tcx.mk_ref(r,
374 mutbl: mt_b.mutbl, // [1] above
376 match self.unify(derefd_ty_a, b) {
378 found = Some((ok, autoderefs));
382 if first_error.is_none() {
383 first_error = Some(err);
389 // Extract type or return an error. We return the first error
390 // we got, which should be from relating the "base" type
391 // (e.g., in example above, the failure from relating `Vec<T>`
392 // to the target type), since that should be the least
394 let (InferOk { value: ty, mut obligations }, autoderefs) = match found {
397 let err = first_error.expect("coerce_borrowed_pointer had no error");
398 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
403 if ty == a && mt_a.mutbl == hir::MutImmutable && autoderefs == 1 {
404 // As a special case, if we would produce `&'a *x`, that's
405 // a total no-op. We end up with the type `&'a T` just as
406 // we started with. In that case, just skip it
407 // altogether. This is just an optimization.
409 // Note that for `&mut`, we DO want to reborrow --
410 // otherwise, this would be a move, which might be an
411 // error. For example `foo(self.x)` where `self` and
412 // `self.x` both have `&mut `type would be a move of
413 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
414 // which is a borrow.
415 assert_eq!(mt_b.mutbl, hir::MutImmutable); // can only coerce &T -> &U
416 return success(identity(), ty, obligations);
419 // Now apply the autoref. We have to extract the region out of
420 // the final ref type we got.
421 let r_borrow = match ty.sty {
422 ty::TyRef(r_borrow, _) => r_borrow,
423 _ => span_bug!(span, "expected a ref type, got {:?}", ty),
425 let autoref = Some(AutoBorrow::Ref(r_borrow, mt_b.mutbl));
426 debug!("coerce_borrowed_pointer: succeeded ty={:?} autoderefs={:?} autoref={:?}",
431 let pref = LvaluePreference::from_mutbl(mt_b.mutbl);
432 obligations.extend(autoderef.finalize_as_infer_ok(pref, exprs).obligations);
434 success(Adjust::DerefRef {
435 autoderefs: autoderefs,
442 // &[T; n] or &mut [T; n] -> &[T]
443 // or &mut [T; n] -> &mut [T]
444 // or &Concrete -> &Trait, etc.
445 fn coerce_unsized(&self, source: Ty<'tcx>, target: Ty<'tcx>) -> CoerceResult<'tcx> {
446 debug!("coerce_unsized(source={:?}, target={:?})", source, target);
448 let traits = (self.tcx.lang_items.unsize_trait(),
449 self.tcx.lang_items.coerce_unsized_trait());
450 let (unsize_did, coerce_unsized_did) = if let (Some(u), Some(cu)) = traits {
453 debug!("Missing Unsize or CoerceUnsized traits");
454 return Err(TypeError::Mismatch);
457 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
458 // a DST unless we have to. This currently comes out in the wash since
459 // we can't unify [T] with U. But to properly support DST, we need to allow
460 // that, at which point we will need extra checks on the target here.
462 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
463 let (source, reborrow) = match (&source.sty, &target.sty) {
464 (&ty::TyRef(_, mt_a), &ty::TyRef(_, mt_b)) => {
465 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
467 let coercion = Coercion(self.cause.span);
468 let r_borrow = self.next_region_var(coercion);
469 (mt_a.ty, Some(AutoBorrow::Ref(r_borrow, mt_b.mutbl)))
471 (&ty::TyRef(_, mt_a), &ty::TyRawPtr(mt_b)) => {
472 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
473 (mt_a.ty, Some(AutoBorrow::RawPtr(mt_b.mutbl)))
477 let coerce_source = source.adjust_for_autoref(self.tcx, reborrow);
479 let adjust = Adjust::DerefRef {
480 autoderefs: if reborrow.is_some() { 1 } else { 0 },
485 // Setup either a subtyping or a LUB relationship between
486 // the `CoerceUnsized` target type and the expected type.
487 // We only have the latter, so we use an inference variable
488 // for the former and let type inference do the rest.
489 let origin = TypeVariableOrigin::MiscVariable(self.cause.span);
490 let coerce_target = self.next_ty_var(origin);
491 let mut coercion = self.unify_and(coerce_target, target, adjust)?;
493 let mut selcx = traits::SelectionContext::new(self);
495 // Use a FIFO queue for this custom fulfillment procedure.
496 let mut queue = VecDeque::new();
498 // Create an obligation for `Source: CoerceUnsized<Target>`.
499 let cause = ObligationCause::misc(self.cause.span, self.body_id);
500 queue.push_back(self.tcx
501 .predicate_for_trait_def(cause, coerce_unsized_did, 0,
502 coerce_source, &[coerce_target]));
504 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
505 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
506 // inference might unify those two inner type variables later.
507 let traits = [coerce_unsized_did, unsize_did];
508 while let Some(obligation) = queue.pop_front() {
509 debug!("coerce_unsized resolve step: {:?}", obligation);
510 let trait_ref = match obligation.predicate {
511 ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => tr.clone(),
513 coercion.obligations.push(obligation);
517 match selcx.select(&obligation.with(trait_ref)) {
518 // Uncertain or unimplemented.
520 Err(traits::Unimplemented) => {
521 debug!("coerce_unsized: early return - can't prove obligation");
522 return Err(TypeError::Mismatch);
525 // Object safety violations or miscellaneous.
527 self.report_selection_error(&obligation, &err);
528 // Treat this like an obligation and follow through
529 // with the unsizing - the lack of a coercion should
530 // be silent, as it causes a type mismatch later.
533 Ok(Some(vtable)) => {
534 for obligation in vtable.nested_obligations() {
535 queue.push_back(obligation);
544 fn coerce_from_safe_fn(&self,
546 fn_ty_a: ty::PolyFnSig<'tcx>,
548 to_unsafe: Adjust<'tcx>,
549 normal: Adjust<'tcx>)
550 -> CoerceResult<'tcx> {
551 if let ty::TyFnPtr(fn_ty_b) = b.sty {
552 match (fn_ty_a.unsafety(), fn_ty_b.unsafety()) {
553 (hir::Unsafety::Normal, hir::Unsafety::Unsafe) => {
554 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
555 return self.unify_and(unsafe_a, b, to_unsafe);
560 self.unify_and(a, b, normal)
563 fn coerce_from_fn_pointer(&self,
565 fn_ty_a: ty::PolyFnSig<'tcx>,
567 -> CoerceResult<'tcx> {
568 //! Attempts to coerce from the type of a Rust function item
569 //! into a closure or a `proc`.
572 let b = self.shallow_resolve(b);
573 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
575 self.coerce_from_safe_fn(a, fn_ty_a, b,
576 Adjust::UnsafeFnPointer, identity())
579 fn coerce_from_fn_item(&self,
581 fn_ty_a: ty::PolyFnSig<'tcx>,
583 -> CoerceResult<'tcx> {
584 //! Attempts to coerce from the type of a Rust function item
585 //! into a closure or a `proc`.
588 let b = self.shallow_resolve(b);
589 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
593 let a_fn_pointer = self.tcx.mk_fn_ptr(fn_ty_a);
594 self.coerce_from_safe_fn(a_fn_pointer, fn_ty_a, b,
595 Adjust::ReifyFnPointer, Adjust::ReifyFnPointer)
597 _ => self.unify_and(a, b, identity()),
601 fn coerce_closure_to_fn(&self,
604 substs_a: ClosureSubsts<'tcx>,
606 -> CoerceResult<'tcx> {
607 //! Attempts to coerce from the type of a non-capturing closure
608 //! into a function pointer.
611 let b = self.shallow_resolve(b);
613 let node_id_a = self.tcx.hir.as_local_node_id(def_id_a).unwrap();
615 ty::TyFnPtr(_) if self.tcx.with_freevars(node_id_a, |v| v.is_empty()) => {
616 if !self.tcx.sess.features.borrow().closure_to_fn_coercion {
617 feature_gate::emit_feature_err(&self.tcx.sess.parse_sess,
618 "closure_to_fn_coercion",
620 feature_gate::GateIssue::Language,
621 feature_gate::CLOSURE_TO_FN_COERCION);
622 return self.unify_and(a, b, identity());
624 // We coerce the closure, which has fn type
625 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
627 // `fn(arg0,arg1,...) -> _`
628 let sig = self.closure_type(def_id_a).subst(self.tcx, substs_a.substs);
629 let converted_sig = sig.map_bound(|s| {
630 let params_iter = match s.inputs()[0].sty {
631 ty::TyTuple(params, _) => {
632 params.into_iter().cloned()
640 hir::Unsafety::Normal,
644 let pointer_ty = self.tcx.mk_fn_ptr(converted_sig);
645 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})",
647 self.unify_and(pointer_ty, b, Adjust::ClosureFnPointer)
649 _ => self.unify_and(a, b, identity()),
653 fn coerce_unsafe_ptr(&self,
656 mutbl_b: hir::Mutability)
657 -> CoerceResult<'tcx> {
658 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
660 let (is_ref, mt_a) = match a.sty {
661 ty::TyRef(_, mt) => (true, mt),
662 ty::TyRawPtr(mt) => (false, mt),
664 return self.unify_and(a, b, identity());
668 // Check that the types which they point at are compatible.
669 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut {
673 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
674 // Although references and unsafe ptrs have the same
675 // representation, we still register an Adjust::DerefRef so that
676 // regionck knows that the region for `a` must be valid here.
677 self.unify_and(a_unsafe, b, if is_ref {
680 autoref: Some(AutoBorrow::RawPtr(mutbl_b)),
683 } else if mt_a.mutbl != mutbl_b {
684 Adjust::MutToConstPointer
691 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
692 /// Attempt to coerce an expression to a type, and return the
693 /// adjusted type of the expression, if successful.
694 /// Adjustments are only recorded if the coercion succeeded.
695 /// The expressions *must not* have any pre-existing adjustments.
696 pub fn try_coerce(&self,
699 expr_diverges: Diverges,
701 -> RelateResult<'tcx, Ty<'tcx>> {
702 let source = self.resolve_type_vars_with_obligations(expr_ty);
703 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
705 // Special-ish case: we can coerce any type `T` into the `!`
706 // type, but only if the source expression diverges.
707 if target.is_never() && expr_diverges.always() {
708 debug!("permit coercion to `!` because expr diverges");
712 let cause = self.cause(expr.span, ObligationCauseCode::ExprAssignable);
713 let coerce = Coerce::new(self, cause);
714 self.commit_if_ok(|_| {
715 let ok = coerce.coerce(&[expr], source, target)?;
716 let adjustment = self.register_infer_ok_obligations(ok);
717 self.apply_adjustment(expr.id, adjustment);
719 // We should now have added sufficient adjustments etc to
720 // ensure that the type of expression, post-adjustment, is
721 // a subtype of target.
726 /// Given some expressions, their known unified type and another expression,
727 /// tries to unify the types, potentially inserting coercions on any of the
728 /// provided expressions and returns their LUB (aka "common supertype").
730 /// This is really an internal helper. From outside the coercion
731 /// module, you should instantiate a `CoerceMany` instance.
732 fn try_find_coercion_lub<E>(&self,
733 cause: &ObligationCause<'tcx>,
738 new_diverges: Diverges)
739 -> RelateResult<'tcx, Ty<'tcx>>
740 where E: AsCoercionSite
742 let prev_ty = self.resolve_type_vars_with_obligations(prev_ty);
743 let new_ty = self.resolve_type_vars_with_obligations(new_ty);
744 debug!("coercion::try_find_coercion_lub({:?}, {:?})", prev_ty, new_ty);
746 // Special-ish case: we can coerce any type `T` into the `!`
747 // type, but only if the source expression diverges.
748 if prev_ty.is_never() && new_diverges.always() {
749 debug!("permit coercion to `!` because expr diverges");
753 let trace = TypeTrace::types(cause, true, prev_ty, new_ty);
755 // Special-case that coercion alone cannot handle:
756 // Two function item types of differing IDs or Substs.
757 match (&prev_ty.sty, &new_ty.sty) {
758 (&ty::TyFnDef(a_def_id, a_substs, a_fty), &ty::TyFnDef(b_def_id, b_substs, b_fty)) => {
759 // The signature must always match.
760 let fty = self.lub(true, trace.clone(), &a_fty, &b_fty)
761 .map(|ok| self.register_infer_ok_obligations(ok))?;
763 if a_def_id == b_def_id {
764 // Same function, maybe the parameters match.
765 let substs = self.commit_if_ok(|_| {
766 self.lub(true, trace.clone(), &a_substs, &b_substs)
767 .map(|ok| self.register_infer_ok_obligations(ok))
770 if let Ok(substs) = substs {
771 // We have a LUB of prev_ty and new_ty, just return it.
772 return Ok(self.tcx.mk_fn_def(a_def_id, substs, fty));
776 // Reify both sides and return the reified fn pointer type.
777 let fn_ptr = self.tcx.mk_fn_ptr(fty);
778 for expr in exprs.iter().map(|e| e.as_coercion_site()).chain(Some(new)) {
779 // The only adjustment that can produce an fn item is
780 // `NeverToAny`, so this should always be valid.
781 self.apply_adjustment(expr.id, Adjustment {
782 kind: Adjust::ReifyFnPointer,
791 let mut coerce = Coerce::new(self, cause.clone());
792 coerce.use_lub = true;
794 // First try to coerce the new expression to the type of the previous ones,
795 // but only if the new expression has no coercion already applied to it.
796 let mut first_error = None;
797 if !self.tables.borrow().adjustments.contains_key(&new.id) {
798 let result = self.commit_if_ok(|_| coerce.coerce(&[new], new_ty, prev_ty));
801 let adjustment = self.register_infer_ok_obligations(ok);
802 self.apply_adjustment(new.id, adjustment);
803 return Ok(adjustment.target);
805 Err(e) => first_error = Some(e),
809 // Then try to coerce the previous expressions to the type of the new one.
810 // This requires ensuring there are no coercions applied to *any* of the
811 // previous expressions, other than noop reborrows (ignoring lifetimes).
813 let expr = expr.as_coercion_site();
814 let noop = match self.tables.borrow().adjustments.get(&expr.id).map(|adj| adj.kind) {
815 Some(Adjust::DerefRef {
817 autoref: Some(AutoBorrow::Ref(_, mutbl_adj)),
820 match self.node_ty(expr.id).sty {
821 ty::TyRef(_, mt_orig) => {
822 // Reborrow that we can safely ignore, because
823 // the next adjustment can only be a DerefRef
824 // which will be merged into it.
825 mutbl_adj == mt_orig.mutbl
830 Some(Adjust::NeverToAny) => true,
836 return self.commit_if_ok(|_| {
837 self.lub(true, trace.clone(), &prev_ty, &new_ty)
838 .map(|ok| self.register_infer_ok_obligations(ok))
843 match self.commit_if_ok(|_| coerce.coerce(&exprs, prev_ty, new_ty)) {
845 // Avoid giving strange errors on failed attempts.
846 if let Some(e) = first_error {
849 self.commit_if_ok(|_| {
850 self.lub(true, trace, &prev_ty, &new_ty)
851 .map(|ok| self.register_infer_ok_obligations(ok))
856 let adjustment = self.register_infer_ok_obligations(ok);
858 let expr = expr.as_coercion_site();
859 self.apply_adjustment(expr.id, adjustment);
861 Ok(adjustment.target)
867 /// CoerceMany encapsulates the pattern you should use when you have
868 /// many expressions that are all getting coerced to a common
869 /// type. This arises, for example, when you have a match (the result
870 /// of each arm is coerced to a common type). It also arises in less
871 /// obvious places, such as when you have many `break foo` expressions
872 /// that target the same loop, or the various `return` expressions in
875 /// The basic protocol is as follows:
877 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
878 /// This will also serve as the "starting LUB". The expectation is
879 /// that this type is something which all of the expressions *must*
880 /// be coercible to. Use a fresh type variable if needed.
881 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
882 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
883 /// unit. This happens for example if you have a `break` with no expression,
884 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
885 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
886 /// from you so that you don't have to worry your pretty head about it.
887 /// But if an error is reported, the final type will be `err`.
888 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
889 /// previously coerced expressions.
890 /// - When all done, invoke `complete()`. This will return the LUB of
891 /// all your expressions.
892 /// - WARNING: I don't believe this final type is guaranteed to be
893 /// related to your initial `expected_ty` in any particular way,
894 /// although it will typically be a subtype, so you should check it.
895 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
896 /// previously coerced expressions.
901 /// let mut coerce = CoerceMany::new(expected_ty);
902 /// for expr in exprs {
903 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
904 /// coerce.coerce(fcx, &cause, expr, expr_ty);
906 /// let final_ty = coerce.complete(fcx);
908 pub struct CoerceMany<'gcx, 'tcx, 'exprs, E>
909 where 'gcx: 'tcx, E: 'exprs + AsCoercionSite,
911 expected_ty: Ty<'tcx>,
912 final_ty: Option<Ty<'tcx>>,
913 expressions: Expressions<'gcx, 'exprs, E>,
917 /// The type of a `CoerceMany` that is storing up the expressions into
918 /// a buffer. We use this in `check/mod.rs` for things like `break`.
919 pub type DynamicCoerceMany<'gcx, 'tcx> = CoerceMany<'gcx, 'tcx, 'gcx, P<hir::Expr>>;
921 enum Expressions<'gcx, 'exprs, E>
922 where E: 'exprs + AsCoercionSite,
924 Dynamic(Vec<&'gcx hir::Expr>),
925 UpFront(&'exprs [E]),
928 impl<'gcx, 'tcx, 'exprs, E> CoerceMany<'gcx, 'tcx, 'exprs, E>
929 where 'gcx: 'tcx, E: 'exprs + AsCoercionSite,
931 /// The usual case; collect the set of expressions dynamically.
932 /// If the full set of coercion sites is known before hand,
933 /// consider `with_coercion_sites()` instead to avoid allocation.
934 pub fn new(expected_ty: Ty<'tcx>) -> Self {
935 Self::make(expected_ty, Expressions::Dynamic(vec![]))
938 /// As an optimization, you can create a `CoerceMany` with a
939 /// pre-existing slice of expressions. In this case, you are
940 /// expected to pass each element in the slice to `coerce(...)` in
941 /// order. This is used with arrays in particular to avoid
942 /// needlessly cloning the slice.
943 pub fn with_coercion_sites(expected_ty: Ty<'tcx>,
944 coercion_sites: &'exprs [E])
946 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
949 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'gcx, 'exprs, E>) -> Self {
958 pub fn is_empty(&self) -> bool {
962 /// Return the "expected type" with which this coercion was
963 /// constructed. This represents the "downward propagated" type
964 /// that was given to us at the start of typing whatever construct
965 /// we are typing (e.g., the match expression).
967 /// Typically, this is used as the expected type when
968 /// type-checking each of the alternative expressions whose types
969 /// we are trying to merge.
970 pub fn expected_ty(&self) -> Ty<'tcx> {
974 /// Returns the current "merged type", representing our best-guess
975 /// at the LUB of the expressions we've seen so far (if any). This
976 /// isn't *final* until you call `self.final()`, which will return
978 pub fn merged_ty(&self) -> Ty<'tcx> {
979 self.final_ty.unwrap_or(self.expected_ty)
982 /// Indicates that the value generated by `expression`, which is
983 /// of type `expression_ty`, is one of the possibility that we
984 /// could coerce from. This will record `expression` and later
985 /// calls to `coerce` may come back and add adjustments and things
987 pub fn coerce<'a>(&mut self,
988 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
989 cause: &ObligationCause<'tcx>,
990 expression: &'gcx hir::Expr,
991 expression_ty: Ty<'tcx>,
992 expression_diverges: Diverges)
994 self.coerce_inner(fcx, cause, Some(expression), expression_ty, expression_diverges, None)
997 /// Indicates that one of the inputs is a "forced unit". This
998 /// occurs in a case like `if foo { ... };`, where the issing else
999 /// generates a "forced unit". Another example is a `loop { break;
1000 /// }`, where the `break` has no argument expression. We treat
1001 /// these cases slightly differently for error-reporting
1002 /// purposes. Note that these tend to correspond to cases where
1003 /// the `()` expression is implicit in the source, and hence we do
1004 /// not take an expression argument.
1006 /// The `augment_error` gives you a chance to extend the error
1007 /// message, in case any results (e.g., we use this to suggest
1008 /// removing a `;`).
1009 pub fn coerce_forced_unit<'a>(&mut self,
1010 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1011 cause: &ObligationCause<'tcx>,
1012 augment_error: &mut FnMut(&mut DiagnosticBuilder))
1014 self.coerce_inner(fcx,
1019 Some(augment_error))
1022 /// The inner coercion "engine". If `expression` is `None`, this
1023 /// is a forced-unit case, and hence `expression_ty` must be
1025 fn coerce_inner<'a>(&mut self,
1026 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1027 cause: &ObligationCause<'tcx>,
1028 expression: Option<&'gcx hir::Expr>,
1029 mut expression_ty: Ty<'tcx>,
1030 expression_diverges: Diverges,
1031 augment_error: Option<&mut FnMut(&mut DiagnosticBuilder)>)
1033 // Incorporate whatever type inference information we have
1034 // until now; in principle we might also want to process
1035 // pending obligations, but doing so should only improve
1036 // compatibility (hopefully that is true) by helping us
1037 // uncover never types better.
1038 if expression_ty.is_ty_var() {
1039 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1042 // If we see any error types, just propagate that error
1044 if expression_ty.references_error() || self.merged_ty().references_error() {
1045 self.final_ty = Some(fcx.tcx.types.err);
1049 // Handle the actual type unification etc.
1050 let result = if let Some(expression) = expression {
1051 if self.pushed == 0 {
1052 // Special-case the first expression we are coercing.
1053 // To be honest, I'm not entirely sure why we do this.
1054 fcx.try_coerce(expression, expression_ty, expression_diverges, self.expected_ty)
1056 match self.expressions {
1057 Expressions::Dynamic(ref exprs) =>
1058 fcx.try_find_coercion_lub(cause,
1063 expression_diverges),
1064 Expressions::UpFront(ref coercion_sites) =>
1065 fcx.try_find_coercion_lub(cause,
1066 &coercion_sites[0..self.pushed],
1070 expression_diverges),
1074 // this is a hack for cases where we default to `()` because
1075 // the expression etc has been omitted from the source. An
1076 // example is an `if let` without an else:
1078 // if let Some(x) = ... { }
1080 // we wind up with a second match arm that is like `_ =>
1081 // ()`. That is the case we are considering here. We take
1082 // a different path to get the right "expected, found"
1083 // message and so forth (and because we know that
1084 // `expression_ty` will be unit).
1086 // Another example is `break` with no argument expression.
1087 assert!(expression_ty.is_nil());
1088 assert!(expression_ty.is_nil(), "if let hack without unit type");
1089 fcx.eq_types(true, cause, expression_ty, self.merged_ty())
1091 fcx.register_infer_ok_obligations(infer_ok);
1098 self.final_ty = Some(v);
1099 if let Some(e) = expression {
1100 match self.expressions {
1101 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1102 Expressions::UpFront(coercion_sites) => {
1103 // if the user gave us an array to validate, check that we got
1104 // the next expression in the list, as expected
1105 assert_eq!(coercion_sites[self.pushed].as_coercion_site().id, e.id);
1112 let (expected, found) = if expression.is_none() {
1113 // In the case where this is a "forced unit", like
1114 // `break`, we want to call the `()` "expected"
1115 // since it is implied by the syntax.
1116 assert!(expression_ty.is_nil());
1117 (expression_ty, self.final_ty.unwrap_or(self.expected_ty))
1119 // Otherwise, the "expected" type for error
1120 // reporting is the current unification type,
1121 // which is basically the LUB of the expressions
1122 // we've seen so far (combined with the expected
1124 (self.final_ty.unwrap_or(self.expected_ty), expression_ty)
1129 ObligationCauseCode::ReturnNoExpression => {
1130 db = struct_span_err!(
1131 fcx.tcx.sess, cause.span, E0069,
1132 "`return;` in a function whose return type is not `()`");
1133 db.span_label(cause.span, &format!("return type is not ()"));
1136 db = fcx.report_mismatched_types(cause, expected, found, err);
1140 if let Some(mut augment_error) = augment_error {
1141 augment_error(&mut db);
1146 self.final_ty = Some(fcx.tcx.types.err);
1151 pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) -> Ty<'tcx> {
1152 if let Some(final_ty) = self.final_ty {
1155 // If we only had inputs that were of type `!` (or no
1156 // inputs at all), then the final type is `!`.
1157 assert_eq!(self.pushed, 0);
1163 /// Something that can be converted into an expression to which we can
1164 /// apply a coercion.
1165 pub trait AsCoercionSite {
1166 fn as_coercion_site(&self) -> &hir::Expr;
1169 impl AsCoercionSite for hir::Expr {
1170 fn as_coercion_site(&self) -> &hir::Expr {
1175 impl AsCoercionSite for P<hir::Expr> {
1176 fn as_coercion_site(&self) -> &hir::Expr {
1181 impl<'a, T> AsCoercionSite for &'a T
1182 where T: AsCoercionSite
1184 fn as_coercion_site(&self) -> &hir::Expr {
1185 (**self).as_coercion_site()
1189 impl AsCoercionSite for ! {
1190 fn as_coercion_site(&self) -> &hir::Expr {
1195 impl AsCoercionSite for hir::Arm {
1196 fn as_coercion_site(&self) -> &hir::Expr {