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;
83 use std::collections::VecDeque;
86 struct Coerce<'a, 'gcx: 'a + 'tcx, 'tcx: 'a> {
87 fcx: &'a FnCtxt<'a, 'gcx, 'tcx>,
88 cause: ObligationCause<'tcx>,
92 impl<'a, 'gcx, 'tcx> Deref for Coerce<'a, 'gcx, 'tcx> {
93 type Target = FnCtxt<'a, 'gcx, 'tcx>;
94 fn deref(&self) -> &Self::Target {
99 type CoerceResult<'tcx> = InferResult<'tcx, Adjustment<'tcx>>;
101 fn coerce_mutbls<'tcx>(from_mutbl: hir::Mutability,
102 to_mutbl: hir::Mutability)
103 -> RelateResult<'tcx, ()> {
104 match (from_mutbl, to_mutbl) {
105 (hir::MutMutable, hir::MutMutable) |
106 (hir::MutImmutable, hir::MutImmutable) |
107 (hir::MutMutable, hir::MutImmutable) => Ok(()),
108 (hir::MutImmutable, hir::MutMutable) => Err(TypeError::Mutability),
112 fn identity<'tcx>() -> Adjust<'tcx> {
120 fn success<'tcx>(kind: Adjust<'tcx>,
122 obligations: traits::PredicateObligations<'tcx>)
123 -> CoerceResult<'tcx> {
133 impl<'f, 'gcx, 'tcx> Coerce<'f, 'gcx, 'tcx> {
134 fn new(fcx: &'f FnCtxt<'f, 'gcx, 'tcx>, cause: ObligationCause<'tcx>) -> Self {
142 fn unify(&self, a: Ty<'tcx>, b: Ty<'tcx>) -> InferResult<'tcx, Ty<'tcx>> {
143 self.commit_if_ok(|_| {
144 let trace = TypeTrace::types(&self.cause, false, a, b);
146 self.lub(false, trace, &a, &b)
148 self.sub(false, trace, &a, &b)
153 /// Unify two types (using sub or lub) and produce a specific coercion.
154 fn unify_and(&self, a: Ty<'tcx>, b: Ty<'tcx>, kind: Adjust<'tcx>)
155 -> CoerceResult<'tcx> {
156 self.unify(&a, &b).and_then(|InferOk { value: ty, obligations }| {
157 success(kind, ty, obligations)
165 -> CoerceResult<'tcx>
166 where E: AsCoercionSite
168 let a = self.shallow_resolve(a);
169 debug!("Coerce.tys({:?} => {:?})", a, b);
171 // Just ignore error types.
172 if a.references_error() || b.references_error() {
173 return success(identity(), b, vec![]);
177 // Subtle: If we are coercing from `!` to `?T`, where `?T` is an unbound
178 // type variable, we want `?T` to fallback to `!` if not
179 // otherwise constrained. An example where this arises:
181 // let _: Option<?T> = Some({ return; });
183 // here, we would coerce from `!` to `?T`.
184 let b = self.shallow_resolve(b);
185 return if self.shallow_resolve(b).is_ty_var() {
186 // micro-optimization: no need for this if `b` is
187 // already resolved in some way.
188 let diverging_ty = self.next_diverging_ty_var(
189 TypeVariableOrigin::AdjustmentType(self.cause.span));
190 self.unify_and(&b, &diverging_ty, Adjust::NeverToAny)
192 success(Adjust::NeverToAny, b, vec![])
196 // Consider coercing the subtype to a DST
197 let unsize = self.coerce_unsized(a, b);
199 debug!("coerce: unsize successful");
202 debug!("coerce: unsize failed");
204 // Examine the supertype and consider auto-borrowing.
206 // Note: does not attempt to resolve type variables we encounter.
207 // See above for details.
209 ty::TyRawPtr(mt_b) => {
210 return self.coerce_unsafe_ptr(a, b, mt_b.mutbl);
213 ty::TyRef(r_b, mt_b) => {
214 return self.coerce_borrowed_pointer(exprs, a, b, r_b, mt_b);
221 ty::TyFnDef(.., a_f) => {
222 // Function items are coercible to any closure
223 // type; function pointers are not (that would
224 // require double indirection).
225 // Additionally, we permit coercion of function
226 // items to drop the unsafe qualifier.
227 self.coerce_from_fn_item(a, a_f, b)
229 ty::TyFnPtr(a_f) => {
230 // We permit coercion of fn pointers to drop the
232 self.coerce_from_fn_pointer(a, a_f, b)
234 ty::TyClosure(def_id_a, substs_a) => {
235 // Non-capturing closures are coercible to
237 self.coerce_closure_to_fn(a, def_id_a, substs_a, b)
240 // Otherwise, just use unification rules.
241 self.unify_and(a, b, identity())
246 /// Reborrows `&mut A` to `&mut B` and `&(mut) A` to `&B`.
247 /// To match `A` with `B`, autoderef will be performed,
248 /// calling `deref`/`deref_mut` where necessary.
249 fn coerce_borrowed_pointer<E>(&self,
253 r_b: &'tcx ty::Region,
254 mt_b: TypeAndMut<'tcx>)
255 -> CoerceResult<'tcx>
256 where E: AsCoercionSite
259 debug!("coerce_borrowed_pointer(a={:?}, b={:?})", a, b);
261 // If we have a parameter of type `&M T_a` and the value
262 // provided is `expr`, we will be adding an implicit borrow,
263 // meaning that we convert `f(expr)` to `f(&M *expr)`. Therefore,
264 // to type check, we will construct the type that `&M*expr` would
267 let (r_a, mt_a) = match a.sty {
268 ty::TyRef(r_a, mt_a) => {
269 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
272 _ => return self.unify_and(a, b, identity()),
275 let span = self.cause.span;
277 let mut first_error = None;
278 let mut r_borrow_var = None;
279 let mut autoderef = self.autoderef(span, a);
280 let mut found = None;
282 for (referent_ty, autoderefs) in autoderef.by_ref() {
284 // Don't let this pass, otherwise it would cause
285 // &T to autoref to &&T.
289 // At this point, we have deref'd `a` to `referent_ty`. So
290 // imagine we are coercing from `&'a mut Vec<T>` to `&'b mut [T]`.
291 // In the autoderef loop for `&'a mut Vec<T>`, we would get
294 // - `&'a mut Vec<T>` -- 0 derefs, just ignore it
295 // - `Vec<T>` -- 1 deref
296 // - `[T]` -- 2 deref
298 // At each point after the first callback, we want to
299 // check to see whether this would match out target type
300 // (`&'b mut [T]`) if we autoref'd it. We can't just
301 // compare the referent types, though, because we still
302 // have to consider the mutability. E.g., in the case
303 // we've been considering, we have an `&mut` reference, so
304 // the `T` in `[T]` needs to be unified with equality.
306 // Therefore, we construct reference types reflecting what
307 // the types will be after we do the final auto-ref and
308 // compare those. Note that this means we use the target
309 // mutability [1], since it may be that we are coercing
310 // from `&mut T` to `&U`.
312 // One fine point concerns the region that we use. We
313 // choose the region such that the region of the final
314 // type that results from `unify` will be the region we
315 // want for the autoref:
317 // - if in sub mode, that means we want to use `'b` (the
318 // region from the target reference) for both
319 // pointers [2]. This is because sub mode (somewhat
320 // arbitrarily) returns the subtype region. In the case
321 // where we are coercing to a target type, we know we
322 // want to use that target type region (`'b`) because --
323 // for the program to type-check -- it must be the
324 // smaller of the two.
325 // - One fine point. It may be surprising that we can
326 // use `'b` without relating `'a` and `'b`. The reason
327 // that this is ok is that what we produce is
328 // effectively a `&'b *x` expression (if you could
329 // annotate the region of a borrow), and regionck has
330 // code that adds edges from the region of a borrow
331 // (`'b`, here) into the regions in the borrowed
332 // expression (`*x`, here). (Search for "link".)
333 // - if in lub mode, things can get fairly complicated. The
334 // easiest thing is just to make a fresh
335 // region variable [4], which effectively means we defer
336 // the decision to region inference (and regionck, which will add
337 // some more edges to this variable). However, this can wind up
338 // creating a crippling number of variables in some cases --
339 // e.g. #32278 -- so we optimize one particular case [3].
340 // Let me try to explain with some examples:
341 // - The "running example" above represents the simple case,
342 // where we have one `&` reference at the outer level and
343 // ownership all the rest of the way down. In this case,
344 // we want `LUB('a, 'b)` as the resulting region.
345 // - However, if there are nested borrows, that region is
346 // too strong. Consider a coercion from `&'a &'x Rc<T>` to
347 // `&'b T`. In this case, `'a` is actually irrelevant.
348 // The pointer we want is `LUB('x, 'b`). If we choose `LUB('a,'b)`
349 // we get spurious errors (`run-pass/regions-lub-ref-ref-rc.rs`).
350 // (The errors actually show up in borrowck, typically, because
351 // this extra edge causes the region `'a` to be inferred to something
352 // too big, which then results in borrowck errors.)
353 // - We could track the innermost shared reference, but there is already
354 // code in regionck that has the job of creating links between
355 // the region of a borrow and the regions in the thing being
356 // borrowed (here, `'a` and `'x`), and it knows how to handle
357 // all the various cases. So instead we just make a region variable
358 // and let regionck figure it out.
359 let r = if !self.use_lub {
361 } else if autoderefs == 1 {
364 if r_borrow_var.is_none() {
365 // create var lazilly, at most once
366 let coercion = Coercion(span);
367 let r = self.next_region_var(coercion);
368 r_borrow_var = Some(r); // [4] above
370 r_borrow_var.unwrap()
372 let derefd_ty_a = self.tcx.mk_ref(r,
375 mutbl: mt_b.mutbl, // [1] above
377 match self.unify(derefd_ty_a, b) {
379 found = Some((ok, autoderefs));
383 if first_error.is_none() {
384 first_error = Some(err);
390 // Extract type or return an error. We return the first error
391 // we got, which should be from relating the "base" type
392 // (e.g., in example above, the failure from relating `Vec<T>`
393 // to the target type), since that should be the least
395 let (InferOk { value: ty, mut obligations }, autoderefs) = match found {
398 let err = first_error.expect("coerce_borrowed_pointer had no error");
399 debug!("coerce_borrowed_pointer: failed with err = {:?}", err);
404 if ty == a && mt_a.mutbl == hir::MutImmutable && autoderefs == 1 {
405 // As a special case, if we would produce `&'a *x`, that's
406 // a total no-op. We end up with the type `&'a T` just as
407 // we started with. In that case, just skip it
408 // altogether. This is just an optimization.
410 // Note that for `&mut`, we DO want to reborrow --
411 // otherwise, this would be a move, which might be an
412 // error. For example `foo(self.x)` where `self` and
413 // `self.x` both have `&mut `type would be a move of
414 // `self.x`, but we auto-coerce it to `foo(&mut *self.x)`,
415 // which is a borrow.
416 assert_eq!(mt_b.mutbl, hir::MutImmutable); // can only coerce &T -> &U
417 return success(identity(), ty, obligations);
420 // Now apply the autoref. We have to extract the region out of
421 // the final ref type we got.
422 let r_borrow = match ty.sty {
423 ty::TyRef(r_borrow, _) => r_borrow,
424 _ => span_bug!(span, "expected a ref type, got {:?}", ty),
426 let autoref = Some(AutoBorrow::Ref(r_borrow, mt_b.mutbl));
427 debug!("coerce_borrowed_pointer: succeeded ty={:?} autoderefs={:?} autoref={:?}",
432 let pref = LvaluePreference::from_mutbl(mt_b.mutbl);
433 obligations.extend(autoderef.finalize_as_infer_ok(pref, exprs).obligations);
435 success(Adjust::DerefRef {
436 autoderefs: autoderefs,
443 // &[T; n] or &mut [T; n] -> &[T]
444 // or &mut [T; n] -> &mut [T]
445 // or &Concrete -> &Trait, etc.
446 fn coerce_unsized(&self, source: Ty<'tcx>, target: Ty<'tcx>) -> CoerceResult<'tcx> {
447 debug!("coerce_unsized(source={:?}, target={:?})", source, target);
449 let traits = (self.tcx.lang_items.unsize_trait(),
450 self.tcx.lang_items.coerce_unsized_trait());
451 let (unsize_did, coerce_unsized_did) = if let (Some(u), Some(cu)) = traits {
454 debug!("Missing Unsize or CoerceUnsized traits");
455 return Err(TypeError::Mismatch);
458 // Note, we want to avoid unnecessary unsizing. We don't want to coerce to
459 // a DST unless we have to. This currently comes out in the wash since
460 // we can't unify [T] with U. But to properly support DST, we need to allow
461 // that, at which point we will need extra checks on the target here.
463 // Handle reborrows before selecting `Source: CoerceUnsized<Target>`.
464 let (source, reborrow) = match (&source.sty, &target.sty) {
465 (&ty::TyRef(_, mt_a), &ty::TyRef(_, mt_b)) => {
466 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
468 let coercion = Coercion(self.cause.span);
469 let r_borrow = self.next_region_var(coercion);
470 (mt_a.ty, Some(AutoBorrow::Ref(r_borrow, mt_b.mutbl)))
472 (&ty::TyRef(_, mt_a), &ty::TyRawPtr(mt_b)) => {
473 coerce_mutbls(mt_a.mutbl, mt_b.mutbl)?;
474 (mt_a.ty, Some(AutoBorrow::RawPtr(mt_b.mutbl)))
478 let coerce_source = source.adjust_for_autoref(self.tcx, reborrow);
480 let adjust = Adjust::DerefRef {
481 autoderefs: if reborrow.is_some() { 1 } else { 0 },
486 // Setup either a subtyping or a LUB relationship between
487 // the `CoerceUnsized` target type and the expected type.
488 // We only have the latter, so we use an inference variable
489 // for the former and let type inference do the rest.
490 let origin = TypeVariableOrigin::MiscVariable(self.cause.span);
491 let coerce_target = self.next_ty_var(origin);
492 let mut coercion = self.unify_and(coerce_target, target, adjust)?;
494 let mut selcx = traits::SelectionContext::new(self);
496 // Use a FIFO queue for this custom fulfillment procedure.
497 let mut queue = VecDeque::new();
499 // Create an obligation for `Source: CoerceUnsized<Target>`.
500 let cause = ObligationCause::misc(self.cause.span, self.body_id);
501 queue.push_back(self.tcx
502 .predicate_for_trait_def(cause, coerce_unsized_did, 0,
503 coerce_source, &[coerce_target]));
505 // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
506 // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
507 // inference might unify those two inner type variables later.
508 let traits = [coerce_unsized_did, unsize_did];
509 while let Some(obligation) = queue.pop_front() {
510 debug!("coerce_unsized resolve step: {:?}", obligation);
511 let trait_ref = match obligation.predicate {
512 ty::Predicate::Trait(ref tr) if traits.contains(&tr.def_id()) => tr.clone(),
514 coercion.obligations.push(obligation);
518 match selcx.select(&obligation.with(trait_ref)) {
519 // Uncertain or unimplemented.
521 Err(traits::Unimplemented) => {
522 debug!("coerce_unsized: early return - can't prove obligation");
523 return Err(TypeError::Mismatch);
526 // Object safety violations or miscellaneous.
528 self.report_selection_error(&obligation, &err);
529 // Treat this like an obligation and follow through
530 // with the unsizing - the lack of a coercion should
531 // be silent, as it causes a type mismatch later.
534 Ok(Some(vtable)) => {
535 for obligation in vtable.nested_obligations() {
536 queue.push_back(obligation);
545 fn coerce_from_safe_fn(&self,
547 fn_ty_a: ty::PolyFnSig<'tcx>,
549 to_unsafe: Adjust<'tcx>,
550 normal: Adjust<'tcx>)
551 -> CoerceResult<'tcx> {
552 if let ty::TyFnPtr(fn_ty_b) = b.sty {
553 match (fn_ty_a.unsafety(), fn_ty_b.unsafety()) {
554 (hir::Unsafety::Normal, hir::Unsafety::Unsafe) => {
555 let unsafe_a = self.tcx.safe_to_unsafe_fn_ty(fn_ty_a);
556 return self.unify_and(unsafe_a, b, to_unsafe);
561 self.unify_and(a, b, normal)
564 fn coerce_from_fn_pointer(&self,
566 fn_ty_a: ty::PolyFnSig<'tcx>,
568 -> CoerceResult<'tcx> {
569 //! Attempts to coerce from the type of a Rust function item
570 //! into a closure or a `proc`.
573 let b = self.shallow_resolve(b);
574 debug!("coerce_from_fn_pointer(a={:?}, b={:?})", a, b);
576 self.coerce_from_safe_fn(a, fn_ty_a, b,
577 Adjust::UnsafeFnPointer, identity())
580 fn coerce_from_fn_item(&self,
582 fn_ty_a: ty::PolyFnSig<'tcx>,
584 -> CoerceResult<'tcx> {
585 //! Attempts to coerce from the type of a Rust function item
586 //! into a closure or a `proc`.
589 let b = self.shallow_resolve(b);
590 debug!("coerce_from_fn_item(a={:?}, b={:?})", a, b);
594 let a_fn_pointer = self.tcx.mk_fn_ptr(fn_ty_a);
595 self.coerce_from_safe_fn(a_fn_pointer, fn_ty_a, b,
596 Adjust::ReifyFnPointer, Adjust::ReifyFnPointer)
598 _ => self.unify_and(a, b, identity()),
602 fn coerce_closure_to_fn(&self,
605 substs_a: ClosureSubsts<'tcx>,
607 -> CoerceResult<'tcx> {
608 //! Attempts to coerce from the type of a non-capturing closure
609 //! into a function pointer.
612 let b = self.shallow_resolve(b);
614 let node_id_a = self.tcx.hir.as_local_node_id(def_id_a).unwrap();
616 ty::TyFnPtr(_) if self.tcx.with_freevars(node_id_a, |v| v.is_empty()) => {
617 if !self.tcx.sess.features.borrow().closure_to_fn_coercion {
618 feature_gate::emit_feature_err(&self.tcx.sess.parse_sess,
619 "closure_to_fn_coercion",
621 feature_gate::GateIssue::Language,
622 feature_gate::CLOSURE_TO_FN_COERCION);
623 return self.unify_and(a, b, identity());
625 // We coerce the closure, which has fn type
626 // `extern "rust-call" fn((arg0,arg1,...)) -> _`
628 // `fn(arg0,arg1,...) -> _`
629 let sig = self.closure_type(def_id_a).subst(self.tcx, substs_a.substs);
630 let converted_sig = sig.map_bound(|s| {
631 let params_iter = match s.inputs()[0].sty {
632 ty::TyTuple(params, _) => {
633 params.into_iter().cloned()
641 hir::Unsafety::Normal,
645 let pointer_ty = self.tcx.mk_fn_ptr(converted_sig);
646 debug!("coerce_closure_to_fn(a={:?}, b={:?}, pty={:?})",
648 self.unify_and(pointer_ty, b, Adjust::ClosureFnPointer)
650 _ => self.unify_and(a, b, identity()),
654 fn coerce_unsafe_ptr(&self,
657 mutbl_b: hir::Mutability)
658 -> CoerceResult<'tcx> {
659 debug!("coerce_unsafe_ptr(a={:?}, b={:?})", a, b);
661 let (is_ref, mt_a) = match a.sty {
662 ty::TyRef(_, mt) => (true, mt),
663 ty::TyRawPtr(mt) => (false, mt),
665 return self.unify_and(a, b, identity());
669 // Check that the types which they point at are compatible.
670 let a_unsafe = self.tcx.mk_ptr(ty::TypeAndMut {
674 coerce_mutbls(mt_a.mutbl, mutbl_b)?;
675 // Although references and unsafe ptrs have the same
676 // representation, we still register an Adjust::DerefRef so that
677 // regionck knows that the region for `a` must be valid here.
678 self.unify_and(a_unsafe, b, if is_ref {
681 autoref: Some(AutoBorrow::RawPtr(mutbl_b)),
684 } else if mt_a.mutbl != mutbl_b {
685 Adjust::MutToConstPointer
692 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
693 /// Attempt to coerce an expression to a type, and return the
694 /// adjusted type of the expression, if successful.
695 /// Adjustments are only recorded if the coercion succeeded.
696 /// The expressions *must not* have any pre-existing adjustments.
697 pub fn try_coerce(&self,
700 expr_diverges: Diverges,
702 -> RelateResult<'tcx, Ty<'tcx>> {
703 let source = self.resolve_type_vars_with_obligations(expr_ty);
704 debug!("coercion::try({:?}: {:?} -> {:?})", expr, source, target);
706 // Special-ish case: we can coerce any type `T` into the `!`
707 // type, but only if the source expression diverges.
708 if target.is_never() && expr_diverges.always() {
709 debug!("permit coercion to `!` because expr diverges");
713 let cause = self.cause(expr.span, ObligationCauseCode::ExprAssignable);
714 let coerce = Coerce::new(self, cause);
715 let ok = self.commit_if_ok(|_| coerce.coerce(&[expr], source, target))?;
717 let adjustment = self.register_infer_ok_obligations(ok);
718 self.apply_adjustment(expr.id, adjustment);
720 // We should now have added sufficient adjustments etc to
721 // ensure that the type of expression, post-adjustment, is
722 // a subtype of target.
726 /// Same as `try_coerce()`, but without side-effects.
727 pub fn can_coerce(&self, expr_ty: Ty<'tcx>, target: Ty<'tcx>) -> bool {
728 let source = self.resolve_type_vars_with_obligations(expr_ty);
729 debug!("coercion::can({:?} -> {:?})", source, target);
731 let cause = self.cause(syntax_pos::DUMMY_SP, ObligationCauseCode::ExprAssignable);
732 let coerce = Coerce::new(self, cause);
733 self.probe(|_| coerce.coerce::<hir::Expr>(&[], source, target)).is_ok()
736 /// Given some expressions, their known unified type and another expression,
737 /// tries to unify the types, potentially inserting coercions on any of the
738 /// provided expressions and returns their LUB (aka "common supertype").
740 /// This is really an internal helper. From outside the coercion
741 /// module, you should instantiate a `CoerceMany` instance.
742 fn try_find_coercion_lub<E>(&self,
743 cause: &ObligationCause<'tcx>,
748 new_diverges: Diverges)
749 -> RelateResult<'tcx, Ty<'tcx>>
750 where E: AsCoercionSite
752 let prev_ty = self.resolve_type_vars_with_obligations(prev_ty);
753 let new_ty = self.resolve_type_vars_with_obligations(new_ty);
754 debug!("coercion::try_find_coercion_lub({:?}, {:?})", prev_ty, new_ty);
756 // Special-ish case: we can coerce any type `T` into the `!`
757 // type, but only if the source expression diverges.
758 if prev_ty.is_never() && new_diverges.always() {
759 debug!("permit coercion to `!` because expr diverges");
763 let trace = TypeTrace::types(cause, true, prev_ty, new_ty);
765 // Special-case that coercion alone cannot handle:
766 // Two function item types of differing IDs or Substs.
767 match (&prev_ty.sty, &new_ty.sty) {
768 (&ty::TyFnDef(a_def_id, a_substs, a_fty), &ty::TyFnDef(b_def_id, b_substs, b_fty)) => {
769 // The signature must always match.
770 let fty = self.lub(true, trace.clone(), &a_fty, &b_fty)
771 .map(|ok| self.register_infer_ok_obligations(ok))?;
773 if a_def_id == b_def_id {
774 // Same function, maybe the parameters match.
775 let substs = self.commit_if_ok(|_| {
776 self.lub(true, trace.clone(), &a_substs, &b_substs)
777 .map(|ok| self.register_infer_ok_obligations(ok))
780 if let Ok(substs) = substs {
781 // We have a LUB of prev_ty and new_ty, just return it.
782 return Ok(self.tcx.mk_fn_def(a_def_id, substs, fty));
786 // Reify both sides and return the reified fn pointer type.
787 let fn_ptr = self.tcx.mk_fn_ptr(fty);
788 for expr in exprs.iter().map(|e| e.as_coercion_site()).chain(Some(new)) {
789 // The only adjustment that can produce an fn item is
790 // `NeverToAny`, so this should always be valid.
791 self.apply_adjustment(expr.id, Adjustment {
792 kind: Adjust::ReifyFnPointer,
801 let mut coerce = Coerce::new(self, cause.clone());
802 coerce.use_lub = true;
804 // First try to coerce the new expression to the type of the previous ones,
805 // but only if the new expression has no coercion already applied to it.
806 let mut first_error = None;
807 if !self.tables.borrow().adjustments.contains_key(&new.id) {
808 let result = self.commit_if_ok(|_| coerce.coerce(&[new], new_ty, prev_ty));
811 let adjustment = self.register_infer_ok_obligations(ok);
812 self.apply_adjustment(new.id, adjustment);
813 return Ok(adjustment.target);
815 Err(e) => first_error = Some(e),
819 // Then try to coerce the previous expressions to the type of the new one.
820 // This requires ensuring there are no coercions applied to *any* of the
821 // previous expressions, other than noop reborrows (ignoring lifetimes).
823 let expr = expr.as_coercion_site();
824 let noop = match self.tables.borrow().adjustments.get(&expr.id).map(|adj| adj.kind) {
825 Some(Adjust::DerefRef {
827 autoref: Some(AutoBorrow::Ref(_, mutbl_adj)),
830 match self.node_ty(expr.id).sty {
831 ty::TyRef(_, mt_orig) => {
832 // Reborrow that we can safely ignore, because
833 // the next adjustment can only be a DerefRef
834 // which will be merged into it.
835 mutbl_adj == mt_orig.mutbl
840 Some(Adjust::NeverToAny) => true,
846 return self.commit_if_ok(|_| {
847 self.lub(true, trace.clone(), &prev_ty, &new_ty)
848 .map(|ok| self.register_infer_ok_obligations(ok))
853 match self.commit_if_ok(|_| coerce.coerce(&exprs, prev_ty, new_ty)) {
855 // Avoid giving strange errors on failed attempts.
856 if let Some(e) = first_error {
859 self.commit_if_ok(|_| {
860 self.lub(true, trace, &prev_ty, &new_ty)
861 .map(|ok| self.register_infer_ok_obligations(ok))
866 let adjustment = self.register_infer_ok_obligations(ok);
868 let expr = expr.as_coercion_site();
869 self.apply_adjustment(expr.id, adjustment);
871 Ok(adjustment.target)
877 /// CoerceMany encapsulates the pattern you should use when you have
878 /// many expressions that are all getting coerced to a common
879 /// type. This arises, for example, when you have a match (the result
880 /// of each arm is coerced to a common type). It also arises in less
881 /// obvious places, such as when you have many `break foo` expressions
882 /// that target the same loop, or the various `return` expressions in
885 /// The basic protocol is as follows:
887 /// - Instantiate the `CoerceMany` with an initial `expected_ty`.
888 /// This will also serve as the "starting LUB". The expectation is
889 /// that this type is something which all of the expressions *must*
890 /// be coercible to. Use a fresh type variable if needed.
891 /// - For each expression whose result is to be coerced, invoke `coerce()` with.
892 /// - In some cases we wish to coerce "non-expressions" whose types are implicitly
893 /// unit. This happens for example if you have a `break` with no expression,
894 /// or an `if` with no `else`. In that case, invoke `coerce_forced_unit()`.
895 /// - `coerce()` and `coerce_forced_unit()` may report errors. They hide this
896 /// from you so that you don't have to worry your pretty head about it.
897 /// But if an error is reported, the final type will be `err`.
898 /// - Invoking `coerce()` may cause us to go and adjust the "adjustments" on
899 /// previously coerced expressions.
900 /// - When all done, invoke `complete()`. This will return the LUB of
901 /// all your expressions.
902 /// - WARNING: I don't believe this final type is guaranteed to be
903 /// related to your initial `expected_ty` in any particular way,
904 /// although it will typically be a subtype, so you should check it.
905 /// - Invoking `complete()` may cause us to go and adjust the "adjustments" on
906 /// previously coerced expressions.
911 /// let mut coerce = CoerceMany::new(expected_ty);
912 /// for expr in exprs {
913 /// let expr_ty = fcx.check_expr_with_expectation(expr, expected);
914 /// coerce.coerce(fcx, &cause, expr, expr_ty);
916 /// let final_ty = coerce.complete(fcx);
918 pub struct CoerceMany<'gcx, 'tcx, 'exprs, E>
919 where 'gcx: 'tcx, E: 'exprs + AsCoercionSite,
921 expected_ty: Ty<'tcx>,
922 final_ty: Option<Ty<'tcx>>,
923 expressions: Expressions<'gcx, 'exprs, E>,
927 /// The type of a `CoerceMany` that is storing up the expressions into
928 /// a buffer. We use this in `check/mod.rs` for things like `break`.
929 pub type DynamicCoerceMany<'gcx, 'tcx> = CoerceMany<'gcx, 'tcx, 'gcx, P<hir::Expr>>;
931 enum Expressions<'gcx, 'exprs, E>
932 where E: 'exprs + AsCoercionSite,
934 Dynamic(Vec<&'gcx hir::Expr>),
935 UpFront(&'exprs [E]),
938 impl<'gcx, 'tcx, 'exprs, E> CoerceMany<'gcx, 'tcx, 'exprs, E>
939 where 'gcx: 'tcx, E: 'exprs + AsCoercionSite,
941 /// The usual case; collect the set of expressions dynamically.
942 /// If the full set of coercion sites is known before hand,
943 /// consider `with_coercion_sites()` instead to avoid allocation.
944 pub fn new(expected_ty: Ty<'tcx>) -> Self {
945 Self::make(expected_ty, Expressions::Dynamic(vec![]))
948 /// As an optimization, you can create a `CoerceMany` with a
949 /// pre-existing slice of expressions. In this case, you are
950 /// expected to pass each element in the slice to `coerce(...)` in
951 /// order. This is used with arrays in particular to avoid
952 /// needlessly cloning the slice.
953 pub fn with_coercion_sites(expected_ty: Ty<'tcx>,
954 coercion_sites: &'exprs [E])
956 Self::make(expected_ty, Expressions::UpFront(coercion_sites))
959 fn make(expected_ty: Ty<'tcx>, expressions: Expressions<'gcx, 'exprs, E>) -> Self {
968 pub fn is_empty(&self) -> bool {
972 /// Return the "expected type" with which this coercion was
973 /// constructed. This represents the "downward propagated" type
974 /// that was given to us at the start of typing whatever construct
975 /// we are typing (e.g., the match expression).
977 /// Typically, this is used as the expected type when
978 /// type-checking each of the alternative expressions whose types
979 /// we are trying to merge.
980 pub fn expected_ty(&self) -> Ty<'tcx> {
984 /// Returns the current "merged type", representing our best-guess
985 /// at the LUB of the expressions we've seen so far (if any). This
986 /// isn't *final* until you call `self.final()`, which will return
988 pub fn merged_ty(&self) -> Ty<'tcx> {
989 self.final_ty.unwrap_or(self.expected_ty)
992 /// Indicates that the value generated by `expression`, which is
993 /// of type `expression_ty`, is one of the possibility that we
994 /// could coerce from. This will record `expression` and later
995 /// calls to `coerce` may come back and add adjustments and things
997 pub fn coerce<'a>(&mut self,
998 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
999 cause: &ObligationCause<'tcx>,
1000 expression: &'gcx hir::Expr,
1001 expression_ty: Ty<'tcx>,
1002 expression_diverges: Diverges)
1004 self.coerce_inner(fcx, cause, Some(expression), expression_ty, expression_diverges, None)
1007 /// Indicates that one of the inputs is a "forced unit". This
1008 /// occurs in a case like `if foo { ... };`, where the issing else
1009 /// generates a "forced unit". Another example is a `loop { break;
1010 /// }`, where the `break` has no argument expression. We treat
1011 /// these cases slightly differently for error-reporting
1012 /// purposes. Note that these tend to correspond to cases where
1013 /// the `()` expression is implicit in the source, and hence we do
1014 /// not take an expression argument.
1016 /// The `augment_error` gives you a chance to extend the error
1017 /// message, in case any results (e.g., we use this to suggest
1018 /// removing a `;`).
1019 pub fn coerce_forced_unit<'a>(&mut self,
1020 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1021 cause: &ObligationCause<'tcx>,
1022 augment_error: &mut FnMut(&mut DiagnosticBuilder))
1024 self.coerce_inner(fcx,
1029 Some(augment_error))
1032 /// The inner coercion "engine". If `expression` is `None`, this
1033 /// is a forced-unit case, and hence `expression_ty` must be
1035 fn coerce_inner<'a>(&mut self,
1036 fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1037 cause: &ObligationCause<'tcx>,
1038 expression: Option<&'gcx hir::Expr>,
1039 mut expression_ty: Ty<'tcx>,
1040 expression_diverges: Diverges,
1041 augment_error: Option<&mut FnMut(&mut DiagnosticBuilder)>)
1043 // Incorporate whatever type inference information we have
1044 // until now; in principle we might also want to process
1045 // pending obligations, but doing so should only improve
1046 // compatibility (hopefully that is true) by helping us
1047 // uncover never types better.
1048 if expression_ty.is_ty_var() {
1049 expression_ty = fcx.infcx.shallow_resolve(expression_ty);
1052 // If we see any error types, just propagate that error
1054 if expression_ty.references_error() || self.merged_ty().references_error() {
1055 self.final_ty = Some(fcx.tcx.types.err);
1059 // Handle the actual type unification etc.
1060 let result = if let Some(expression) = expression {
1061 if self.pushed == 0 {
1062 // Special-case the first expression we are coercing.
1063 // To be honest, I'm not entirely sure why we do this.
1064 fcx.try_coerce(expression, expression_ty, expression_diverges, self.expected_ty)
1066 match self.expressions {
1067 Expressions::Dynamic(ref exprs) =>
1068 fcx.try_find_coercion_lub(cause,
1073 expression_diverges),
1074 Expressions::UpFront(ref coercion_sites) =>
1075 fcx.try_find_coercion_lub(cause,
1076 &coercion_sites[0..self.pushed],
1080 expression_diverges),
1084 // this is a hack for cases where we default to `()` because
1085 // the expression etc has been omitted from the source. An
1086 // example is an `if let` without an else:
1088 // if let Some(x) = ... { }
1090 // we wind up with a second match arm that is like `_ =>
1091 // ()`. That is the case we are considering here. We take
1092 // a different path to get the right "expected, found"
1093 // message and so forth (and because we know that
1094 // `expression_ty` will be unit).
1096 // Another example is `break` with no argument expression.
1097 assert!(expression_ty.is_nil());
1098 assert!(expression_ty.is_nil(), "if let hack without unit type");
1099 fcx.eq_types(true, cause, expression_ty, self.merged_ty())
1101 fcx.register_infer_ok_obligations(infer_ok);
1108 self.final_ty = Some(v);
1109 if let Some(e) = expression {
1110 match self.expressions {
1111 Expressions::Dynamic(ref mut buffer) => buffer.push(e),
1112 Expressions::UpFront(coercion_sites) => {
1113 // if the user gave us an array to validate, check that we got
1114 // the next expression in the list, as expected
1115 assert_eq!(coercion_sites[self.pushed].as_coercion_site().id, e.id);
1122 let (expected, found) = if expression.is_none() {
1123 // In the case where this is a "forced unit", like
1124 // `break`, we want to call the `()` "expected"
1125 // since it is implied by the syntax.
1126 assert!(expression_ty.is_nil());
1127 (expression_ty, self.final_ty.unwrap_or(self.expected_ty))
1129 // Otherwise, the "expected" type for error
1130 // reporting is the current unification type,
1131 // which is basically the LUB of the expressions
1132 // we've seen so far (combined with the expected
1134 (self.final_ty.unwrap_or(self.expected_ty), expression_ty)
1139 ObligationCauseCode::ReturnNoExpression => {
1140 db = struct_span_err!(
1141 fcx.tcx.sess, cause.span, E0069,
1142 "`return;` in a function whose return type is not `()`");
1143 db.span_label(cause.span, &format!("return type is not ()"));
1146 db = fcx.report_mismatched_types(cause, expected, found, err);
1150 if let Some(mut augment_error) = augment_error {
1151 augment_error(&mut db);
1156 self.final_ty = Some(fcx.tcx.types.err);
1161 pub fn complete<'a>(self, fcx: &FnCtxt<'a, 'gcx, 'tcx>) -> Ty<'tcx> {
1162 if let Some(final_ty) = self.final_ty {
1165 // If we only had inputs that were of type `!` (or no
1166 // inputs at all), then the final type is `!`.
1167 assert_eq!(self.pushed, 0);
1173 /// Something that can be converted into an expression to which we can
1174 /// apply a coercion.
1175 pub trait AsCoercionSite {
1176 fn as_coercion_site(&self) -> &hir::Expr;
1179 impl AsCoercionSite for hir::Expr {
1180 fn as_coercion_site(&self) -> &hir::Expr {
1185 impl AsCoercionSite for P<hir::Expr> {
1186 fn as_coercion_site(&self) -> &hir::Expr {
1191 impl<'a, T> AsCoercionSite for &'a T
1192 where T: AsCoercionSite
1194 fn as_coercion_site(&self) -> &hir::Expr {
1195 (**self).as_coercion_site()
1199 impl AsCoercionSite for ! {
1200 fn as_coercion_site(&self) -> &hir::Expr {
1205 impl AsCoercionSite for hir::Arm {
1206 fn as_coercion_site(&self) -> &hir::Expr {