1 use crate::errors::OpaqueHiddenTypeDiag;
2 use crate::infer::{DefiningAnchor, InferCtxt, InferOk};
4 use hir::def_id::{DefId, LocalDefId};
5 use hir::{HirId, OpaqueTyOrigin};
6 use rustc_data_structures::sync::Lrc;
7 use rustc_data_structures::vec_map::VecMap;
9 use rustc_middle::traits::ObligationCause;
10 use rustc_middle::ty::error::{ExpectedFound, TypeError};
11 use rustc_middle::ty::fold::BottomUpFolder;
12 use rustc_middle::ty::GenericArgKind;
13 use rustc_middle::ty::{
14 self, OpaqueHiddenType, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable,
15 TypeVisitable, TypeVisitor,
19 use std::ops::ControlFlow;
21 pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;
25 pub use table::{OpaqueTypeStorage, OpaqueTypeTable};
27 use super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
28 use super::InferResult;
30 /// Information about the opaque types whose values we
31 /// are inferring in this function (these are the `impl Trait` that
32 /// appear in the return type).
33 #[derive(Clone, Debug)]
34 pub struct OpaqueTypeDecl<'tcx> {
35 /// The hidden types that have been inferred for this opaque type.
36 /// There can be multiple, but they are all `lub`ed together at the end
37 /// to obtain the canonical hidden type.
38 pub hidden_type: OpaqueHiddenType<'tcx>,
40 /// The origin of the opaque type.
41 pub origin: hir::OpaqueTyOrigin,
44 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
45 /// This is a backwards compatibility hack to prevent breaking changes from
46 /// lazy TAIT around RPIT handling.
47 pub fn replace_opaque_types_with_inference_vars<T: TypeFoldable<'tcx>>(
52 param_env: ty::ParamEnv<'tcx>,
53 ) -> InferOk<'tcx, T> {
54 if !value.has_opaque_types() {
55 return InferOk { value, obligations: vec![] };
57 let mut obligations = vec![];
58 let replace_opaque_type = |def_id: DefId| {
61 .map_or(false, |def_id| self.opaque_type_origin(def_id, span).is_some())
63 let value = value.fold_with(&mut ty::fold::BottomUpFolder {
67 ty_op: |ty| match *ty.kind() {
68 ty::Opaque(def_id, _substs) if replace_opaque_type(def_id) => {
69 let def_span = self.tcx.def_span(def_id);
70 let span = if span.contains(def_span) { def_span } else { span };
71 let code = traits::ObligationCauseCode::OpaqueReturnType(None);
72 let cause = ObligationCause::new(span, body_id, code);
73 // FIXME(compiler-errors): We probably should add a new TypeVariableOriginKind
74 // for opaque types, and then use that kind to fix the spans for type errors
75 // that we see later on.
76 let ty_var = self.next_ty_var(TypeVariableOrigin {
77 kind: TypeVariableOriginKind::OpaqueTypeInference(def_id),
81 self.handle_opaque_type(ty, ty_var, true, &cause, param_env)
90 InferOk { value, obligations }
93 pub fn handle_opaque_type(
98 cause: &ObligationCause<'tcx>,
99 param_env: ty::ParamEnv<'tcx>,
100 ) -> InferResult<'tcx, ()> {
101 if a.references_error() || b.references_error() {
102 return Ok(InferOk { value: (), obligations: vec![] });
104 let (a, b) = if a_is_expected { (a, b) } else { (b, a) };
105 let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() {
106 ty::Opaque(def_id, substs) if def_id.is_local() => {
107 let def_id = def_id.expect_local();
108 let origin = match self.defining_use_anchor {
109 DefiningAnchor::Bind(_) => {
110 // Check that this is `impl Trait` type is
111 // declared by `parent_def_id` -- i.e., one whose
112 // value we are inferring. At present, this is
113 // always true during the first phase of
114 // type-check, but not always true later on during
115 // NLL. Once we support named opaque types more fully,
116 // this same scenario will be able to arise during all phases.
118 // Here is an example using type alias `impl Trait`
119 // that indicates the distinction we are checking for:
123 // pub type Foo = impl Iterator;
124 // pub fn make_foo() -> Foo { .. }
128 // fn foo() -> a::Foo { a::make_foo() }
132 // Here, the return type of `foo` references an
133 // `Opaque` indeed, but not one whose value is
134 // presently being inferred. You can get into a
135 // similar situation with closure return types
139 // fn foo() -> impl Iterator { .. }
141 // let x = || foo(); // returns the Opaque assoc with `foo`
144 self.opaque_type_origin(def_id, cause.span)?
146 DefiningAnchor::Bubble => self.opaque_ty_origin_unchecked(def_id, cause.span),
147 DefiningAnchor::Error => return None,
149 if let ty::Opaque(did2, _) = *b.kind() {
150 // We could accept this, but there are various ways to handle this situation, and we don't
151 // want to make a decision on it right now. Likely this case is so super rare anyway, that
152 // no one encounters it in practice.
153 // It does occur however in `fn fut() -> impl Future<Output = i32> { async { 42 } }`,
154 // where it is of no concern, so we only check for TAITs.
155 if let Some(OpaqueTyOrigin::TyAlias) =
156 did2.as_local().and_then(|did2| self.opaque_type_origin(did2, cause.span))
158 self.tcx.sess.emit_err(OpaqueHiddenTypeDiag {
160 hidden_type: self.tcx.def_span(did2),
161 opaque_type: self.tcx.def_span(def_id),
165 Some(self.register_hidden_type(
166 OpaqueTypeKey { def_id, substs },
175 if let Some(res) = process(a, b) {
177 } else if let Some(res) = process(b, a) {
180 let (a, b) = self.resolve_vars_if_possible((a, b));
181 Err(TypeError::Sorts(ExpectedFound::new(true, a, b)))
185 /// Given the map `opaque_types` containing the opaque
186 /// `impl Trait` types whose underlying, hidden types are being
187 /// inferred, this method adds constraints to the regions
188 /// appearing in those underlying hidden types to ensure that they
189 /// at least do not refer to random scopes within the current
190 /// function. These constraints are not (quite) sufficient to
191 /// guarantee that the regions are actually legal values; that
192 /// final condition is imposed after region inference is done.
196 /// Let's work through an example to explain how it works. Assume
197 /// the current function is as follows:
200 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
203 /// Here, we have two `impl Trait` types whose values are being
204 /// inferred (the `impl Bar<'a>` and the `impl
205 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
206 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
207 /// the return type of `foo`, we *reference* those definitions:
210 /// type Foo1<'x> = impl Bar<'x>;
211 /// type Foo2<'x> = impl Bar<'x>;
212 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
219 /// As indicating in the comments above, each of those references
220 /// is (in the compiler) basically a substitution (`substs`)
221 /// applied to the type of a suitable `def_id` (which identifies
222 /// `Foo1` or `Foo2`).
224 /// Now, at this point in compilation, what we have done is to
225 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
226 /// fresh inference variables C1 and C2. We wish to use the values
227 /// of these variables to infer the underlying types of `Foo1` and
228 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
229 /// constraints like:
232 /// for<'a> (Foo1<'a> = C1)
233 /// for<'b> (Foo1<'b> = C2)
236 /// For these equation to be satisfiable, the types `C1` and `C2`
237 /// can only refer to a limited set of regions. For example, `C1`
238 /// can only refer to `'static` and `'a`, and `C2` can only refer
239 /// to `'static` and `'b`. The job of this function is to impose that
242 /// Up to this point, C1 and C2 are basically just random type
243 /// inference variables, and hence they may contain arbitrary
244 /// regions. In fact, it is fairly likely that they do! Consider
245 /// this possible definition of `foo`:
248 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
253 /// Here, the values for the concrete types of the two impl
254 /// traits will include inference variables:
261 /// Ordinarily, the subtyping rules would ensure that these are
262 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
263 /// type per se, we don't get such constraints by default. This
264 /// is where this function comes into play. It adds extra
265 /// constraints to ensure that all the regions which appear in the
266 /// inferred type are regions that could validly appear.
268 /// This is actually a bit of a tricky constraint in general. We
269 /// want to say that each variable (e.g., `'0`) can only take on
270 /// values that were supplied as arguments to the opaque type
271 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
272 /// scope. We don't have a constraint quite of this kind in the current
277 /// We generally prefer to make `<=` constraints, since they
278 /// integrate best into the region solver. To do that, we find the
279 /// "minimum" of all the arguments that appear in the substs: that
280 /// is, some region which is less than all the others. In the case
281 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
282 /// all). Then we apply that as a least bound to the variables
283 /// (e.g., `'a <= '0`).
285 /// In some cases, there is no minimum. Consider this example:
288 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
291 /// Here we would report a more complex "in constraint", like `'r
292 /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
293 /// the hidden type).
295 /// # Constrain regions, not the hidden concrete type
297 /// Note that generating constraints on each region `Rc` is *not*
298 /// the same as generating an outlives constraint on `Tc` itself.
299 /// For example, if we had a function like this:
302 /// # #![feature(type_alias_impl_trait)]
304 /// # trait Foo<'a> {}
305 /// # impl<'a, T> Foo<'a> for (&'a u32, T) {}
306 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
310 /// // Equivalent to:
311 /// # mod dummy { use super::*;
312 /// type FooReturn<'a, T> = impl Foo<'a>;
313 /// fn foo<'a, T>(x: &'a u32, y: T) -> FooReturn<'a, T> {
319 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
320 /// is an inference variable). If we generated a constraint that
321 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
322 /// but this is not necessary, because the opaque type we
323 /// create will be allowed to reference `T`. So we only generate a
324 /// constraint that `'0: 'a`.
325 #[instrument(level = "debug", skip(self))]
326 pub fn register_member_constraints(
328 param_env: ty::ParamEnv<'tcx>,
329 opaque_type_key: OpaqueTypeKey<'tcx>,
330 concrete_ty: Ty<'tcx>,
333 let def_id = opaque_type_key.def_id;
337 let concrete_ty = self.resolve_vars_if_possible(concrete_ty);
339 debug!(?concrete_ty);
341 let first_own_region = match self.opaque_ty_origin_unchecked(def_id, span) {
342 hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {
345 // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
349 // type foo::<'p0..'pn>::Foo<'q0..'qm>
350 // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
352 // For these types we only iterate over `'l0..lm` below.
353 tcx.generics_of(def_id).parent_count
355 // These opaque type inherit all lifetime parameters from their
356 // parent, so we have to check them all.
357 hir::OpaqueTyOrigin::TyAlias => 0,
360 // For a case like `impl Foo<'a, 'b>`, we would generate a constraint
361 // `'r in ['a, 'b, 'static]` for each region `'r` that appears in the
362 // hidden type (i.e., it must be equal to `'a`, `'b`, or `'static`).
364 // `conflict1` and `conflict2` are the two region bounds that we
365 // detected which were unrelated. They are used for diagnostics.
367 // Create the set of choice regions: each region in the hidden
368 // type can be equal to any of the region parameters of the
369 // opaque type definition.
370 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
371 opaque_type_key.substs[first_own_region..]
373 .filter_map(|arg| match arg.unpack() {
374 GenericArgKind::Lifetime(r) => Some(r),
375 GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
377 .chain(std::iter::once(self.tcx.lifetimes.re_static))
381 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
382 op: |r| self.member_constraint(opaque_type_key, span, concrete_ty, r, &choice_regions),
386 #[instrument(skip(self), level = "trace", ret)]
387 pub fn opaque_type_origin(&self, def_id: LocalDefId, span: Span) -> Option<OpaqueTyOrigin> {
388 let opaque_hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
389 let parent_def_id = match self.defining_use_anchor {
390 DefiningAnchor::Bubble | DefiningAnchor::Error => return None,
391 DefiningAnchor::Bind(bind) => bind,
393 let item_kind = &self.tcx.hir().expect_item(def_id).kind;
395 let hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) = item_kind else {
398 "weird opaque type: {:#?}, {:#?}",
403 let in_definition_scope = match *origin {
404 // Async `impl Trait`
405 hir::OpaqueTyOrigin::AsyncFn(parent) => parent == parent_def_id,
406 // Anonymous `impl Trait`
407 hir::OpaqueTyOrigin::FnReturn(parent) => parent == parent_def_id,
408 // Named `type Foo = impl Bar;`
409 hir::OpaqueTyOrigin::TyAlias => {
410 may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id)
414 in_definition_scope.then_some(*origin)
417 #[instrument(skip(self), level = "trace", ret)]
418 fn opaque_ty_origin_unchecked(&self, def_id: LocalDefId, span: Span) -> OpaqueTyOrigin {
419 match self.tcx.hir().expect_item(def_id).kind {
420 hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => origin,
422 span_bug!(span, "weird opaque type: {:?}, {:#?}", def_id, itemkind)
428 // Visitor that requires that (almost) all regions in the type visited outlive
429 // `least_region`. We cannot use `push_outlives_components` because regions in
430 // closure signatures are not included in their outlives components. We need to
431 // ensure all regions outlive the given bound so that we don't end up with,
432 // say, `ReVar` appearing in a return type and causing ICEs when other
433 // functions end up with region constraints involving regions from other
436 // We also cannot use `for_each_free_region` because for closures it includes
437 // the regions parameters from the enclosing item.
439 // We ignore any type parameters because impl trait values are assumed to
440 // capture all the in-scope type parameters.
441 struct ConstrainOpaqueTypeRegionVisitor<OP> {
445 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
447 OP: FnMut(ty::Region<'tcx>),
449 fn visit_binder<T: TypeVisitable<'tcx>>(
451 t: &ty::Binder<'tcx, T>,
452 ) -> ControlFlow<Self::BreakTy> {
453 t.super_visit_with(self);
454 ControlFlow::CONTINUE
457 fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
459 // ignore bound regions, keep visiting
460 ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
463 ControlFlow::CONTINUE
468 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
469 // We're only interested in types involving regions
470 if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
471 return ControlFlow::CONTINUE;
475 ty::Closure(_, ref substs) => {
476 // Skip lifetime parameters of the enclosing item(s)
478 substs.as_closure().tupled_upvars_ty().visit_with(self);
479 substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
482 ty::Generator(_, ref substs, _) => {
483 // Skip lifetime parameters of the enclosing item(s)
484 // Also skip the witness type, because that has no free regions.
486 substs.as_generator().tupled_upvars_ty().visit_with(self);
487 substs.as_generator().return_ty().visit_with(self);
488 substs.as_generator().yield_ty().visit_with(self);
489 substs.as_generator().resume_ty().visit_with(self);
492 ty.super_visit_with(self);
496 ControlFlow::CONTINUE
506 pub fn is_defining(self) -> bool {
508 UseKind::DefiningUse => true,
509 UseKind::OpaqueUse => false,
514 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
515 #[instrument(skip(self), level = "debug")]
516 pub fn register_hidden_type(
518 opaque_type_key: OpaqueTypeKey<'tcx>,
519 cause: ObligationCause<'tcx>,
520 param_env: ty::ParamEnv<'tcx>,
522 origin: hir::OpaqueTyOrigin,
523 ) -> InferResult<'tcx, ()> {
525 let OpaqueTypeKey { def_id, substs } = opaque_type_key;
527 // Ideally, we'd get the span where *this specific `ty` came
528 // from*, but right now we just use the span from the overall
529 // value being folded. In simple cases like `-> impl Foo`,
530 // these are the same span, but not in cases like `-> (impl
532 let span = cause.span;
534 let mut obligations = vec![];
535 let prev = self.inner.borrow_mut().opaque_types().register(
536 OpaqueTypeKey { def_id, substs },
537 OpaqueHiddenType { ty: hidden_ty, span },
540 if let Some(prev) = prev {
541 obligations = self.at(&cause, param_env).eq(prev, hidden_ty)?.obligations;
544 let item_bounds = tcx.bound_explicit_item_bounds(def_id.to_def_id());
546 for predicate in item_bounds.transpose_iter().map(|e| e.map_bound(|(p, _)| *p)) {
548 let predicate = predicate.subst(tcx, substs);
550 let predicate = predicate.fold_with(&mut BottomUpFolder {
552 ty_op: |ty| match *ty.kind() {
553 // We can't normalize associated types from `rustc_infer`,
554 // but we can eagerly register inference variables for them.
555 ty::Projection(projection_ty) if !projection_ty.has_escaping_bound_vars() => {
556 self.infer_projection(
564 // Replace all other mentions of the same opaque type with the hidden type,
565 // as the bounds must hold on the hidden type after all.
566 ty::Opaque(def_id2, substs2)
567 if def_id.to_def_id() == def_id2 && substs == substs2 =>
577 if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
578 if projection.term.references_error() {
579 // No point on adding these obligations since there's a type error involved.
580 return Ok(InferOk { value: (), obligations: vec![] });
582 trace!("{:#?}", projection.term);
584 // Require that the predicate holds for the concrete type.
586 obligations.push(traits::Obligation::new(cause.clone(), param_env, predicate));
588 Ok(InferOk { value: (), obligations })
592 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
595 /// ```ignore UNSOLVED (is this a bug?)
596 /// # #![feature(type_alias_impl_trait)]
599 /// pub trait Bar { /* ... */ }
600 /// pub type Baz = impl Bar;
602 /// # impl Bar for () {}
603 /// fn f1() -> Baz { /* ... */ }
605 /// fn f2() -> bar::Baz { /* ... */ }
609 /// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
610 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
611 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
612 fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: LocalDefId, opaque_hir_id: hir::HirId) -> bool {
613 let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
615 // Named opaque types can be defined by any siblings or children of siblings.
616 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
617 // We walk up the node tree until we hit the root or the scope of the opaque type.
618 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
619 hir_id = tcx.hir().local_def_id_to_hir_id(tcx.hir().get_parent_item(hir_id));
621 // Syntactically, we are allowed to define the concrete type if:
622 let res = hir_id == scope;
624 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
625 tcx.hir().find(hir_id),
626 tcx.hir().get(opaque_hir_id),