1 use crate::infer::{InferCtxt, InferOk};
3 use hir::def_id::{DefId, LocalDefId};
4 use hir::{HirId, OpaqueTyOrigin};
5 use rustc_data_structures::sync::Lrc;
6 use rustc_data_structures::vec_map::VecMap;
8 use rustc_middle::traits::ObligationCause;
9 use rustc_middle::ty::fold::BottomUpFolder;
10 use rustc_middle::ty::subst::{GenericArgKind, Subst};
11 use rustc_middle::ty::{
12 self, OpaqueHiddenType, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable,
13 TypeVisitable, TypeVisitor,
17 use std::ops::ControlFlow;
19 pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;
23 pub use table::{OpaqueTypeStorage, OpaqueTypeTable};
25 use super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
26 use super::InferResult;
28 /// Information about the opaque types whose values we
29 /// are inferring in this function (these are the `impl Trait` that
30 /// appear in the return type).
31 #[derive(Clone, Debug)]
32 pub struct OpaqueTypeDecl<'tcx> {
33 /// The hidden types that have been inferred for this opaque type.
34 /// There can be multiple, but they are all `lub`ed together at the end
35 /// to obtain the canonical hidden type.
36 pub hidden_type: OpaqueHiddenType<'tcx>,
38 /// The origin of the opaque type.
39 pub origin: hir::OpaqueTyOrigin,
42 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
43 pub fn replace_opaque_types_with_inference_vars(
48 param_env: ty::ParamEnv<'tcx>,
49 ) -> InferOk<'tcx, Ty<'tcx>> {
50 if !ty.has_opaque_types() {
51 return InferOk { value: ty, obligations: vec![] };
53 let mut obligations = vec![];
54 let replace_opaque_type = |def_id| self.opaque_type_origin(def_id, span).is_some();
55 let value = ty.fold_with(&mut ty::fold::BottomUpFolder {
59 ty_op: |ty| match *ty.kind() {
60 ty::Opaque(def_id, _substs) if replace_opaque_type(def_id) => {
61 let def_span = self.tcx.def_span(def_id);
62 let span = if span.contains(def_span) { def_span } else { span };
63 let code = traits::ObligationCauseCode::OpaqueReturnType(None);
64 let cause = ObligationCause::new(span, body_id, code);
65 // FIXME(compiler-errors): We probably should add a new TypeVariableOriginKind
66 // for opaque types, and then use that kind to fix the spans for type errors
67 // that we see later on.
68 let ty_var = self.next_ty_var(TypeVariableOrigin {
69 kind: TypeVariableOriginKind::TypeInference,
73 self.handle_opaque_type(ty, ty_var, true, &cause, param_env)
82 InferOk { value, obligations }
85 pub fn handle_opaque_type(
90 cause: &ObligationCause<'tcx>,
91 param_env: ty::ParamEnv<'tcx>,
92 ) -> InferResult<'tcx, ()> {
93 if a.references_error() || b.references_error() {
94 return Ok(InferOk { value: (), obligations: vec![] });
96 let (a, b) = if a_is_expected { (a, b) } else { (b, a) };
97 let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() {
98 ty::Opaque(def_id, substs) => {
99 let origin = if self.defining_use_anchor.is_some() {
100 // Check that this is `impl Trait` type is
101 // declared by `parent_def_id` -- i.e., one whose
102 // value we are inferring. At present, this is
103 // always true during the first phase of
104 // type-check, but not always true later on during
105 // NLL. Once we support named opaque types more fully,
106 // this same scenario will be able to arise during all phases.
108 // Here is an example using type alias `impl Trait`
109 // that indicates the distinction we are checking for:
113 // pub type Foo = impl Iterator;
114 // pub fn make_foo() -> Foo { .. }
118 // fn foo() -> a::Foo { a::make_foo() }
122 // Here, the return type of `foo` references an
123 // `Opaque` indeed, but not one whose value is
124 // presently being inferred. You can get into a
125 // similar situation with closure return types
129 // fn foo() -> impl Iterator { .. }
131 // let x = || foo(); // returns the Opaque assoc with `foo`
134 self.opaque_type_origin(def_id, cause.span)?
136 self.opaque_ty_origin_unchecked(def_id, cause.span)
138 if let ty::Opaque(did2, _) = *b.kind() {
139 // We could accept this, but there are various ways to handle this situation, and we don't
140 // want to make a decision on it right now. Likely this case is so super rare anyway, that
141 // no one encounters it in practice.
142 // It does occur however in `fn fut() -> impl Future<Output = i32> { async { 42 } }`,
143 // where it is of no concern, so we only check for TAITs.
144 if let Some(OpaqueTyOrigin::TyAlias) = self.opaque_type_origin(did2, cause.span)
150 "opaque type's hidden type cannot be another opaque type from the same scope",
152 .span_label(cause.span, "one of the two opaque types used here has to be outside its defining scope")
154 self.tcx.def_span(def_id),
155 "opaque type whose hidden type is being assigned",
158 self.tcx.def_span(did2),
159 "opaque type being used as hidden type",
164 Some(self.register_hidden_type(
165 OpaqueTypeKey { def_id, substs },
174 if let Some(res) = process(a, b) {
176 } else if let Some(res) = process(b, a) {
179 // Rerun equality check, but this time error out due to
181 match self.at(cause, param_env).define_opaque_types(false).eq(a, b) {
184 "opaque types are never equal to anything but themselves: {:#?}",
192 /// Given the map `opaque_types` containing the opaque
193 /// `impl Trait` types whose underlying, hidden types are being
194 /// inferred, this method adds constraints to the regions
195 /// appearing in those underlying hidden types to ensure that they
196 /// at least do not refer to random scopes within the current
197 /// function. These constraints are not (quite) sufficient to
198 /// guarantee that the regions are actually legal values; that
199 /// final condition is imposed after region inference is done.
203 /// Let's work through an example to explain how it works. Assume
204 /// the current function is as follows:
207 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
210 /// Here, we have two `impl Trait` types whose values are being
211 /// inferred (the `impl Bar<'a>` and the `impl
212 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
213 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
214 /// the return type of `foo`, we *reference* those definitions:
217 /// type Foo1<'x> = impl Bar<'x>;
218 /// type Foo2<'x> = impl Bar<'x>;
219 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
226 /// As indicating in the comments above, each of those references
227 /// is (in the compiler) basically a substitution (`substs`)
228 /// applied to the type of a suitable `def_id` (which identifies
229 /// `Foo1` or `Foo2`).
231 /// Now, at this point in compilation, what we have done is to
232 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
233 /// fresh inference variables C1 and C2. We wish to use the values
234 /// of these variables to infer the underlying types of `Foo1` and
235 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
236 /// constraints like:
239 /// for<'a> (Foo1<'a> = C1)
240 /// for<'b> (Foo1<'b> = C2)
243 /// For these equation to be satisfiable, the types `C1` and `C2`
244 /// can only refer to a limited set of regions. For example, `C1`
245 /// can only refer to `'static` and `'a`, and `C2` can only refer
246 /// to `'static` and `'b`. The job of this function is to impose that
249 /// Up to this point, C1 and C2 are basically just random type
250 /// inference variables, and hence they may contain arbitrary
251 /// regions. In fact, it is fairly likely that they do! Consider
252 /// this possible definition of `foo`:
255 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
260 /// Here, the values for the concrete types of the two impl
261 /// traits will include inference variables:
268 /// Ordinarily, the subtyping rules would ensure that these are
269 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
270 /// type per se, we don't get such constraints by default. This
271 /// is where this function comes into play. It adds extra
272 /// constraints to ensure that all the regions which appear in the
273 /// inferred type are regions that could validly appear.
275 /// This is actually a bit of a tricky constraint in general. We
276 /// want to say that each variable (e.g., `'0`) can only take on
277 /// values that were supplied as arguments to the opaque type
278 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
279 /// scope. We don't have a constraint quite of this kind in the current
284 /// We generally prefer to make `<=` constraints, since they
285 /// integrate best into the region solver. To do that, we find the
286 /// "minimum" of all the arguments that appear in the substs: that
287 /// is, some region which is less than all the others. In the case
288 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
289 /// all). Then we apply that as a least bound to the variables
290 /// (e.g., `'a <= '0`).
292 /// In some cases, there is no minimum. Consider this example:
295 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
298 /// Here we would report a more complex "in constraint", like `'r
299 /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
300 /// the hidden type).
302 /// # Constrain regions, not the hidden concrete type
304 /// Note that generating constraints on each region `Rc` is *not*
305 /// the same as generating an outlives constraint on `Tc` itself.
306 /// For example, if we had a function like this:
309 /// # #![feature(type_alias_impl_trait)]
311 /// # trait Foo<'a> {}
312 /// # impl<'a, T> Foo<'a> for (&'a u32, T) {}
313 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
317 /// // Equivalent to:
318 /// # mod dummy { use super::*;
319 /// type FooReturn<'a, T> = impl Foo<'a>;
320 /// fn foo<'a, T>(x: &'a u32, y: T) -> FooReturn<'a, T> {
326 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
327 /// is an inference variable). If we generated a constraint that
328 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
329 /// but this is not necessary, because the opaque type we
330 /// create will be allowed to reference `T`. So we only generate a
331 /// constraint that `'0: 'a`.
332 #[instrument(level = "debug", skip(self))]
333 pub fn register_member_constraints(
335 param_env: ty::ParamEnv<'tcx>,
336 opaque_type_key: OpaqueTypeKey<'tcx>,
337 concrete_ty: Ty<'tcx>,
340 let def_id = opaque_type_key.def_id;
344 let concrete_ty = self.resolve_vars_if_possible(concrete_ty);
346 debug!(?concrete_ty);
348 let first_own_region = match self.opaque_ty_origin_unchecked(def_id, span) {
349 hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {
352 // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
356 // type foo::<'p0..'pn>::Foo<'q0..'qm>
357 // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
359 // For these types we only iterate over `'l0..lm` below.
360 tcx.generics_of(def_id).parent_count
362 // These opaque type inherit all lifetime parameters from their
363 // parent, so we have to check them all.
364 hir::OpaqueTyOrigin::TyAlias => 0,
367 // For a case like `impl Foo<'a, 'b>`, we would generate a constraint
368 // `'r in ['a, 'b, 'static]` for each region `'r` that appears in the
369 // hidden type (i.e., it must be equal to `'a`, `'b`, or `'static`).
371 // `conflict1` and `conflict2` are the two region bounds that we
372 // detected which were unrelated. They are used for diagnostics.
374 // Create the set of choice regions: each region in the hidden
375 // type can be equal to any of the region parameters of the
376 // opaque type definition.
377 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
378 opaque_type_key.substs[first_own_region..]
380 .filter_map(|arg| match arg.unpack() {
381 GenericArgKind::Lifetime(r) => Some(r),
382 GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
384 .chain(std::iter::once(self.tcx.lifetimes.re_static))
388 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
390 self.member_constraint(
391 opaque_type_key.def_id,
401 #[instrument(skip(self), level = "trace")]
402 pub fn opaque_type_origin(&self, opaque_def_id: DefId, span: Span) -> Option<OpaqueTyOrigin> {
403 let def_id = opaque_def_id.as_local()?;
404 let opaque_hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
405 let parent_def_id = self.defining_use_anchor?;
406 let item_kind = &self.tcx.hir().expect_item(def_id).kind;
408 let hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) = item_kind else {
411 "weird opaque type: {:#?}, {:#?}",
416 let in_definition_scope = match *origin {
417 // Async `impl Trait`
418 hir::OpaqueTyOrigin::AsyncFn(parent) => parent == parent_def_id,
419 // Anonymous `impl Trait`
420 hir::OpaqueTyOrigin::FnReturn(parent) => parent == parent_def_id,
421 // Named `type Foo = impl Bar;`
422 hir::OpaqueTyOrigin::TyAlias => {
423 may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id)
427 in_definition_scope.then_some(*origin)
430 #[instrument(skip(self), level = "trace")]
431 fn opaque_ty_origin_unchecked(&self, opaque_def_id: DefId, span: Span) -> OpaqueTyOrigin {
432 let def_id = opaque_def_id.as_local().unwrap();
433 let origin = match self.tcx.hir().expect_item(def_id).kind {
434 hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => origin,
436 span_bug!(span, "weird opaque type: {:?}, {:#?}", opaque_def_id, itemkind)
444 // Visitor that requires that (almost) all regions in the type visited outlive
445 // `least_region`. We cannot use `push_outlives_components` because regions in
446 // closure signatures are not included in their outlives components. We need to
447 // ensure all regions outlive the given bound so that we don't end up with,
448 // say, `ReVar` appearing in a return type and causing ICEs when other
449 // functions end up with region constraints involving regions from other
452 // We also cannot use `for_each_free_region` because for closures it includes
453 // the regions parameters from the enclosing item.
455 // We ignore any type parameters because impl trait values are assumed to
456 // capture all the in-scope type parameters.
457 struct ConstrainOpaqueTypeRegionVisitor<OP> {
461 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
463 OP: FnMut(ty::Region<'tcx>),
465 fn visit_binder<T: TypeVisitable<'tcx>>(
467 t: &ty::Binder<'tcx, T>,
468 ) -> ControlFlow<Self::BreakTy> {
469 t.super_visit_with(self);
470 ControlFlow::CONTINUE
473 fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
475 // ignore bound regions, keep visiting
476 ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
479 ControlFlow::CONTINUE
484 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
485 // We're only interested in types involving regions
486 if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
487 return ControlFlow::CONTINUE;
491 ty::Closure(_, ref substs) => {
492 // Skip lifetime parameters of the enclosing item(s)
494 substs.as_closure().tupled_upvars_ty().visit_with(self);
495 substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
498 ty::Generator(_, ref substs, _) => {
499 // Skip lifetime parameters of the enclosing item(s)
500 // Also skip the witness type, because that has no free regions.
502 substs.as_generator().tupled_upvars_ty().visit_with(self);
503 substs.as_generator().return_ty().visit_with(self);
504 substs.as_generator().yield_ty().visit_with(self);
505 substs.as_generator().resume_ty().visit_with(self);
508 ty.super_visit_with(self);
512 ControlFlow::CONTINUE
522 pub fn is_defining(self) -> bool {
524 UseKind::DefiningUse => true,
525 UseKind::OpaqueUse => false,
530 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
531 #[instrument(skip(self), level = "debug")]
532 pub fn register_hidden_type(
534 opaque_type_key: OpaqueTypeKey<'tcx>,
535 cause: ObligationCause<'tcx>,
536 param_env: ty::ParamEnv<'tcx>,
538 origin: hir::OpaqueTyOrigin,
539 ) -> InferResult<'tcx, ()> {
541 let OpaqueTypeKey { def_id, substs } = opaque_type_key;
543 // Ideally, we'd get the span where *this specific `ty` came
544 // from*, but right now we just use the span from the overall
545 // value being folded. In simple cases like `-> impl Foo`,
546 // these are the same span, but not in cases like `-> (impl
548 let span = cause.span;
550 let mut obligations = vec![];
551 let prev = self.inner.borrow_mut().opaque_types().register(
552 OpaqueTypeKey { def_id, substs },
553 OpaqueHiddenType { ty: hidden_ty, span },
556 if let Some(prev) = prev {
557 obligations = self.at(&cause, param_env).eq(prev, hidden_ty)?.obligations;
560 let item_bounds = tcx.bound_explicit_item_bounds(def_id);
562 for predicate in item_bounds.transpose_iter().map(|e| e.map_bound(|(p, _)| *p)) {
564 let predicate = predicate.subst(tcx, substs);
566 let predicate = predicate.fold_with(&mut BottomUpFolder {
568 ty_op: |ty| match *ty.kind() {
569 // We can't normalize associated types from `rustc_infer`,
570 // but we can eagerly register inference variables for them.
571 ty::Projection(projection_ty) if !projection_ty.has_escaping_bound_vars() => {
572 self.infer_projection(
580 // Replace all other mentions of the same opaque type with the hidden type,
581 // as the bounds must hold on the hidden type after all.
582 ty::Opaque(def_id2, substs2) if def_id == def_id2 && substs == substs2 => {
591 if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
592 if projection.term.references_error() {
593 // No point on adding these obligations since there's a type error involved.
594 return Ok(InferOk { value: (), obligations: vec![] });
596 trace!("{:#?}", projection.term);
598 // Require that the predicate holds for the concrete type.
600 obligations.push(traits::Obligation::new(cause.clone(), param_env, predicate));
602 Ok(InferOk { value: (), obligations })
606 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
609 /// ```ignore UNSOLVED (is this a bug?)
610 /// # #![feature(type_alias_impl_trait)]
613 /// pub trait Bar { /* ... */ }
614 /// pub type Baz = impl Bar;
616 /// # impl Bar for () {}
617 /// fn f1() -> Baz { /* ... */ }
619 /// fn f2() -> bar::Baz { /* ... */ }
623 /// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
624 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
625 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
626 fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: LocalDefId, opaque_hir_id: hir::HirId) -> bool {
627 let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
629 // Named opaque types can be defined by any siblings or children of siblings.
630 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
631 // We walk up the node tree until we hit the root or the scope of the opaque type.
632 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
633 hir_id = tcx.hir().local_def_id_to_hir_id(tcx.hir().get_parent_item(hir_id));
635 // Syntactically, we are allowed to define the concrete type if:
636 let res = hir_id == scope;
638 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
639 tcx.hir().find(hir_id),
640 tcx.hir().get(opaque_hir_id),