1 use crate::errors::OpaqueHiddenTypeDiag;
2 use crate::infer::{DefiningAnchor, InferCtxt, InferOk};
5 use hir::def_id::{DefId, LocalDefId};
6 use hir::{HirId, OpaqueTyOrigin};
7 use rustc_data_structures::sync::Lrc;
8 use rustc_data_structures::vec_map::VecMap;
10 use rustc_middle::traits::ObligationCause;
11 use rustc_middle::ty::error::{ExpectedFound, TypeError};
12 use rustc_middle::ty::fold::BottomUpFolder;
13 use rustc_middle::ty::GenericArgKind;
14 use rustc_middle::ty::{
15 self, OpaqueHiddenType, OpaqueTypeKey, Ty, TyCtxt, TypeFoldable, TypeSuperVisitable,
16 TypeVisitable, TypeVisitor,
20 use std::ops::ControlFlow;
22 pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;
26 pub use table::{OpaqueTypeStorage, OpaqueTypeTable};
28 use super::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
29 use super::InferResult;
31 /// Information about the opaque types whose values we
32 /// are inferring in this function (these are the `impl Trait` that
33 /// appear in the return type).
34 #[derive(Clone, Debug)]
35 pub struct OpaqueTypeDecl<'tcx> {
36 /// The hidden types that have been inferred for this opaque type.
37 /// There can be multiple, but they are all `lub`ed together at the end
38 /// to obtain the canonical hidden type.
39 pub hidden_type: OpaqueHiddenType<'tcx>,
41 /// The origin of the opaque type.
42 pub origin: hir::OpaqueTyOrigin,
45 impl<'tcx> InferCtxt<'tcx> {
46 /// This is a backwards compatibility hack to prevent breaking changes from
47 /// lazy TAIT around RPIT handling.
48 pub fn replace_opaque_types_with_inference_vars<T: TypeFoldable<'tcx>>(
53 param_env: ty::ParamEnv<'tcx>,
54 ) -> InferOk<'tcx, T> {
55 if !value.has_opaque_types() {
56 return InferOk { value, obligations: vec![] };
58 let mut obligations = vec![];
59 let replace_opaque_type = |def_id: DefId| {
62 .map_or(false, |def_id| self.opaque_type_origin(def_id, span).is_some())
64 let value = value.fold_with(&mut ty::fold::BottomUpFolder {
68 ty_op: |ty| match *ty.kind() {
69 ty::Opaque(def_id, _substs) if replace_opaque_type(def_id) => {
70 let def_span = self.tcx.def_span(def_id);
71 let span = if span.contains(def_span) { def_span } else { span };
72 let code = traits::ObligationCauseCode::OpaqueReturnType(None);
73 let cause = ObligationCause::new(span, body_id, code);
74 // FIXME(compiler-errors): We probably should add a new TypeVariableOriginKind
75 // for opaque types, and then use that kind to fix the spans for type errors
76 // that we see later on.
77 let ty_var = self.next_ty_var(TypeVariableOrigin {
78 kind: TypeVariableOriginKind::OpaqueTypeInference(def_id),
82 self.handle_opaque_type(ty, ty_var, true, &cause, param_env)
91 InferOk { value, obligations }
94 pub fn handle_opaque_type(
99 cause: &ObligationCause<'tcx>,
100 param_env: ty::ParamEnv<'tcx>,
101 ) -> InferResult<'tcx, ()> {
102 if a.references_error() || b.references_error() {
103 return Ok(InferOk { value: (), obligations: vec![] });
105 let (a, b) = if a_is_expected { (a, b) } else { (b, a) };
106 let process = |a: Ty<'tcx>, b: Ty<'tcx>, a_is_expected| match *a.kind() {
107 ty::Opaque(def_id, substs) if def_id.is_local() => {
108 let def_id = def_id.expect_local();
109 let origin = match self.defining_use_anchor {
110 DefiningAnchor::Bind(_) => {
111 // Check that this is `impl Trait` type is
112 // declared by `parent_def_id` -- i.e., one whose
113 // value we are inferring. At present, this is
114 // always true during the first phase of
115 // type-check, but not always true later on during
116 // NLL. Once we support named opaque types more fully,
117 // this same scenario will be able to arise during all phases.
119 // Here is an example using type alias `impl Trait`
120 // that indicates the distinction we are checking for:
124 // pub type Foo = impl Iterator;
125 // pub fn make_foo() -> Foo { .. }
129 // fn foo() -> a::Foo { a::make_foo() }
133 // Here, the return type of `foo` references an
134 // `Opaque` indeed, but not one whose value is
135 // presently being inferred. You can get into a
136 // similar situation with closure return types
140 // fn foo() -> impl Iterator { .. }
142 // let x = || foo(); // returns the Opaque assoc with `foo`
145 self.opaque_type_origin(def_id, cause.span)?
147 DefiningAnchor::Bubble => self.opaque_ty_origin_unchecked(def_id, cause.span),
148 DefiningAnchor::Error => return None,
150 if let ty::Opaque(did2, _) = *b.kind() {
151 // We could accept this, but there are various ways to handle this situation, and we don't
152 // want to make a decision on it right now. Likely this case is so super rare anyway, that
153 // no one encounters it in practice.
154 // It does occur however in `fn fut() -> impl Future<Output = i32> { async { 42 } }`,
155 // where it is of no concern, so we only check for TAITs.
156 if let Some(OpaqueTyOrigin::TyAlias) =
157 did2.as_local().and_then(|did2| self.opaque_type_origin(did2, cause.span))
159 self.tcx.sess.emit_err(OpaqueHiddenTypeDiag {
161 hidden_type: self.tcx.def_span(did2),
162 opaque_type: self.tcx.def_span(def_id),
166 Some(self.register_hidden_type(
167 OpaqueTypeKey { def_id, substs },
177 if let Some(res) = process(a, b, true) {
179 } else if let Some(res) = process(b, a, false) {
182 let (a, b) = self.resolve_vars_if_possible((a, b));
183 Err(TypeError::Sorts(ExpectedFound::new(true, a, b)))
187 /// Given the map `opaque_types` containing the opaque
188 /// `impl Trait` types whose underlying, hidden types are being
189 /// inferred, this method adds constraints to the regions
190 /// appearing in those underlying hidden types to ensure that they
191 /// at least do not refer to random scopes within the current
192 /// function. These constraints are not (quite) sufficient to
193 /// guarantee that the regions are actually legal values; that
194 /// final condition is imposed after region inference is done.
198 /// Let's work through an example to explain how it works. Assume
199 /// the current function is as follows:
202 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
205 /// Here, we have two `impl Trait` types whose values are being
206 /// inferred (the `impl Bar<'a>` and the `impl
207 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
208 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
209 /// the return type of `foo`, we *reference* those definitions:
212 /// type Foo1<'x> = impl Bar<'x>;
213 /// type Foo2<'x> = impl Bar<'x>;
214 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
221 /// As indicating in the comments above, each of those references
222 /// is (in the compiler) basically a substitution (`substs`)
223 /// applied to the type of a suitable `def_id` (which identifies
224 /// `Foo1` or `Foo2`).
226 /// Now, at this point in compilation, what we have done is to
227 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
228 /// fresh inference variables C1 and C2. We wish to use the values
229 /// of these variables to infer the underlying types of `Foo1` and
230 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
231 /// constraints like:
234 /// for<'a> (Foo1<'a> = C1)
235 /// for<'b> (Foo1<'b> = C2)
238 /// For these equation to be satisfiable, the types `C1` and `C2`
239 /// can only refer to a limited set of regions. For example, `C1`
240 /// can only refer to `'static` and `'a`, and `C2` can only refer
241 /// to `'static` and `'b`. The job of this function is to impose that
244 /// Up to this point, C1 and C2 are basically just random type
245 /// inference variables, and hence they may contain arbitrary
246 /// regions. In fact, it is fairly likely that they do! Consider
247 /// this possible definition of `foo`:
250 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
255 /// Here, the values for the concrete types of the two impl
256 /// traits will include inference variables:
263 /// Ordinarily, the subtyping rules would ensure that these are
264 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
265 /// type per se, we don't get such constraints by default. This
266 /// is where this function comes into play. It adds extra
267 /// constraints to ensure that all the regions which appear in the
268 /// inferred type are regions that could validly appear.
270 /// This is actually a bit of a tricky constraint in general. We
271 /// want to say that each variable (e.g., `'0`) can only take on
272 /// values that were supplied as arguments to the opaque type
273 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
274 /// scope. We don't have a constraint quite of this kind in the current
279 /// We generally prefer to make `<=` constraints, since they
280 /// integrate best into the region solver. To do that, we find the
281 /// "minimum" of all the arguments that appear in the substs: that
282 /// is, some region which is less than all the others. In the case
283 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
284 /// all). Then we apply that as a least bound to the variables
285 /// (e.g., `'a <= '0`).
287 /// In some cases, there is no minimum. Consider this example:
290 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
293 /// Here we would report a more complex "in constraint", like `'r
294 /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
295 /// the hidden type).
297 /// # Constrain regions, not the hidden concrete type
299 /// Note that generating constraints on each region `Rc` is *not*
300 /// the same as generating an outlives constraint on `Tc` itself.
301 /// For example, if we had a function like this:
304 /// # #![feature(type_alias_impl_trait)]
306 /// # trait Foo<'a> {}
307 /// # impl<'a, T> Foo<'a> for (&'a u32, T) {}
308 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
312 /// // Equivalent to:
313 /// # mod dummy { use super::*;
314 /// type FooReturn<'a, T> = impl Foo<'a>;
315 /// fn foo<'a, T>(x: &'a u32, y: T) -> FooReturn<'a, T> {
321 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
322 /// is an inference variable). If we generated a constraint that
323 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
324 /// but this is not necessary, because the opaque type we
325 /// create will be allowed to reference `T`. So we only generate a
326 /// constraint that `'0: 'a`.
327 #[instrument(level = "debug", skip(self))]
328 pub fn register_member_constraints(
330 param_env: ty::ParamEnv<'tcx>,
331 opaque_type_key: OpaqueTypeKey<'tcx>,
332 concrete_ty: Ty<'tcx>,
335 let def_id = opaque_type_key.def_id;
339 let concrete_ty = self.resolve_vars_if_possible(concrete_ty);
341 debug!(?concrete_ty);
343 let first_own_region = match self.opaque_ty_origin_unchecked(def_id, span) {
344 hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {
347 // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
351 // type foo::<'p0..'pn>::Foo<'q0..'qm>
352 // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
354 // For these types we only iterate over `'l0..lm` below.
355 tcx.generics_of(def_id).parent_count
357 // These opaque type inherit all lifetime parameters from their
358 // parent, so we have to check them all.
359 hir::OpaqueTyOrigin::TyAlias => 0,
362 // For a case like `impl Foo<'a, 'b>`, we would generate a constraint
363 // `'r in ['a, 'b, 'static]` for each region `'r` that appears in the
364 // hidden type (i.e., it must be equal to `'a`, `'b`, or `'static`).
366 // `conflict1` and `conflict2` are the two region bounds that we
367 // detected which were unrelated. They are used for diagnostics.
369 // Create the set of choice regions: each region in the hidden
370 // type can be equal to any of the region parameters of the
371 // opaque type definition.
372 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
373 opaque_type_key.substs[first_own_region..]
375 .filter_map(|arg| match arg.unpack() {
376 GenericArgKind::Lifetime(r) => Some(r),
377 GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
379 .chain(std::iter::once(self.tcx.lifetimes.re_static))
383 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
384 op: |r| self.member_constraint(opaque_type_key, span, concrete_ty, r, &choice_regions),
388 #[instrument(skip(self), level = "trace", ret)]
389 pub fn opaque_type_origin(&self, def_id: LocalDefId, span: Span) -> Option<OpaqueTyOrigin> {
390 let opaque_hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
391 let parent_def_id = match self.defining_use_anchor {
392 DefiningAnchor::Bubble | DefiningAnchor::Error => return None,
393 DefiningAnchor::Bind(bind) => bind,
395 let item_kind = &self.tcx.hir().expect_item(def_id).kind;
397 let hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) = item_kind else {
400 "weird opaque type: {:#?}, {:#?}",
405 let in_definition_scope = match *origin {
406 // Async `impl Trait`
407 hir::OpaqueTyOrigin::AsyncFn(parent) => parent == parent_def_id,
408 // Anonymous `impl Trait`
409 hir::OpaqueTyOrigin::FnReturn(parent) => parent == parent_def_id,
410 // Named `type Foo = impl Bar;`
411 hir::OpaqueTyOrigin::TyAlias => {
412 may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id)
416 in_definition_scope.then_some(*origin)
419 #[instrument(skip(self), level = "trace", ret)]
420 fn opaque_ty_origin_unchecked(&self, def_id: LocalDefId, span: Span) -> OpaqueTyOrigin {
421 match self.tcx.hir().expect_item(def_id).kind {
422 hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => origin,
424 span_bug!(span, "weird opaque type: {:?}, {:#?}", def_id, itemkind)
430 // Visitor that requires that (almost) all regions in the type visited outlive
431 // `least_region`. We cannot use `push_outlives_components` because regions in
432 // closure signatures are not included in their outlives components. We need to
433 // ensure all regions outlive the given bound so that we don't end up with,
434 // say, `ReVar` appearing in a return type and causing ICEs when other
435 // functions end up with region constraints involving regions from other
438 // We also cannot use `for_each_free_region` because for closures it includes
439 // the regions parameters from the enclosing item.
441 // We ignore any type parameters because impl trait values are assumed to
442 // capture all the in-scope type parameters.
443 struct ConstrainOpaqueTypeRegionVisitor<OP> {
447 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
449 OP: FnMut(ty::Region<'tcx>),
451 fn visit_binder<T: TypeVisitable<'tcx>>(
453 t: &ty::Binder<'tcx, T>,
454 ) -> ControlFlow<Self::BreakTy> {
455 t.super_visit_with(self);
456 ControlFlow::CONTINUE
459 fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
461 // ignore bound regions, keep visiting
462 ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
465 ControlFlow::CONTINUE
470 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
471 // We're only interested in types involving regions
472 if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
473 return ControlFlow::CONTINUE;
477 ty::Closure(_, ref substs) => {
478 // Skip lifetime parameters of the enclosing item(s)
480 substs.as_closure().tupled_upvars_ty().visit_with(self);
481 substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
484 ty::Generator(_, ref substs, _) => {
485 // Skip lifetime parameters of the enclosing item(s)
486 // Also skip the witness type, because that has no free regions.
488 substs.as_generator().tupled_upvars_ty().visit_with(self);
489 substs.as_generator().return_ty().visit_with(self);
490 substs.as_generator().yield_ty().visit_with(self);
491 substs.as_generator().resume_ty().visit_with(self);
494 ty.super_visit_with(self);
498 ControlFlow::CONTINUE
508 pub fn is_defining(self) -> bool {
510 UseKind::DefiningUse => true,
511 UseKind::OpaqueUse => false,
516 impl<'tcx> InferCtxt<'tcx> {
517 #[instrument(skip(self), level = "debug")]
518 fn register_hidden_type(
520 opaque_type_key: OpaqueTypeKey<'tcx>,
521 cause: ObligationCause<'tcx>,
522 param_env: ty::ParamEnv<'tcx>,
524 origin: hir::OpaqueTyOrigin,
526 ) -> InferResult<'tcx, ()> {
528 let OpaqueTypeKey { def_id, substs } = opaque_type_key;
530 // Ideally, we'd get the span where *this specific `ty` came
531 // from*, but right now we just use the span from the overall
532 // value being folded. In simple cases like `-> impl Foo`,
533 // these are the same span, but not in cases like `-> (impl
535 let span = cause.span;
537 let mut obligations = vec![];
538 let prev = self.inner.borrow_mut().opaque_types().register(
539 OpaqueTypeKey { def_id, substs },
540 OpaqueHiddenType { ty: hidden_ty, span },
543 if let Some(prev) = prev {
545 self.at(&cause, param_env).eq_exp(a_is_expected, prev, hidden_ty)?.obligations;
548 let item_bounds = tcx.bound_explicit_item_bounds(def_id.to_def_id());
550 for (predicate, _) in item_bounds.subst_iter_copied(tcx, substs) {
551 let predicate = predicate.fold_with(&mut BottomUpFolder {
553 ty_op: |ty| match *ty.kind() {
554 // We can't normalize associated types from `rustc_infer`,
555 // but we can eagerly register inference variables for them.
556 // FIXME(RPITIT): Don't replace RPITITs with inference vars.
557 ty::Projection(projection_ty)
558 if !projection_ty.has_escaping_bound_vars()
559 && tcx.def_kind(projection_ty.item_def_id)
560 != DefKind::ImplTraitPlaceholder =>
562 self.infer_projection(
570 // Replace all other mentions of the same opaque type with the hidden type,
571 // as the bounds must hold on the hidden type after all.
572 ty::Opaque(def_id2, substs2)
573 if def_id.to_def_id() == def_id2 && substs == substs2 =>
577 // FIXME(RPITIT): This can go away when we move to associated types
579 if def_id.to_def_id() == proj.item_def_id && substs == proj.substs =>
589 if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
590 if projection.term.references_error() {
591 // No point on adding these obligations since there's a type error involved.
592 return Ok(InferOk { value: (), obligations: vec![] });
594 trace!("{:#?}", projection.term);
596 // Require that the predicate holds for the concrete type.
598 obligations.push(traits::Obligation::new(cause.clone(), param_env, predicate));
600 Ok(InferOk { value: (), obligations })
604 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
607 /// ```ignore UNSOLVED (is this a bug?)
608 /// # #![feature(type_alias_impl_trait)]
611 /// pub trait Bar { /* ... */ }
612 /// pub type Baz = impl Bar;
614 /// # impl Bar for () {}
615 /// fn f1() -> Baz { /* ... */ }
617 /// fn f2() -> bar::Baz { /* ... */ }
621 /// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
622 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
623 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
624 fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: LocalDefId, opaque_hir_id: hir::HirId) -> bool {
625 let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
627 // Named opaque types can be defined by any siblings or children of siblings.
628 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
629 // We walk up the node tree until we hit the root or the scope of the opaque type.
630 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
631 hir_id = tcx.hir().get_parent_item(hir_id).into();
633 // Syntactically, we are allowed to define the concrete type if:
634 let res = hir_id == scope;
636 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
637 tcx.hir().find(hir_id),
638 tcx.hir().get(opaque_hir_id),