1 use crate::infer::{InferCtxt, InferOk};
3 use hir::def_id::{DefId, LocalDefId};
4 use hir::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, TypeVisitor,
16 use std::ops::ControlFlow;
18 pub type OpaqueTypeMap<'tcx> = VecMap<OpaqueTypeKey<'tcx>, OpaqueTypeDecl<'tcx>>;
22 pub use table::{OpaqueTypeStorage, OpaqueTypeTable};
24 use super::InferResult;
26 /// Information about the opaque types whose values we
27 /// are inferring in this function (these are the `impl Trait` that
28 /// appear in the return type).
29 #[derive(Clone, Debug)]
30 pub struct OpaqueTypeDecl<'tcx> {
31 /// The hidden types that have been inferred for this opaque type.
32 /// There can be multiple, but they are all `lub`ed together at the end
33 /// to obtain the canonical hidden type.
34 pub hidden_type: OpaqueHiddenType<'tcx>,
36 /// The origin of the opaque type.
37 pub origin: hir::OpaqueTyOrigin,
40 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
41 pub fn handle_opaque_type(
46 cause: &ObligationCause<'tcx>,
47 param_env: ty::ParamEnv<'tcx>,
48 ) -> InferResult<'tcx, ()> {
49 if a.references_error() || b.references_error() {
50 return Ok(InferOk { value: (), obligations: vec![] });
52 let (a, b) = if a_is_expected { (a, b) } else { (b, a) };
53 let process = |a: Ty<'tcx>, b: Ty<'tcx>| match *a.kind() {
54 ty::Opaque(def_id, substs) => {
55 if let ty::Opaque(did2, _) = *b.kind() {
56 // We could accept this, but there are various ways to handle this situation, and we don't
57 // want to make a decision on it right now. Likely this case is so super rare anyway, that
58 // no one encounters it in practice.
59 // It does occur however in `fn fut() -> impl Future<Output = i32> { async { 42 } }`,
60 // where it is of no concern, so we only check for TAITs.
61 if let Some(OpaqueTyOrigin::TyAlias) = self.opaque_type_origin(did2, cause.span)
67 "opaque type's hidden type cannot be another opaque type from the same scope",
69 .span_label(cause.span, "one of the two opaque types used here has to be outside its defining scope")
71 self.tcx.def_span(def_id),
72 "opaque type whose hidden type is being assigned",
75 self.tcx.def_span(did2),
76 "opaque type being used as hidden type",
81 Some(self.register_hidden_type(
82 OpaqueTypeKey { def_id, substs },
86 if self.defining_use_anchor.is_some() {
87 // Check that this is `impl Trait` type is
88 // declared by `parent_def_id` -- i.e., one whose
89 // value we are inferring. At present, this is
90 // always true during the first phase of
91 // type-check, but not always true later on during
92 // NLL. Once we support named opaque types more fully,
93 // this same scenario will be able to arise during all phases.
95 // Here is an example using type alias `impl Trait`
96 // that indicates the distinction we are checking for:
100 // pub type Foo = impl Iterator;
101 // pub fn make_foo() -> Foo { .. }
105 // fn foo() -> a::Foo { a::make_foo() }
109 // Here, the return type of `foo` references an
110 // `Opaque` indeed, but not one whose value is
111 // presently being inferred. You can get into a
112 // similar situation with closure return types
116 // fn foo() -> impl Iterator { .. }
118 // let x = || foo(); // returns the Opaque assoc with `foo`
121 self.opaque_type_origin(def_id, cause.span)?
123 self.opaque_ty_origin_unchecked(def_id, cause.span)
129 if let Some(res) = process(a, b) {
131 } else if let Some(res) = process(b, a) {
134 // Rerun equality check, but this time error out due to
136 match self.at(cause, param_env).define_opaque_types(false).eq(a, b) {
139 "opaque types are never equal to anything but themselves: {:#?}",
147 /// Given the map `opaque_types` containing the opaque
148 /// `impl Trait` types whose underlying, hidden types are being
149 /// inferred, this method adds constraints to the regions
150 /// appearing in those underlying hidden types to ensure that they
151 /// at least do not refer to random scopes within the current
152 /// function. These constraints are not (quite) sufficient to
153 /// guarantee that the regions are actually legal values; that
154 /// final condition is imposed after region inference is done.
158 /// Let's work through an example to explain how it works. Assume
159 /// the current function is as follows:
162 /// fn foo<'a, 'b>(..) -> (impl Bar<'a>, impl Bar<'b>)
165 /// Here, we have two `impl Trait` types whose values are being
166 /// inferred (the `impl Bar<'a>` and the `impl
167 /// Bar<'b>`). Conceptually, this is sugar for a setup where we
168 /// define underlying opaque types (`Foo1`, `Foo2`) and then, in
169 /// the return type of `foo`, we *reference* those definitions:
172 /// type Foo1<'x> = impl Bar<'x>;
173 /// type Foo2<'x> = impl Bar<'x>;
174 /// fn foo<'a, 'b>(..) -> (Foo1<'a>, Foo2<'b>) { .. }
181 /// As indicating in the comments above, each of those references
182 /// is (in the compiler) basically a substitution (`substs`)
183 /// applied to the type of a suitable `def_id` (which identifies
184 /// `Foo1` or `Foo2`).
186 /// Now, at this point in compilation, what we have done is to
187 /// replace each of the references (`Foo1<'a>`, `Foo2<'b>`) with
188 /// fresh inference variables C1 and C2. We wish to use the values
189 /// of these variables to infer the underlying types of `Foo1` and
190 /// `Foo2`. That is, this gives rise to higher-order (pattern) unification
191 /// constraints like:
194 /// for<'a> (Foo1<'a> = C1)
195 /// for<'b> (Foo1<'b> = C2)
198 /// For these equation to be satisfiable, the types `C1` and `C2`
199 /// can only refer to a limited set of regions. For example, `C1`
200 /// can only refer to `'static` and `'a`, and `C2` can only refer
201 /// to `'static` and `'b`. The job of this function is to impose that
204 /// Up to this point, C1 and C2 are basically just random type
205 /// inference variables, and hence they may contain arbitrary
206 /// regions. In fact, it is fairly likely that they do! Consider
207 /// this possible definition of `foo`:
210 /// fn foo<'a, 'b>(x: &'a i32, y: &'b i32) -> (impl Bar<'a>, impl Bar<'b>) {
215 /// Here, the values for the concrete types of the two impl
216 /// traits will include inference variables:
223 /// Ordinarily, the subtyping rules would ensure that these are
224 /// sufficiently large. But since `impl Bar<'a>` isn't a specific
225 /// type per se, we don't get such constraints by default. This
226 /// is where this function comes into play. It adds extra
227 /// constraints to ensure that all the regions which appear in the
228 /// inferred type are regions that could validly appear.
230 /// This is actually a bit of a tricky constraint in general. We
231 /// want to say that each variable (e.g., `'0`) can only take on
232 /// values that were supplied as arguments to the opaque type
233 /// (e.g., `'a` for `Foo1<'a>`) or `'static`, which is always in
234 /// scope. We don't have a constraint quite of this kind in the current
239 /// We generally prefer to make `<=` constraints, since they
240 /// integrate best into the region solver. To do that, we find the
241 /// "minimum" of all the arguments that appear in the substs: that
242 /// is, some region which is less than all the others. In the case
243 /// of `Foo1<'a>`, that would be `'a` (it's the only choice, after
244 /// all). Then we apply that as a least bound to the variables
245 /// (e.g., `'a <= '0`).
247 /// In some cases, there is no minimum. Consider this example:
250 /// fn baz<'a, 'b>() -> impl Trait<'a, 'b> { ... }
253 /// Here we would report a more complex "in constraint", like `'r
254 /// in ['a, 'b, 'static]` (where `'r` is some region appearing in
255 /// the hidden type).
257 /// # Constrain regions, not the hidden concrete type
259 /// Note that generating constraints on each region `Rc` is *not*
260 /// the same as generating an outlives constraint on `Tc` iself.
261 /// For example, if we had a function like this:
264 /// fn foo<'a, T>(x: &'a u32, y: T) -> impl Foo<'a> {
268 /// // Equivalent to:
269 /// type FooReturn<'a, T> = impl Foo<'a>;
270 /// fn foo<'a, T>(..) -> FooReturn<'a, T> { .. }
273 /// then the hidden type `Tc` would be `(&'0 u32, T)` (where `'0`
274 /// is an inference variable). If we generated a constraint that
275 /// `Tc: 'a`, then this would incorrectly require that `T: 'a` --
276 /// but this is not necessary, because the opaque type we
277 /// create will be allowed to reference `T`. So we only generate a
278 /// constraint that `'0: 'a`.
279 #[instrument(level = "debug", skip(self))]
280 pub fn register_member_constraints(
282 param_env: ty::ParamEnv<'tcx>,
283 opaque_type_key: OpaqueTypeKey<'tcx>,
284 concrete_ty: Ty<'tcx>,
287 let def_id = opaque_type_key.def_id;
291 let concrete_ty = self.resolve_vars_if_possible(concrete_ty);
293 debug!(?concrete_ty);
295 let first_own_region = match self.opaque_ty_origin_unchecked(def_id, span) {
296 hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {
299 // fn foo<'l0..'ln>() -> impl Trait<'l0..'lm>
303 // type foo::<'p0..'pn>::Foo<'q0..'qm>
304 // fn foo<l0..'ln>() -> foo::<'static..'static>::Foo<'l0..'lm>.
306 // For these types we only iterate over `'l0..lm` below.
307 tcx.generics_of(def_id).parent_count
309 // These opaque type inherit all lifetime parameters from their
310 // parent, so we have to check them all.
311 hir::OpaqueTyOrigin::TyAlias => 0,
314 // For a case like `impl Foo<'a, 'b>`, we would generate a constraint
315 // `'r in ['a, 'b, 'static]` for each region `'r` that appears in the
316 // hidden type (i.e., it must be equal to `'a`, `'b`, or `'static`).
318 // `conflict1` and `conflict2` are the two region bounds that we
319 // detected which were unrelated. They are used for diagnostics.
321 // Create the set of choice regions: each region in the hidden
322 // type can be equal to any of the region parameters of the
323 // opaque type definition.
324 let choice_regions: Lrc<Vec<ty::Region<'tcx>>> = Lrc::new(
325 opaque_type_key.substs[first_own_region..]
327 .filter_map(|arg| match arg.unpack() {
328 GenericArgKind::Lifetime(r) => Some(r),
329 GenericArgKind::Type(_) | GenericArgKind::Const(_) => None,
331 .chain(std::iter::once(self.tcx.lifetimes.re_static))
335 concrete_ty.visit_with(&mut ConstrainOpaqueTypeRegionVisitor {
337 self.member_constraint(
338 opaque_type_key.def_id,
348 #[instrument(skip(self), level = "trace")]
349 pub fn opaque_type_origin(&self, opaque_def_id: DefId, span: Span) -> Option<OpaqueTyOrigin> {
350 let def_id = opaque_def_id.as_local()?;
351 let opaque_hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
352 let parent_def_id = self.defining_use_anchor?;
353 let item_kind = &self.tcx.hir().expect_item(def_id).kind;
355 let hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) = item_kind else {
358 "weird opaque type: {:#?}, {:#?}",
363 let in_definition_scope = match *origin {
364 // Async `impl Trait`
365 hir::OpaqueTyOrigin::AsyncFn(parent) => parent == parent_def_id,
366 // Anonymous `impl Trait`
367 hir::OpaqueTyOrigin::FnReturn(parent) => parent == parent_def_id,
368 // Named `type Foo = impl Bar;`
369 hir::OpaqueTyOrigin::TyAlias => {
370 may_define_opaque_type(self.tcx, parent_def_id, opaque_hir_id)
374 in_definition_scope.then_some(*origin)
377 #[instrument(skip(self), level = "trace")]
378 fn opaque_ty_origin_unchecked(&self, opaque_def_id: DefId, span: Span) -> OpaqueTyOrigin {
379 let def_id = opaque_def_id.as_local().unwrap();
380 let origin = match self.tcx.hir().expect_item(def_id).kind {
381 hir::ItemKind::OpaqueTy(hir::OpaqueTy { origin, .. }) => origin,
383 span_bug!(span, "weird opaque type: {:?}, {:#?}", opaque_def_id, itemkind)
391 // Visitor that requires that (almost) all regions in the type visited outlive
392 // `least_region`. We cannot use `push_outlives_components` because regions in
393 // closure signatures are not included in their outlives components. We need to
394 // ensure all regions outlive the given bound so that we don't end up with,
395 // say, `ReVar` appearing in a return type and causing ICEs when other
396 // functions end up with region constraints involving regions from other
399 // We also cannot use `for_each_free_region` because for closures it includes
400 // the regions parameters from the enclosing item.
402 // We ignore any type parameters because impl trait values are assumed to
403 // capture all the in-scope type parameters.
404 struct ConstrainOpaqueTypeRegionVisitor<OP> {
408 impl<'tcx, OP> TypeVisitor<'tcx> for ConstrainOpaqueTypeRegionVisitor<OP>
410 OP: FnMut(ty::Region<'tcx>),
412 fn visit_binder<T: TypeFoldable<'tcx>>(
414 t: &ty::Binder<'tcx, T>,
415 ) -> ControlFlow<Self::BreakTy> {
416 t.as_ref().skip_binder().visit_with(self);
417 ControlFlow::CONTINUE
420 fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
422 // ignore bound regions, keep visiting
423 ty::ReLateBound(_, _) => ControlFlow::CONTINUE,
426 ControlFlow::CONTINUE
431 fn visit_ty(&mut self, ty: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
432 // We're only interested in types involving regions
433 if !ty.flags().intersects(ty::TypeFlags::HAS_FREE_REGIONS) {
434 return ControlFlow::CONTINUE;
438 ty::Closure(_, ref substs) => {
439 // Skip lifetime parameters of the enclosing item(s)
441 substs.as_closure().tupled_upvars_ty().visit_with(self);
442 substs.as_closure().sig_as_fn_ptr_ty().visit_with(self);
445 ty::Generator(_, ref substs, _) => {
446 // Skip lifetime parameters of the enclosing item(s)
447 // Also skip the witness type, because that has no free regions.
449 substs.as_generator().tupled_upvars_ty().visit_with(self);
450 substs.as_generator().return_ty().visit_with(self);
451 substs.as_generator().yield_ty().visit_with(self);
452 substs.as_generator().resume_ty().visit_with(self);
455 ty.super_visit_with(self);
459 ControlFlow::CONTINUE
469 pub fn is_defining(self) -> bool {
471 UseKind::DefiningUse => true,
472 UseKind::OpaqueUse => false,
477 impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
478 #[instrument(skip(self), level = "debug")]
479 pub fn register_hidden_type(
481 opaque_type_key: OpaqueTypeKey<'tcx>,
482 cause: ObligationCause<'tcx>,
483 param_env: ty::ParamEnv<'tcx>,
485 origin: hir::OpaqueTyOrigin,
486 ) -> InferResult<'tcx, ()> {
488 let OpaqueTypeKey { def_id, substs } = opaque_type_key;
490 // Ideally, we'd get the span where *this specific `ty` came
491 // from*, but right now we just use the span from the overall
492 // value being folded. In simple cases like `-> impl Foo`,
493 // these are the same span, but not in cases like `-> (impl
495 let span = cause.span;
497 let mut obligations = vec![];
498 let prev = self.inner.borrow_mut().opaque_types().register(
499 OpaqueTypeKey { def_id, substs },
500 OpaqueHiddenType { ty: hidden_ty, span },
503 if let Some(prev) = prev {
504 obligations = self.at(&cause, param_env).eq(prev, hidden_ty)?.obligations;
507 let item_bounds = tcx.explicit_item_bounds(def_id);
509 for (predicate, _) in item_bounds {
511 let predicate = predicate.subst(tcx, substs);
513 let predicate = predicate.fold_with(&mut BottomUpFolder {
515 ty_op: |ty| match *ty.kind() {
516 // We can't normalize associated types from `rustc_infer`,
517 // but we can eagerly register inference variables for them.
518 ty::Projection(projection_ty) if !projection_ty.has_escaping_bound_vars() => {
519 self.infer_projection(
527 // Replace all other mentions of the same opaque type with the hidden type,
528 // as the bounds must hold on the hidden type after all.
529 ty::Opaque(def_id2, substs2) if def_id == def_id2 && substs == substs2 => {
538 if let ty::PredicateKind::Projection(projection) = predicate.kind().skip_binder() {
539 if projection.term.references_error() {
540 // No point on adding these obligations since there's a type error involved.
541 return Ok(InferOk { value: (), obligations: vec![] });
543 trace!("{:#?}", projection.term);
545 // Require that the predicate holds for the concrete type.
547 obligations.push(traits::Obligation::new(cause.clone(), param_env, predicate));
549 Ok(InferOk { value: (), obligations })
553 /// Returns `true` if `opaque_hir_id` is a sibling or a child of a sibling of `def_id`.
559 /// pub trait Bar { .. }
561 /// pub type Baz = impl Bar;
563 /// fn f1() -> Baz { .. }
566 /// fn f2() -> bar::Baz { .. }
570 /// Here, `def_id` is the `LocalDefId` of the defining use of the opaque type (e.g., `f1` or `f2`),
571 /// and `opaque_hir_id` is the `HirId` of the definition of the opaque type `Baz`.
572 /// For the above example, this function returns `true` for `f1` and `false` for `f2`.
573 fn may_define_opaque_type(tcx: TyCtxt<'_>, def_id: LocalDefId, opaque_hir_id: hir::HirId) -> bool {
574 let mut hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
576 // Named opaque types can be defined by any siblings or children of siblings.
577 let scope = tcx.hir().get_defining_scope(opaque_hir_id);
578 // We walk up the node tree until we hit the root or the scope of the opaque type.
579 while hir_id != scope && hir_id != hir::CRATE_HIR_ID {
580 hir_id = tcx.hir().local_def_id_to_hir_id(tcx.hir().get_parent_item(hir_id));
582 // Syntactically, we are allowed to define the concrete type if:
583 let res = hir_id == scope;
585 "may_define_opaque_type(def={:?}, opaque_node={:?}) = {}",
586 tcx.hir().find(hir_id),
587 tcx.hir().get(opaque_hir_id),