Recurse into nested impl-trait when computing variance.

compiler/rustc_hir_analysis/src/variance/mod.rs

@@ -7,7 +7,8 @@ use rustc_arena::DroplessArena;
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::ty::query::Providers;
-use rustc_middle::ty::{self, CrateVariancesMap, TyCtxt, TypeSuperVisitable, TypeVisitable};
+use rustc_middle::ty::{self, CrateVariancesMap, SubstsRef, Ty, TyCtxt};
+use rustc_middle::ty::{DefIdTree, TypeSuperVisitable, TypeVisitable};
 use std::ops::ControlFlow;
 
 /// Defines the `TermsContext` basically houses an arena where we can
@@ -75,18 +76,50 @@ fn variance_of_opaque(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Varianc
  // type Foo<'a, 'b, 'c> = impl Trait<'a> + 'b;
  // ```
  // we may not use `'c` in the hidden type.
- struct OpaqueTypeLifetimeCollector {
+ struct OpaqueTypeLifetimeCollector<'tcx> {
+ tcx: TyCtxt<'tcx>,
+ root_def_id: DefId,
  variances: Vec<ty::Variance>,
  }
 
- impl<'tcx> ty::TypeVisitor<'tcx> for OpaqueTypeLifetimeCollector {
+ impl<'tcx> OpaqueTypeLifetimeCollector<'tcx> {
+ #[instrument(level = "trace", skip(self), ret)]
+ fn visit_opaque(&mut self, def_id: DefId, substs: SubstsRef<'tcx>) -> ControlFlow<!> {
+ if def_id != self.root_def_id && self.tcx.is_descendant_of(def_id, self.root_def_id) {
+ let child_variances = self.tcx.variances_of(def_id);
+ for (a, v) in substs.iter().zip(child_variances) {
+ if *v != ty::Bivariant {
+ a.visit_with(self)?;
+ }
+ }
+ ControlFlow::CONTINUE
+ } else {
+ substs.visit_with(self)
+ }
+ }
+ }
+
+ impl<'tcx> ty::TypeVisitor<'tcx> for OpaqueTypeLifetimeCollector<'tcx> {
  #[instrument(level = "trace", skip(self), ret)]
  fn visit_region(&mut self, r: ty::Region<'tcx>) -> ControlFlow<Self::BreakTy> {
  if let ty::RegionKind::ReEarlyBound(ebr) = r.kind() {
  self.variances[ebr.index as usize] = ty::Invariant;
  }
  r.super_visit_with(self)
  }
+
+ #[instrument(level = "trace", skip(self), ret)]
+ fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
+ match t.kind() {
+ ty::Opaque(def_id, substs) => self.visit_opaque(*def_id, substs),
+ ty::Projection(proj)
+ if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
+ {
+ self.visit_opaque(proj.item_def_id, proj.substs)
+ }
+ _ => t.super_visit_with(self),
+ }
+ }
  }
 
  // By default, RPIT are invariant wrt type and const generics, but they are bivariant wrt
@@ -111,7 +144,8 @@ fn variance_of_opaque(tcx: TyCtxt<'_>, item_def_id: LocalDefId) -> &[ty::Varianc
  }
  }
 
- let mut collector = OpaqueTypeLifetimeCollector { variances };
+ let mut collector =
+ OpaqueTypeLifetimeCollector { tcx, root_def_id: item_def_id.to_def_id(), variances };
  let id_substs = ty::InternalSubsts::identity_for_item(tcx, item_def_id.to_def_id());
  for pred in tcx.bound_explicit_item_bounds(item_def_id.to_def_id()).transpose_iter() {
  let pred = pred.map_bound(|(pred, _)| *pred).subst(tcx, id_substs);

src/test/ui/impl-trait/nested-return-type4.rs

@@ -0,0 +1,8 @@
+// edition: 2021
+
+fn test<'s: 's>(s: &'s str) -> impl std::future::Future<Output = impl Sized> {
+ async move { let _s = s; }
+ //~^ ERROR hidden type for `impl Future<Output = impl Sized>` captures lifetime that does not appear in bounds
+}
+
+fn main() {}

src/test/ui/impl-trait/nested-return-type4.stderr

@@ -0,0 +1,20 @@
+error[E0700]: hidden type for `impl Future<Output = impl Sized>` captures lifetime that does not appear in bounds
+ --> $DIR/nested-return-type4.rs:4:5
+ |
+LL | fn test<'s: 's>(s: &'s str) -> impl std::future::Future<Output = impl Sized> {
+ | -- hidden type `[async block@$DIR/nested-return-type4.rs:4:5: 4:31]` captures the lifetime `'s` as defined here
+LL | async move { let _s = s; }
+ | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+ |
+help: to declare that `impl Future<Output = impl Sized>` captures `'s`, you can add an explicit `'s` lifetime bound
+ |
+LL | fn test<'s: 's>(s: &'s str) -> impl std::future::Future<Output = impl Sized> + 's {
+ | ++++
+help: to declare that `impl Sized` captures `'s`, you can add an explicit `'s` lifetime bound
+ |
+LL | fn test<'s: 's>(s: &'s str) -> impl std::future::Future<Output = impl Sized + 's> {
+ | ++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0700`.