use implied bounds when checking opaque types

compiler/rustc_hir_analysis/src/check/check.rs

@@ -31,6 +31,7 @@ use rustc_target::abi::FieldIdx;
 use rustc_target::spec::abi::Abi;
 use rustc_trait_selection::traits::error_reporting::on_unimplemented::OnUnimplementedDirective;
 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt as _;
+use rustc_trait_selection::traits::outlives_bounds::InferCtxtExt as _;
 use rustc_trait_selection::traits::{self, ObligationCtxt, TraitEngine, TraitEngineExt as _};
 
 use std::ops::ControlFlow;
@@ -222,7 +223,7 @@ fn check_opaque(tcx: TyCtxt<'_>, id: hir::ItemId) {
     if check_opaque_for_cycles(tcx, item.owner_id.def_id, substs, span, &origin).is_err() {
         return;
     }
-    check_opaque_meets_bounds(tcx, item.owner_id.def_id, substs, span, &origin);
+    check_opaque_meets_bounds(tcx, item.owner_id.def_id, span, &origin);
 }
 
 /// Checks that an opaque type does not use `Self` or `T::Foo` projections that would result
@@ -391,7 +392,6 @@ pub(super) fn check_opaque_for_cycles<'tcx>(
 fn check_opaque_meets_bounds<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
-    substs: SubstsRef<'tcx>,
     span: Span,
     origin: &hir::OpaqueTyOrigin,
 ) {
@@ -406,6 +406,8 @@ fn check_opaque_meets_bounds<'tcx>(
         .with_opaque_type_inference(DefiningAnchor::Bind(defining_use_anchor))
         .build();
     let ocx = ObligationCtxt::new(&infcx);
+
+    let substs = InternalSubsts::identity_for_item(tcx, def_id.to_def_id());
     let opaque_ty = tcx.mk_opaque(def_id.to_def_id(), substs);
 
     // `ReErased` regions appear in the "parent_substs" of closures/generators.
@@ -448,9 +450,18 @@ fn check_opaque_meets_bounds<'tcx>(
     match origin {
         // Checked when type checking the function containing them.
         hir::OpaqueTyOrigin::FnReturn(..) | hir::OpaqueTyOrigin::AsyncFn(..) => {}
+        // Nested opaque types occur only in associated types:
+        // ` type Opaque<T> = impl Trait<&'static T, AssocTy = impl Nested>; `
+        // They can only be referenced as `<Opaque<T> as Trait<&'static T>>::AssocTy`.
+        // We don't have to check them here because their well-formedness follows from the WF of
+        // the projection input types in the defining- and use-sites.
+        hir::OpaqueTyOrigin::TyAlias
+            if tcx.def_kind(tcx.parent(def_id.to_def_id())) == DefKind::OpaqueTy => {}
         // Can have different predicates to their defining use
         hir::OpaqueTyOrigin::TyAlias => {
-            let outlives_env = OutlivesEnvironment::new(param_env);
+            let wf_tys = ocx.assumed_wf_types(param_env, span, def_id);
+            let implied_bounds = infcx.implied_bounds_tys(param_env, def_id, wf_tys);
+            let outlives_env = OutlivesEnvironment::with_bounds(param_env, implied_bounds);
             let _ = ocx.resolve_regions_and_report_errors(defining_use_anchor, &outlives_env);
         }
     }

compiler/rustc_ty_utils/src/implied_bounds.rs

@@ -31,6 +31,18 @@ fn assumed_wf_types(tcx: TyCtxt<'_>, def_id: DefId) -> &ty::List<Ty<'_>> {
             }
         }
         DefKind::AssocConst | DefKind::AssocTy => tcx.assumed_wf_types(tcx.parent(def_id)),
+        DefKind::OpaqueTy => match tcx.def_kind(tcx.parent(def_id)) {
+            DefKind::TyAlias => ty::List::empty(),
+            DefKind::AssocTy => tcx.assumed_wf_types(tcx.parent(def_id)),
+            // Nested opaque types only occur in associated types:
+            // ` type Opaque<T> = impl Trait<&'static T, AssocTy = impl Nested>; `
+            // assumed_wf_types should include those of `Opaque<T>`, `Opaque<T>` itself
+            // and `&'static T`.
+            DefKind::OpaqueTy => bug!("unimplemented implied bounds for neseted opaque types"),
+            def_kind @ _ => {
+                bug!("unimplemented implied bounds for opaque types with parent {def_kind:?}")
+            }
+        },
         DefKind::Mod
         | DefKind::Struct
         | DefKind::Union
@@ -51,7 +63,6 @@ fn assumed_wf_types(tcx: TyCtxt<'_>, def_id: DefId) -> &ty::List<Ty<'_>> {
         | DefKind::ForeignMod
         | DefKind::AnonConst
         | DefKind::InlineConst
-        | DefKind::OpaqueTy
         | DefKind::ImplTraitPlaceholder
         | DefKind::Field
         | DefKind::LifetimeParam

tests/ui/type-alias-impl-trait/wf-in-associated-type.fail.stderr

@@ -0,0 +1,25 @@
+error[E0309]: the parameter type `T` may not live long enough
+  --> $DIR/wf-in-associated-type.rs:36:23
+   |
+LL |         type Opaque = impl Sized + 'a;
+   |                       ^^^^^^^^^^^^^^^ ...so that the type `&'a T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound...
+   |
+LL |     impl<'a, T: 'a> Trait<'a, T> for () {
+   |               ++++
+
+error[E0309]: the parameter type `T` may not live long enough
+  --> $DIR/wf-in-associated-type.rs:36:23
+   |
+LL |         type Opaque = impl Sized + 'a;
+   |                       ^^^^^^^^^^^^^^^ ...so that the reference type `&'a T` does not outlive the data it points at
+   |
+help: consider adding an explicit lifetime bound...
+   |
+LL |     impl<'a, T: 'a> Trait<'a, T> for () {
+   |               ++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0309`.

tests/ui/type-alias-impl-trait/wf-in-associated-type.rs

@@ -0,0 +1,45 @@
+// WF check for impl Trait in associated type position.
+//
+// revisions: pass fail
+// [pass] check-pass
+// [fail] check-fail
+
+#![feature(impl_trait_in_assoc_type)]
+
+// The hidden type here (`&'a T`) requires proving `T: 'a`.
+// We know it holds because of implied bounds from the impl header.
+#[cfg(pass)]
+mod pass {
+    trait Trait<Req> {
+        type Opaque1;
+        fn constrain_opaque1(req: Req) -> Self::Opaque1;
+    }
+
+    impl<'a, T> Trait<&'a T> for () {
+        type Opaque1 = impl IntoIterator<Item = impl Sized + 'a>;
+        fn constrain_opaque1(req: &'a T) -> Self::Opaque1 {
+            [req]
+        }
+    }
+}
+
+// The hidden type here (`&'a T`) requires proving `T: 'a`,
+// but that is not known to hold in the impl.
+#[cfg(fail)]
+mod fail {
+    trait Trait<'a, T> {
+        type Opaque;
+        fn constrain_opaque(req: &'a T) -> Self::Opaque;
+    }
+
+    impl<'a, T> Trait<'a, T> for () {
+        type Opaque = impl Sized + 'a;
+        //[fail]~^ ERROR the parameter type `T` may not live long enough
+        //[fail]~| ERROR the parameter type `T` may not live long enough
+        fn constrain_opaque(req: &'a T) -> Self::Opaque {
+            req
+        }
+    }
+}
+
+fn main() {}

tests/ui/type-alias-impl-trait/wf-nested.fail.stderr

@@ -0,0 +1,19 @@
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/wf-nested.rs:55:27
+   |
+LL |     type InnerOpaque<T> = impl Sized;
+   |                           ^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds...
+   |
+note: ...that is required by this bound
+  --> $DIR/wf-nested.rs:12:20
+   |
+LL | struct IsStatic<T: 'static>(T);
+   |                    ^^^^^^^
+help: consider adding an explicit lifetime bound...
+   |
+LL |     type InnerOpaque<T: 'static> = impl Sized;
+   |                       +++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0310`.

tests/ui/type-alias-impl-trait/wf-nested.pass_sound.stderr

@@ -0,0 +1,14 @@
+error[E0310]: the parameter type `T` may not live long enough
+  --> $DIR/wf-nested.rs:46:17
+   |
+LL |         let _ = outer.get();
+   |                 ^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds
+   |
+help: consider adding an explicit lifetime bound...
+   |
+LL |     fn test<T: 'static>() {
+   |              +++++++++
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0310`.

tests/ui/type-alias-impl-trait/wf-nested.rs

@@ -0,0 +1,60 @@
+// Well-formedness of nested opaque types, i.e. `impl Sized` in
+// `type Outer = impl Trait<Assoc = impl Sized>`.
+// See the comments below.
+//
+// revisions: pass pass_sound fail
+// [pass] check-pass
+// [pass_sound] check-fail
+// [fail] check-fail
+
+#![feature(type_alias_impl_trait)]
+
+struct IsStatic<T: 'static>(T);
+
+trait Trait<In> {
+    type Out;
+
+    fn get(&self) -> Result<Self::Out, ()> {
+        Err(())
+    }
+}
+
+impl<T> Trait<&'static T> for () {
+    type Out = IsStatic<T>;
+}
+
+// The hidden type for `impl Sized` is `IsStatic<T>`, which requires `T: 'static`.
+// We know it is well-formed because it can *only* be referenced as a projection:
+// <OuterOpaque<T> as Trait<&'static T>>::Out`.
+// So any instantiation of the type already requires proving `T: 'static`.
+#[cfg(pass)]
+mod pass {
+    use super::*;
+    type OuterOpaque<T> = impl Trait<&'static T, Out = impl Sized>;
+    fn define<T>() -> OuterOpaque<T> {}
+}
+
+// Test the soundness of `pass` - We should require `T: 'static` at the use site.
+#[cfg(pass_sound)]
+mod pass_sound {
+    use super::*;
+    type OuterOpaque<T> = impl Trait<&'static T, Out = impl Sized>;
+    fn define<T>() -> OuterOpaque<T> {}
+
+    fn test<T>() {
+        let outer = define::<T>();
+        let _ = outer.get(); //[pass_sound]~ ERROR `T` may not live long enough
+    }
+}
+
+// Similar to `pass` but here `impl Sized` can be referenced directly as
+// InnerOpaque<T>, so we require an explicit bound `T: 'static`.
+#[cfg(fail)]
+mod fail {
+    use super::*;
+    type InnerOpaque<T> = impl Sized; //[fail]~ ERROR `T` may not live long enough
+    type OuterOpaque<T> = impl Trait<&'static T, Out = InnerOpaque<T>>;
+    fn define<T>() -> OuterOpaque<T> {}
+}
+
+fn main() {}