typeck: Prohibit RPIT types that inherit lifetimes

This commit prohibits return position `impl Trait` types that "inherit
lifetimes" from the parent scope. The intent is to forbid cases that are
challenging until they can be addressed properly.

src/librustc_typeck/check/mod.rs

@@ -1315,7 +1315,88 @@ fn check_union(tcx: TyCtxt<'_>, id: hir::HirId, span: Span) {
     check_packed(tcx, span, def_id);
 }
 
+/// Checks that an opaque type does not contain cycles and does not use `Self` or `T::Foo`
+/// projections that would result in "inheriting lifetimes".
 fn check_opaque<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId, substs: SubstsRef<'tcx>, span: Span) {
+    check_opaque_for_inheriting_lifetimes(tcx, def_id, span);
+    check_opaque_for_cycles(tcx, def_id, substs, span);
+}
+
+/// Checks that an opaque type does not use `Self` or `T::Foo` projections that would result
+/// in "inheriting lifetimes".
+fn check_opaque_for_inheriting_lifetimes(
+    tcx: TyCtxt<'tcx>,
+    def_id: DefId,
+    span: Span,
+) {
+    let item = tcx.hir().expect_item(
+        tcx.hir().as_local_hir_id(def_id).expect("existential type is not local"));
+    debug!("check_opaque_for_inheriting_lifetimes: def_id={:?} span={:?} item={:?}",
+           def_id, span, item);
+
+    #[derive(Debug)]
+    struct ProhibitOpaqueVisitor<'tcx> {
+        opaque_identity_ty: Ty<'tcx>,
+        generics: &'tcx ty::Generics,
+    };
+
+    impl<'tcx> ty::fold::TypeVisitor<'tcx> for ProhibitOpaqueVisitor<'tcx> {
+        fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
+            debug!("check_opaque_for_inheriting_lifetimes: (visit_ty) t={:?}", t);
+            if t == self.opaque_identity_ty { false } else { t.super_visit_with(self) }
+        }
+
+        fn visit_region(&mut self, r: ty::Region<'tcx>) -> bool {
+            debug!("check_opaque_for_inheriting_lifetimes: (visit_region) r={:?}", r);
+            if let RegionKind::ReEarlyBound(ty::EarlyBoundRegion { index, .. }) = r {
+                return *index < self.generics.parent_count as u32;
+            }
+
+            r.super_visit_with(self)
+        }
+    }
+
+    let prohibit_opaque = match item.node {
+        ItemKind::Existential(hir::ExistTy { origin: hir::ExistTyOrigin::AsyncFn, .. }) |
+        ItemKind::Existential(hir::ExistTy { origin: hir::ExistTyOrigin::ReturnImplTrait, .. }) => {
+            let mut visitor = ProhibitOpaqueVisitor {
+                opaque_identity_ty: tcx.mk_opaque(
+                    def_id, InternalSubsts::identity_for_item(tcx, def_id)),
+                generics: tcx.generics_of(def_id),
+            };
+            debug!("check_opaque_for_inheriting_lifetimes: visitor={:?}", visitor);
+
+            tcx.predicates_of(def_id).predicates.iter().any(
+                |(predicate, _)| predicate.visit_with(&mut visitor))
+        },
+        _ => false,
+    };
+
+    debug!("check_opaque_for_inheriting_lifetimes: prohibit_opaque={:?}", prohibit_opaque);
+    if prohibit_opaque {
+        let is_async = match item.node {
+            ItemKind::Existential(hir::ExistTy { origin, .. }) => match origin {
+                hir::ExistTyOrigin::AsyncFn => true,
+                _ => false,
+            },
+            _ => unreachable!(),
+        };
+
+        tcx.sess.span_err(span, &format!(
+            "`{}` return type cannot contain a projection or `Self` that references lifetimes from \
+             a parent scope",
+            if is_async { "async fn" } else { "impl Trait" },
+        ));
+    }
+}
+
+/// Checks that an opaque type does not contain cycles.
+fn check_opaque_for_cycles<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    def_id: DefId,
+    substs: SubstsRef<'tcx>,
+    span: Span,
+) {
     if let Err(partially_expanded_type) = tcx.try_expand_impl_trait_type(def_id, substs) {
         let mut err = struct_span_err!(
             tcx.sess, span, E0720,

src/test/ui/async-await/issue-61949-self-return-type.rs

@@ -0,0 +1,28 @@
+// ignore-tidy-linelength
+// edition:2018
+#![feature(async_await)]
+
+// This test checks that `Self` is prohibited as a return type. See #61949 for context.
+
+pub struct Foo<'a> {
+    pub bar: &'a i32,
+}
+
+impl<'a> Foo<'a> {
+    pub async fn new(_bar: &'a i32) -> Self {
+    //~^ ERROR `async fn` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
+        Foo {
+            bar: &22
+        }
+    }
+}
+
+async fn foo() {
+    let x = {
+        let bar = 22;
+        Foo::new(&bar).await
+    };
+    drop(x);
+}
+
+fn main() { }

src/test/ui/async-await/issue-61949-self-return-type.stderr

@@ -0,0 +1,8 @@
+error: `async fn` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
+  --> $DIR/issue-61949-self-return-type.rs:12:40
+   |
+LL |     pub async fn new(_bar: &'a i32) -> Self {
+   |                                        ^^^^
+
+error: aborting due to previous error
+

src/test/ui/impl-trait/bound-normalization-fail.rs

@@ -1,4 +1,5 @@
 // compile-fail
+// ignore-tidy-linelength
 // edition:2018
 
 #![feature(async_await)]
@@ -45,7 +46,8 @@ mod lifetimes {
 
     /// Missing bound constraining `Assoc`, `T::Assoc` can't be normalized further.
     fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output=T::Assoc> {
-        //~^ ERROR: type mismatch
+    //~^ ERROR: type mismatch
+    //~^^ ERROR `impl Trait` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
         Foo(())
     }
 }

src/test/ui/impl-trait/bound-normalization-fail.stderr

@@ -1,11 +1,11 @@
 warning: the feature `impl_trait_in_bindings` is incomplete and may cause the compiler to crash
-  --> $DIR/bound-normalization-fail.rs:6:12
+  --> $DIR/bound-normalization-fail.rs:7:12
    |
 LL | #![feature(impl_trait_in_bindings)]
    |            ^^^^^^^^^^^^^^^^^^^^^^
 
 error[E0271]: type mismatch resolving `<Foo<()> as FooLike>::Output == <T as impl_trait::Trait>::Assoc`
-  --> $DIR/bound-normalization-fail.rs:30:32
+  --> $DIR/bound-normalization-fail.rs:31:32
    |
 LL |     fn foo_fail<T: Trait>() -> impl FooLike<Output=T::Assoc> {
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found associated type
@@ -14,8 +14,14 @@ LL |     fn foo_fail<T: Trait>() -> impl FooLike<Output=T::Assoc> {
               found type `<T as impl_trait::Trait>::Assoc`
    = note: the return type of a function must have a statically known size
 
+error: `impl Trait` return type cannot contain a projection or `Self` that references lifetimes from a parent scope
+  --> $DIR/bound-normalization-fail.rs:48:41
+   |
+LL |     fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output=T::Assoc> {
+   |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0271]: type mismatch resolving `<Foo<()> as FooLike>::Output == <T as lifetimes::Trait<'static>>::Assoc`
-  --> $DIR/bound-normalization-fail.rs:47:41
+  --> $DIR/bound-normalization-fail.rs:48:41
    |
 LL |     fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output=T::Assoc> {
    |                                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected (), found associated type
@@ -24,6 +30,6 @@ LL |     fn foo2_fail<'a, T: Trait<'a>>() -> impl FooLike<Output=T::Assoc> {
               found type `<T as lifetimes::Trait<'static>>::Assoc`
    = note: the return type of a function must have a statically known size
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0271`.