compiler: report error when trait object type param reference self

Fixes #139082.

Emits an error when `Self` is found in the projection bounds of a trait
object. In type aliases, `Self` has no meaning, so `type A = &'static
dyn B` where `trait B = Fn() -> Self` will expands to `type A = &'static
Fn() -> Self` which is illegal, causing the region solver to bail out
when hitting the uninferred Self.

Bug: #139082
Signed-off-by: xtex <xtexchooser@duck.com>

2 files changed, 21 insertions, 0 deletions

diff --git a/tests/ui/dyn-compatibility/trait-alias-self-projection.rs b/tests/ui/dyn-compatibility/trait-alias-self-projection.rs
new file mode 100644
index 00000000000..0badb738809
--- /dev/null
+++ b/tests/ui/dyn-compatibility/trait-alias-self-projection.rs
@@ -0,0 +1,12 @@
+#![feature(trait_alias)]
+trait B = Fn() -> Self;
+type D = &'static dyn B;
+//~^ ERROR E0411
+
+fn a() -> D {
+    unreachable!();
+}
+
+fn main() {
+    _ = a();
+}
diff --git a/tests/ui/dyn-compatibility/trait-alias-self-projection.stderr b/tests/ui/dyn-compatibility/trait-alias-self-projection.stderr
new file mode 100644
index 00000000000..dccee02e9cd
--- /dev/null
+++ b/tests/ui/dyn-compatibility/trait-alias-self-projection.stderr
@@ -0,0 +1,9 @@
+error[E0411]: `Self` is not allowed in type aliases
+  --> $DIR/trait-alias-self-projection.rs:3:19
+   |
+LL | type D = &'static dyn B;
+   |                   ^^^^^ `Self` is only available in impls, traits, and concrete type definitions
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0411`.