add tests for metadata for unsized generics and opaques

src/test/ui/traits/pointee-tail-is-generic-errors.rs

@@ -0,0 +1,22 @@
+// edition:2018
+
+#![feature(ptr_metadata)]
+#![feature(type_alias_impl_trait)]
+
+type Opaque = impl std::fmt::Debug + ?Sized;
+
+fn opaque() -> &'static Opaque {
+    &[1] as &[i32]
+}
+
+fn a<T: ?Sized>() {
+    is_thin::<T>();
+    //~^ ERROR type mismatch resolving `<T as Pointee>::Metadata == ()`
+
+    is_thin::<Opaque>();
+    //~^ ERROR type mismatch resolving `<impl Debug + ?Sized as Pointee>::Metadata == ()`
+}
+
+fn is_thin<T: std::ptr::Pointee<Metadata = ()> + ?Sized>() {}
+
+fn main() {}

src/test/ui/traits/pointee-tail-is-generic-errors.stderr

@@ -0,0 +1,40 @@
+error[E0271]: type mismatch resolving `<T as Pointee>::Metadata == ()`
+  --> $DIR/pointee-tail-is-generic-errors.rs:13:5
+   |
+LL |     is_thin::<T>();
+   |     ^^^^^^^^^^^^ expected `()`, found associated type
+   |
+   = note:    expected unit type `()`
+           found associated type `<T as Pointee>::Metadata`
+   = help: consider constraining the associated type `<T as Pointee>::Metadata` to `()`
+   = note: for more information, visit https://doc.rust-lang.org/book/ch19-03-advanced-traits.html
+note: required by a bound in `is_thin`
+  --> $DIR/pointee-tail-is-generic-errors.rs:20:33
+   |
+LL | fn is_thin<T: std::ptr::Pointee<Metadata = ()> + ?Sized>() {}
+   |                                 ^^^^^^^^^^^^^ required by this bound in `is_thin`
+
+error[E0271]: type mismatch resolving `<impl Debug + ?Sized as Pointee>::Metadata == ()`
+  --> $DIR/pointee-tail-is-generic-errors.rs:16:5
+   |
+LL | type Opaque = impl std::fmt::Debug + ?Sized;
+   |               ----------------------------- the found opaque type
+...
+LL |     is_thin::<Opaque>();
+   |     ^^^^^^^^^^^^^^^^^ expected `()`, found associated type
+   |
+   = note:    expected unit type `()`
+           found associated type `<impl Debug + ?Sized as Pointee>::Metadata`
+note: required by a bound in `is_thin`
+  --> $DIR/pointee-tail-is-generic-errors.rs:20:33
+   |
+LL | fn is_thin<T: std::ptr::Pointee<Metadata = ()> + ?Sized>() {}
+   |                                 ^^^^^^^^^^^^^ required by this bound in `is_thin`
+help: consider constraining the associated type `<impl Debug + ?Sized as Pointee>::Metadata` to `()`
+   |
+LL | type Opaque = impl std::fmt::Debug<Metadata = ()> + ?Sized;
+   |                                   +++++++++++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0271`.