Add auto trait that shouldn't leak TAIT test

Author: spastorino

src/test/ui/type-alias-impl-trait/auto-trait-leakage2.rs

@@ -0,0 +1,19 @@
+#![feature(type_alias_impl_trait)]
+#![allow(dead_code)]
+
+mod m {
+    use std::rc::Rc;
+
+    type Foo = impl std::fmt::Debug;
+
+    pub fn foo() -> Foo {
+        Rc::new(22_u32)
+    }
+}
+
+fn is_send<T: Send>(_: T) {}
+
+fn main() {
+    is_send(m::foo());
+    //~^ ERROR: `Rc<u32>` cannot be sent between threads safely [E0277]
+}

src/test/ui/type-alias-impl-trait/auto-trait-leakage2.stderr

@@ -0,0 +1,20 @@
+error[E0277]: `Rc<u32>` cannot be sent between threads safely
+  --> $DIR/auto-trait-leakage2.rs:17:5
+   |
+LL |     type Foo = impl std::fmt::Debug;
+   |                -------------------- within this `impl Debug`
+...
+LL |     is_send(m::foo());
+   |     ^^^^^^^ `Rc<u32>` cannot be sent between threads safely
+   |
+   = help: within `impl Debug`, the trait `Send` is not implemented for `Rc<u32>`
+   = note: required because it appears within the type `impl Debug`
+note: required by a bound in `is_send`
+  --> $DIR/auto-trait-leakage2.rs:14:15
+   |
+LL | fn is_send<T: Send>(_: T) {}
+   |               ^^^^ required by this bound in `is_send`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.