Add tests for opaque types

Author: lcnr

src/test/ui/type-alias-impl-trait/structural-match-no-leak.rs

@@ -0,0 +1,20 @@
+#![feature(const_fn, type_alias_impl_trait)]
+
+type Bar = impl Send;
+
+// While i32 is structural-match, we do not want to leak this information.
+// (See https://github.com/rust-lang/rust/issues/72156)
+const fn leak_free() -> Bar {
+    7i32
+}
+const LEAK_FREE: Bar = leak_free();
+
+fn leak_free_test() {
+    match todo!() {
+        LEAK_FREE => (),
+        //~^ opaque types cannot be used in patterns
+        _ => (),
+    }
+}
+
+fn main() { }

src/test/ui/type-alias-impl-trait/structural-match-no-leak.stderr

@@ -0,0 +1,8 @@
+error: opaque types cannot be used in patterns
+  --> $DIR/structural-match-no-leak.rs:14:9
+   |
+LL |         LEAK_FREE => (),
+   |         ^^^^^^^^^
+
+error: aborting due to previous error
+

src/test/ui/type-alias-impl-trait/structural-match.rs

@@ -0,0 +1,21 @@
+#![feature(const_fn, type_alias_impl_trait)]
+
+type Foo = impl Send;
+
+// This is not structural-match
+struct A;
+
+const fn value() -> Foo {
+    A
+}
+const VALUE: Foo = value();
+
+fn test() {
+    match todo!() {
+        VALUE => (),
+        //~^ opaque types cannot be used in patterns
+        _ => (),
+    }
+}
+
+fn main() { }

src/test/ui/type-alias-impl-trait/structural-match.stderr

@@ -0,0 +1,8 @@
+error: opaque types cannot be used in patterns
+  --> $DIR/structural-match.rs:15:9
+   |
+LL |         VALUE => (),
+   |         ^^^^^
+
+error: aborting due to previous error
+