Implement #[define_opaque] attribute for functions.

example/issue-72793.rs

@@ -2,23 +2,21 @@
 
 #![feature(type_alias_impl_trait)]
 
-mod helper {
+pub trait T {
-    pub trait T {
+    type Item;
-        type Item;
+}
-    }
+
-
+pub type Alias<'a> = impl T<Item = &'a ()>;
-    pub type Alias<'a> = impl T<Item = &'a ()>;
+
-
+struct S;
-    struct S;
+impl<'a> T for &'a S {
-    impl<'a> T for &'a S {
+    type Item = &'a ();
-        type Item = &'a ();
+}
-    }
+
-
+#[define_opaque(Alias)]
-    pub fn filter_positive<'a>() -> Alias<'a> {
+pub fn filter_positive<'a>() -> Alias<'a> {
-        &S
+    &S
-    }
 }
-use helper::*;
 
 fn with_positive(fun: impl Fn(Alias<'_>)) {
     fun(filter_positive());