Add a compatibility clause for UFCS method calls

Author: sfackler

active/0000-enum-namespacing.md

@@ -254,6 +254,32 @@ This should not affect much if any code in practice, as Rust's naming
 conventions state that variants should be capitalized and method names should
 not.
 
+## With other post-1.0 work
+
+Part of the UFCS work will (or at least discussed) allowing this:
+```
+trait Foo {
+    fn Baz();
+}
+
+enum Bar {
+    Baz,
+}
+
+impl Foo for Bar {
+    fn Baz() {}
+}
+
+fn main() {
+    Bar::Baz(); // instead of Foo::<for Bar>::Baz()
+}
+```
+If this is implemented before namespaced enums, we must add a restriction,
+similar to the one for intrinsic methods, that prohibits something like
+`Foo::Baz` from being called as in the above example. Note that the trait may
+be implemented for `Bar` without any problems, and the method may be called via
+the long form. It's just `Bar::Baz()` that must be disallowed.
+
 ## Later
 
 The compiler's resolve stage will be altered in two ways. It will place an