Allow the Self type to be used in impls.

Closes rust-lang#310

Author: nrc

0000-self-impl.md

@@ -0,0 +1,49 @@
+- Start Date: 2014-12-13
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Allow `Self` type to be used in impls.
+
+# Motivation
+
+Allows macros which operate on methods to do more, more easily without having to
+rebuild the concrete self type. Macros could use the literal self type like
+programmers do, but that requires extra machinery in the macro expansion code
+and extra work by the macro author.
+
+Allows easier copy and pasting of method signatures from trait declarations to
+implementations.
+
+Is more succinct where the self type is complex.
+
+## Motivation for doing this now
+
+I'm hitting the macro problem in a side project. I wrote and hope to land the
+compiler code to make it work, but it is ugly and this is a much nicer solution.
+It is also really easy to implement, and since it is just a desugaring, it
+should not add any additional complexity to the compiler. Obviously, this should
+not block 1.0.
+
+# Detailed design
+
+When used inside an impl, `Self` is desugared during syntactic expansion to the
+concrete type being implemented. `Self` can be used anywhere the desugared type
+could be used.
+
+# Drawbacks
+
+There are some advantages to being explicit about the self type where it is
+possible - clarity and fewer type aliases.
+
+# Alternatives
+
+We could just force authors to use the concrete type as we do currently. This
+would require macro expansion code to make available the concrete type (or the
+whole impl AST) to macros working on methods. The macro author would then
+extract/construct the self type and use it instead of `Self`.
+
+# Unresolved questions
+
+None.