Add a lifetime specifier to macro_rules!

Author: sgrif

text/0000-macro-lifetimes.md

@@ -0,0 +1,49 @@
+- Feature Name: Allow `lifetime` specifiers to be passed to macros
+- Start Date: 2016-04-22
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Add a `lifetime` specifier for `macro_rules!` patterns, that matches any valid
+lifetime.
+
+# Motivation
+[motivation]: #motivation
+
+Certain classes of macros are completely impossible without the ability to pass
+lifetimes. Specifically, anything that wants to implement a trait from inside of
+a macro is going to need to deal with lifetimes eventually. They're also
+commonly needed for any macros that need to deal with types in a more granular
+way than just `ty`.
+
+Since a lifetime is a single token, there is currently no way to accept one
+without an explicit matcher. Something like `'$lifetime:ident` will fail to
+compile.
+
+# Detailed design
+[design]: #detailed-design
+
+This RFC proposes adding `lifetime` as an additional specifier to
+`macro_rules!` (alternatively: `life` or `lt`). Since a lifetime acts very much
+like an identifier, and can appear in almost as many places, it can be handled
+almost identically. A preliminary implementation can be found at
+https://github.com/rust-lang/rust/pull/33135
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+None
+
+# Alternatives
+[alternatives]: #alternatives
+
+A more general specifier, such as a "type parameter list", which would roughly
+map to `ast::Generics` would cover most of the cases that matching lifetimes
+individually would cover.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None