Merge remote-tracking branch 'sgrif/sg-macro-lifetimes'

Author: nikomatsakis

text/0000-macro-lifetimes.md

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+- 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, the only way to match against a lifetime is
+by capturing it as `tt`. Something like `'$lifetime:ident` would fail to
+compile. This is extremely limiting, as it becomes difficult to sanitize input,
+and `tt` is extremely difficult to use in a sequence without using awkward
+separators.
+
+# Detailed design
+[design]: #detailed-design
+
+This RFC proposes adding `lifetime` as an additional specifier to
+`macro_rules!` (alternatively: `life` or `lt`). As it is a single token, it is
+able to be followed by any other specifier. 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