- Feature Name: Allow
lifetimespecifiers to be passed to macros - Start Date: 2016-04-22
- RFC PR: (leave this empty)
- Rust Issue: (leave this empty)
Add a lifetime specifier for macro_rules! patterns, that matches any valid
lifetime.
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.
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 rust-lang/rust#33135
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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.
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