`macro_rules!` metavariable for generic

I was working on a macro which implements a trait for a type, which I wanted to be a declarative macro, but this doesn't work if the type has generic arguments that I want to use as generic.

Toy example of what I want to do

This case is simple enough that I could match the generics with $( <$param:tt> )?, but more generally you can produce a knot of lifetimes, types, and const values with potentially quite complicated constraints on them, that is very difficult (maybe impossible?) to parse properly.

So it got me thinking: is there any reason why there isn't a :generic metavariable to use in declarative macros? I looked up and saw a couple of discussions on here that fizzled out without anything coming of them, and nothing in issue tickets or RFCs on GitHub, though it's possible I missed something.


The usual hack for this is to adjust the grammar you give to the macro a bit, and match essentially for [$($For:tt)*] where [$($Where:tt)*], with just declarations in the former with any and all bounds in the latter.

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I've definitely wanted this on multiple occasions, and would fully support an RFC proposing it.


Another metavariable type that I've wanted a few times is :bound, for opaquely taking lifetime or trait bounds.

I have written a few macro_rules! that need to manually parse generic parameter lists, and parsing bounds is the hardest part of it. Parsing generic parameters opaquely would not have been useful for me because I needed to use them in both the generic parameter list and as generic arguments.


I made this into an RFC: Fragment Specifiers for Generic Arguments by JarredAllen · Pull Request #3442 · rust-lang/rfcs · GitHub

Also thanks to @matt1985 for suggesting taking pieces of the generic parameter list and making them into their own pieces, I've incorporated that idea into the RFC also,

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