Macro to run a macro over each tuple type

Originally filed by @joshtriplett:

In the course of implementing the Bounded trait for tuples (https://github.com/rust-lang/num/pull/89), I implemented a generic macro that runs the macro passed as its argument over every tuple type up to 20:

macro_rules! for_each_tuple_ {
 ( $m:ident !! ) => (
 $m! { }
 );
 ( $m:ident !! $h:ident, $($t:ident,)* ) => (
 $m! { $h $($t)* }
 for_each_tuple_! { $m !! $($t,)* }
 );
}
macro_rules! for_each_tuple {
 ( $m:ident ) => (
 for_each_tuple_! { $m !! A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, }
 );
}

There are other instances of this pattern in Rust libraries; for instance, see https://github.com/rust-lang/rust/blob/8f5b5f94dcdb9884737dfbc8efd893d1d70f0b14/src/libcore/hash/mod.rs#L239 and https://github.com/rust-lang/rust/blob/8f5b5f94dcdb9884737dfbc8efd893d1d70f0b14/src/libcore/hash/mod.rs#L259 . The latter could be replaced with for_each_tuple!(impl_hash_tuple);

Would it make sense to put something like this somewhere in the Rust standard libraries, marked as unstable since it depends on defining macros? This would be useful for anyone implementing traits on tuples.

Review the proposed macro pattern and the related tuple implementations in libcore/hash/mod.rs referenced by the issue. Determine whether a reusable standard-library facility for applying macros to tuple types is appropriate, including its unstable status and placement. Done means reaching and documenting a clear implementation or rejection decision.