Equivalent of `cfg_select`?

[WorldSEnder]

There are some contexts where the grammar of rust allows one macro, but not arbitrary expressions, e.g. attributes on expressions and impl heads.

Is there an interest in a cfg_select! macro that does the equivalent of std::cfg_select! for rust versions?

rustversion::cfg_select! {
    nightly => { Vec<T, A> }, 
    _       => { Vec<T> },
}
// expands on nightly to
// Vec<T, A>
// on stable to
// Vec<T>

Note in the example above that attributes are not allowed on generic arguments, hence naming the full type. A type alias is also not a good option since rust checks that all parameters are used and errors when the optional allocator argument is unused.

Would be interested in this and follow up with a PR if you are too.