Tracking issue for `concat_idents`

[aturon]

Tracks stabilization for the concat_idents macro.

Update(fmease): Please see #124225 (comment) (an alternative to this feature).

[retep998]

concat_idents is so fundamentally useless at the moment because macros cannot be used in ident positions.

Yeah it is pretty much useless right now (see #12249 #13294).

[nrc]

Relevant blog for future plans: http://www.ncameron.org/blog/untitledconcat_idents-and-macros-in-ident-position/

[eddyb]

A solution that has been thrown around on IRC (but never ended up anywhere else AFAIK):
Have a macro invocation form for "early expansion", i.e. the following would work without macros in ident position:

macro_rules! wrap {
    ($name:ident) => { struct concat_idents!!(Wrapped, $name)($name); }
}

The "early expansion" macro_name!!(args) syntax was originally $*macro_name!(args) and is a subject of bikeshed.

If we move towards having all macros produce token streams that are parsed on-demand, such "early expansion" could be used in many more locations without adding macro support for each one.
Generic parameters, ADT fields, function signatures and match arms come to mind - there are so many recursive push-down hacks macros can end up with, just to construct lists of things and whatnot, that would simply not be necessary with "early expansion".

The only disadvantage is that concat_idents!! and friends would only work inside the RHS of macro_rules!, but I don't really see why you would need to use such a capability outside of a macro.

[skade]

https://github.com/mikkyang/rust-blas uses it for quick notation of FFI functions that follow a certain scheme: https://github.com/mikkyang/rust-blas/blob/master/src/prefix.rs https://github.com/mikkyang/rust-blas/blob/master/src/matrix/ll.rs

[skade]

Well, given that mikkyang/rust-blas#12 is a nicer solution within the current language, I'd also like to raise my hand for "useless".

[nrc]

I think we should never stabilise the current version - as others have noted it is useless.

[aidanhs]

Since this is the tracking issue, other who stumble across it might be interested in https://github.com/SkylerLipthay/interpolate_idents which works on nightly rust.

[retep998]

Unfortunately that solution will always depend on nightly Rust (until the day that syntax extensions become stable and pigs fly).

[skade]

Given that the consensus seems to be "useless" and theres nicer solutions (e.g. [1], maybe we can remove this feature?

[1] https://github.com/mikkyang/rust-blas/pull/12/files