concat_idents! is not very useful

[alexcrichton]

It cannot access local variables due to hygiene, it can't be used to create names of a function (not where a macro can appear), and it can basically only be used to access non-hygienic things.

I believe the utility of this macro has been greatly reduced over time, and it may be time to just remove it (or feature gate it)

Nominating (it's a core built-in macro)

[huonw]

(Feature gated in #13295.)

[pnkfelix]

not accepted for 1.0 milestone as this is now feature gated.

Assigning P-low (rather than closing) so that we can determine what else, if anything, to do here.

[kmcallister]

I think a good concat_idents! is important, although not urgent. Otherwise it's a big regression from C.