ICE on Beta and Nightly; Underscores in macro definitions

[zrneely]

Underscores as "variable" names in macro definitions cause an ICE on Beta and Nightly, but the error message works as expected on Stable. A quick example can be seen here: https://is.gd/3sWZP5

[TimNN]

This has been fixed in the latest nightly, probably by #33870 (which is not yet available on the playpen).

[alexcrichton]

Yay!