Macro invocations allow type parameters in the path

[Manishearth]

vec::<T,U,V,W::X::Y<Z>>![1,2,3,4]; is valid syntax and compiles to a regular vector.

This is because macros use Paths, not Idents as their names. Multi-segment paths are disallowed, but the path can have type parameters.

We can move macros to Idents, however this would not be backwards compatible since anyone using vec::<T>![1,2] would get a syntax error (even though the original syntax is meaningless). It's still a bug, so perhaps we can get away with this breaking change.

Thoughts?

cc @eddyb @alexcrichton

[eddyb]

I'd say fix it and do a crater run.

[alexcrichton]

Sounds like a plan to me

[DemiMarie]

Ping?

[eddyb]

@drbo I don't think anyone worked on it.
In case someone wants to work on it, the two relevant points are type macros and expressions macros.
It should be possible to error if the path is absolute, has more than one component, or any lifetime/type parameters. We can then do a crater run to see if these are used in the wild for some reason.

[eddyb]

cc @jseyfried @petrochenkov @nrc

[jseyfried]

I fixed this in #34495.