Deps: proc-macro

[ehuss]

This issue is for working through the implementation issues for compiling the proc-macro crate.

Usually proc-macro is built for the host compiler, and thus isn't much of a concern for std-aware cargo. However, some crates like proc-macro2 link against it to provide the same facility to normal code.

There may also be issues with how dependencies on proc-macro are expressed. If there is an explicit dependency listed on proc-macro, Cargo will need to be careful to consider if it is a host or target dependency.

[alexcrichton]

@ehuss do you think there's remaining issues here? I think that this is largely solved modulo #5 myself

[ehuss]

I can't think of any potential issues right now, so I'll go ahead and close.