Unit tests of proc-macro crates

[dtolnay]

Running cargo miri test --lib on a crate that has [lib] proc-macro = true in Cargo.toml prints the following and does not run the unit tests:

Running unit tests of `proc-macro` crates is not currently supported by Miri.

Unit tests inside proc macro crates are allowed and supported by cargo test --lib.

I found this message originated a year ago from #1675 but I did not find a corresponding issue that I could follow, so here is one.

[dtolnay]

I'm not sure whether this would be blocked on #1542. Proc macro unit tests when run by cargo test are not allowed to instantiate proc macro bridge objects (it's the same panic as if you had tried to instantiate one from a bin crate) so implementing the proc macro server API wouldn't be necessary to support running unit tests.

That said, it would be really slick if cargo miri test would allow working with proc macro objects from a unit test.

[RalfJung]

Good point, we should certainly track this somewhere.

Conceptually cargo treats proc-macros as being on the host while Miri is responsible for the target, so I am not sure if and how this will be possible. I am not deeply familiar with proc-macros and how cargo treats them.