Publishing cargo verification

[ehuss]

Cargo's CI should verify that its packages can build before being published.

Cargo is published to crates.io with --no-verify, in part because the publishing system isn't prepared for doing verification. However, that would also only verify for one platform.

Ever since switching cargo to a workspace, we've had problems with workspace feature unification causing problems that aren't discovered until after cargo is published (#12017, #12565). Our CI can't detect these issues because all tests are done with feature unification.

Unfortunately cargo package can't be used due to version bumps of local dependencies (#9260).

I'm not sure if or how this could be fixed without changes to cargo itself.

[epage]

Unsure why I didn't recommend this before but cargo-hack (which we already use for MSRV verification) is able to run Cargo commands on packages in isolation and can automatically enumerate feature combintions.

Granted, once someone implements the non-unified version of #14774, we can use that.