`cargo rustc` should have an `--all-targets` option

[Manishearth]

See also: https://github.com/Manishearth/rust-clippy/issues/935

Tools like cargo check and cargo clippy use cargo rustc, but they don't need the extra restriction of having to specify a single target. It would be nice if cargo rustc/rustdoc had an --all-targets or --allow-multiple-targets option that could be used here.

[alexcrichton]

Got a chance to talk about this with @wycats today and the conclusion was that this flag shouldn't be added for now. The workflow being created, cargo clippy, is using cargo rustc as an implementation detail, and it can just run multiple cargo rustc invocations with the relevant filters. Targets can already be learned about programmatically through cargo metadata so the plumbing should all be there.

Along those lines it doesn't quite make sense just yet to add new functionality to cargo rustc for just these use cases as it's intended to be a primitive, not necessarily ergonomic to call