Extend `cargo _path` to allow `+<toolchain>`

[wcampbell0x2a]

Could MetadataCommand::cargo_path be extended to add some sort of +<toolchain> syntax? I use guppy and need a nightly compiler when I invoke cargo metadata, but there isn't a way of easily setting $CARGO and making the user find the correct +nightly compiler path, let alone myself.

[sunshowers]

Thanks for the report!

So one challenge here is that the + syntax only works if cargo points to the rustup shim for it. That is often, but not always, the case.

Could you run rustup which cargo --toolchain nightly and pass that in?