rustup show: add flag to show only active toolchain

[TimNN]

I often want to know which toolchain is active for a certain directory (and which directory has the override). Having rustup show first print a few dozen lines of installed toolchains is rather annoying in that situation.

It would be nice to have a flag which would cause rustup show to only print the active toolchain. (For me it would even be preferable to make that the default behavior, since the list of installed toolchains is easily available via rustup toolchain list).

[das-sein]

@Diggsey I think this is resolved by rustup show active-toolchain, right?

[dwijnand]

Possibly resolved by #1633 (using rustup default instead)?

[TimNN]

@Diggsey I think this is resolved by rustup show active-toolchain, right?

This does not quite address my original issue: It shows the active toolchain, but not why it is active (e.g. which directory the override is coming from, or that the default is active).

Possibly resolved by #1633 (using rustup default instead)?

I don't have a build with that patch, but from the name of the command, I don't believe that this will resolve my issue: I'm interested in the active toolchain, not the default one.

[dwijnand]

Good point.

[das-sein]

@Diggsey I think I figured out what you mean. The rustup show command outputs the reason for the override for any override you have set, but rustup show active-toolchain didn't do that. I've started a PR with the more standard behavior. Let me know how I did.