Command for "install stuff from `rust-toolchain.toml`"

[jmaargh]

Apologies if this already exists, but I couldn't find it in the docs or shell help. Or if there is an alternate workflow for this.

I love rust-toolchain.toml, it's a great idea. It would be even more useful if there were a command such as rustup override install, which would look at the current rust-toolchain.toml and install the toolchain and components specified there. It would be super useful both when exploring projects for the first time, but also when using disposable container-based development environments.

I'm unopinionated on the exact syntax of this command. Also happy to help implement this if the maintainers want it.

### Tasks
- [ ] https://github.com/rust-lang/rustup/issues/3972

[rbtcollins]

See #1397

[jmaargh]

I feel like there's value in keeping this issue different from #1397 for a couple of reasons:

1. The main point of rustup show should not force-install the default toolchain if it is not installed. #1397 is to disable a certain behaviour, this is to add that behaviour somewhere else (something that was discussed in rustup show should not force-install the default toolchain if it is not installed. #1397, but isn't the main point)

2. It's not clear to me from rustup show should not force-install the default toolchain if it is not installed. #1397 whether what's being discussed is installing just the toolchain per-se, or also the components that can be set in a rust-toolchain.toml file. This issue is suggesting ensuring that everything specified in rust-toolchain.toml is installed and available.

[kinnison]

I think that ensuring rust-toolchain{,.toml} is properly honoured is an all-or-nothing thing - it couldn't "just" be the toolchain itself, no matter what.

You're right that #1397 is about ensuring that we don't over-indulge in autoinstallation, and I think this is a good home for how we then permit manual triggering of what was previously automatic.

I continue to think something like rustup toolchain ensure [PATH] seems like a good approach, but further straw men for discussion would be good.

[jmaargh]

I'm easy about the syntax. rustup toolchain ensure [path] works for me.

I was thinking re-using rustup toolchain install, so:

• rustup toolchain install [toolchain] works as currently
• rustup toolchain install [path to rust-toolchain.toml] installs according to the specified file
• rustup toolchian install [path to directory] finds the rust-toolchain.toml that would apply to that directory (if any) and asks for confirmation before installing, with the confirmation prompt specifying exactly which file and that you could have specified a toolchain instead.
• rustup toolchain install acts as rustup toolchian install .

[kinnison]

Given all the other arguments rustup toolchain install can be given, I'd favour a separate verb.

[rbtcollins]

There's a request to permit this during rustup-init as well; I've marked them as dup because I think its basically the same work... getting to a oneline for rustup not being installed is doable via:
rustup-init -y --no-update-default-toolchain && rustup toolchain ensure . (or whatever design we end up settling on).