Better document what exactly the action does

[RalfJung]

The summary says that this "installs a Rust toolchain using rustup". However it clearly does more than that, since the toolchain also becomes the default toolchain to use afterwards. How exactly is that done? Is it a rustup default or a rustup override set or something else? The details matter when there are rust-toolchain files in the folder, so it would be great to have this documented.

[szepeviktor]

rust-toolchain/action.yml

Line 75 in f361669

- run: rustup default ${{steps.parse.outputs.toolchain}}

[MarijnS95]

Would love it if the rustup default step could be disabled; I typically only use this action to quickly install nightly to run -Zminimal-versions generate-lockfile before compiling with stable (or any other MSRV).

[zvonkok]

If your project has a rust-toolchain.yml than this action will honor it and the default will be that your projects ships via rust-toolchain.yml

[baxterjo]

If your project has a rust-toolchain.yml than this action will honor it and the default will be that your projects ships via rust-toolchain.yml

I don't think that's accurate @dtolnay has been pretty adamant about not adding rust-toolchain.toml support to this action.

What you're seeing is likely that this action is installing the toolchain specified and then subsequent calls to rustup or rustc are using rust-toolchain.toml