Get the Rust version for a specific directory

[TonalidadeHidrica]

Problem you are trying to solve

If we run rustc --version for a directory when the version specified in e.g. rust-toolchain is not installed, rustup automatically downloads the version.

Solution you'd like

It's better to have a way to obtain the Rust version that would be used for current working directory (or any other directory) without obtaining actual binary.

Notes

This would be especially useful for shell extensions that shows the current Rust version. The real-world examples are:

• starship, which re-implements the algorithm to resolve the Rust version in order to avoid running anything like rustup or rustc;
• tide plugin for fish shell, which has a feature to show the Rust version in the prompt, which triggers inevitable download when cd-ing to a working directory containing rust-toolchain specifying not-currently-installed toolchain

[rbtcollins]

rustc itself cannot be run without being downloaded - and the proxy doesn't know what version an arbitrary binary will output.

We could make rustup show work without downloading the toolchain, and there is an open feature request to enable that. Would that be sufficient?

[TonalidadeHidrica]

@rbtcollins Yes, making rustup show wor without downloading the toolchain will do.

[TonalidadeHidrica]

@rbtcollins Which specific ticket requests the feature?

[TonalidadeHidrica]

(After two years) I found the issue about this topic, #1397, and by luck it has been closed just three days ago. So I think I can close the issue, horray!