Make "discover" the default for rust-analyzer.rustc.source?

[RalfJung]

Currently, to get RA to work on a project that uses rustc-private crates, one has to do two things:

• add a snippet to the Cargo.toml (as in https://github.com/rust-lang/miri/pull/1736)
• add "rust-analyzer.rustc.source": "discover" to the vscode settings

I wonder if the 2nd step could be made unnecessary, by making discover the default value for rust-analyzer.rustc.source? If I understand https://github.com/rust-lang/miri/pull/1736#issuecomment-792979551 correctly, that setting has no negative effects on crates without rustc_private, and for crates with rustc_private it is the value one usually needs.

[Veykril]

We eagerly fetch the metadata of the rustc workspace when the setting is set to discover (or a concrete path). So while we don't semantically load it (that is we don't build its crategraph), we do some other loading stuff (including loading the files into the VFS even though we don't need them)

[RalfJung]

Ah, so there still is some negative effect on other crates then. Bummer.

[JohnScience]

Is it possible to make a suggestion about enabling those when there's an unknown extern crate error?