If the user doesn't have rustc on $PATH, rust-analyzer won't be able
to run `rustc --print cfg`. This isn't really an error, as
rust-analyzer can still proceed without it.
This is particularly noticeable when loading crates defined in a
rust-project.json. Until the configuration is loaded, the opened files
are briefly treated as detached files and users see this error.
Environments with rust-project.json generally have a sysroot and rustc
elsewhere, so the error confuses users.
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