Currently, rust-analyzer highlights the entire region when a `cfg` is
inactive (e.g. `#[cfg(windows)]` on a Linux machine). However,
unlinked files only highlight the first three characters of the file.
This was introduced in #8444, but users have repeatedly found
themselves with no rust-analyzer support for a file and unsure
why (see e.g. #13226 and the intentionally prominent pop-up added in
PR #14366).
(Anecdotally, we see this issue bite our users regularly, particularly
people new to Rust.)
Instead, highlight the entire inactive file, but mark it as all as
unused. This allows users to hover and run the quickfix from any line.
Whilst this is marginally more prominent, it's less invasive than a
pop-up, and users do want to know why they're getting no rust-analyzer
support in certain files.
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