Note that this is not actually *used* by default; it is a matter of
configuration still, because you might want to:
- Compile all .rs files with `rustc %` (where each can be built itself)
- Compile all .rs files with `rustc some-file.rs` (where you are editing
part of a crate)
- Compile with a different tool, such as `make`. (In this case you might
put a `~/.vim/after/compiler/rustc.vim` to match such cases, set
makeprg and extend errorformat as appropriate. That should probably go
in a different compiler mode, e.g. make-rustc.)
To try using it, `:compiler rustc`. Then, `:make` on a file you would
run `rustc` on will work its magic, invoking rustc. To automate this,
you could have something like `autocmd FileType rust compiler rustc` in
your Vim config.
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