Bug 1646256 - Make rust-analyzer and YouCompleteMe work with mozilla-central. r=sylvestre

The default configuration recommended here[1], while being a good
generic default, it doesn't quite work for mozilla-central as that peeks
the closest ancestor directory with a `Cargo.toml`.

For example, when editing under `servo/components/style`, it'd try to
use that directory as a project root and fail like:

```
    0: Failed to run `cargo metadata --manifest-path /home/emilio/src/moz/gecko/servo/components/style/Cargo.toml`
    1: Error during execution of `cargo metadata`:     Updating crates.io index
       error: failed to get `servo_atoms` as a dependency of package `style v0.0.1 (/home/emilio/src/moz/gecko/servo/components/style)`
       Caused by:
         failed to load source for dependency `servo_atoms`
```

Instead, we want to override it using the `'project_directory'` setting
so that we hit this path[2] and it works out of the box.

[1]: https://rust-analyzer.github.io/manual.html#youcompleteme
[2]: 63db7ea379/ycmd/completers/language_server/language_server_completer.py (L1914-L1916)

Differential Revision: https://phabricator.services.mozilla.com/D79938

.ycm_extra_conf.py

@@ -36,7 +36,12 @@ def _is_likely_cpp_header(filename):
 def Settings(**kwargs):
     if kwargs[ 'language' ] == 'cfamily':
         return FlagsForFile(kwargs['filename'])
-    return {}
+    # This is useful for generic language server protocols, like rust-analyzer,
+    # to discover the right project root instead of guessing based on where the
+    # closest Cargo.toml is.
+    return {
+      'project_directory': '.',
+    }
 
 
 def FlagsForFile(filename):

docs/contributing/editor.rst

@@ -64,6 +64,9 @@ As long as that is installed and you have run :code:`./mach build` or
 :code:`./mach configure`, it should work out of the box. Configuration for this lives
 in :code:`.ycm_extra_conf` at the root of the repo.
 
+Rust auto-completion should work both with the default completer (RLS, as of
+this writing), or with `rust-analyzer <https://rust-analyzer.github.io/manual.html#youcompleteme>`__.
+
 ESLint
 ~~~~~~