`ruff server`: Resolve configuration for each document individually (#10950)
## Summary
Configuration is no longer the property of a workspace but rather of
individual documents. Just like the Ruff CLI, each document is
configured based on the 'nearest' project configuration. See [the Ruff
documentation](https://docs.astral.sh/ruff/configuration/#config-file-discovery)
for more details.
To reduce the amount of times we resolve configuration for a file, we
have an index for each workspace that stores a reference-counted pointer
to a configuration for a given folder. If another file in the same
folder is opened, the configuration is simply re-used rather than us
re-resolving it.
## Guide for reviewing
The first commit is just the restructuring work, which adds some noise
to the diff. If you want to quickly understand what's actually changed,
I recommend looking at the two commits that come after it.
f7c073d4412242dfe09fd620371432735ffa0c18 makes configuration a property
of `DocumentController`/`DocumentRef`, moving it out of `Workspace`, and
it also sets up the `ConfigurationIndex`, though it doesn't implement
its key function, `get_or_insert`. In the commit after it,
fc35618f17f761de170125f089ce7fc054df356b, we implement `get_or_insert`.
## Test Plan
The best way to test this would be to ensure that the behavior matches
the Ruff CLI. Open a project with multiple configuration files (or add
them yourself), and then introduce problems in certain files that won't
show due to their configuration. Add those same problems to a section of
the project where those rules are run. Confirm that the lint rules are
run as expected with `ruff check`. Then, open your editor and confirm
that the diagnostics shown match the CLI output.
As an example - I have a workspace with two separate folders, `pandas`
and `scipy`. I created a `pyproject.toml` file in `pandas/pandas/io` and
a `ruff.toml` file in `pandas/pandas/api`. I changed the `select` and
`preview` settings in the sub-folder configuration files and confirmed
that these were reflected in the diagnostics. I also confirmed that this
did not change the diagnostics for the `scipy` folder whatsoever.
ruff
cffc5557 - `ruff server`: Resolve configuration for each document individually (#10950)
