Add support for using uv as an alternative formatter backend (#19665)

This adds a new `backend: internal | uv` option to the LSP
`FormatOptions` allowing users to perform document and range formatting
operations though uv. The idea here is to prototype a solution for users
to transition to a `uv format` command without encountering version
mismatches (and consequently, formatting differences) between the LSP's
version of `ruff` and uv's version of `ruff`.

The primarily alternative to this would be to use uv to discover the
`ruff` version used to start the LSP in the first place. However, this
would increase the scope of a minimal `uv format` command beyond "run a
formatter", and raise larger questions about how uv should be used to
coordinate toolchain discovery. I think those are good things to
explore, but I'm hesitant to let them block a `uv format`
implementation. Another downside of using uv to discover `ruff`, is that
it needs to be implemented _outside_ the LSP; e.g., we'd need to change
the instructions on how to run the LSP and implement it in each editor
integration, like the VS Code plugin.

---------

Co-authored-by: Dhruv Manilawala <dhruvmanila@gmail.com>

crates/ruff_server/src/session/index/ruff_settings.rs

@@ -401,6 +401,7 @@ impl ConfigurationTransformer for EditorConfigurationTransformer<'_> {
             configuration,
             format_preview,
             lint_preview,
+            format_backend: _,
             select,
             extend_select,
             ignore,

crates/ruff_server/src/session/options.rs

@@ -7,9 +7,12 @@ use serde_json::{Map, Value};
 
 use ruff_linter::{RuleSelector, line_width::LineLength, rule_selector::ParseError};
 
-use crate::session::{
-    Client,
-    settings::{ClientSettings, EditorSettings, GlobalClientSettings, ResolvedConfiguration},
+use crate::{
+    format::FormatBackend,
+    session::{
+        Client,
+        settings::{ClientSettings, EditorSettings, GlobalClientSettings, ResolvedConfiguration},
+    },
 };
 
 pub(crate) type WorkspaceOptionsMap = FxHashMap<Url, ClientOptions>;
@@ -124,6 +127,7 @@ impl ClientOptions {
             configuration,
             lint_preview: lint.preview,
             format_preview: format.preview,
+            format_backend: format.backend,
             select: lint.select.and_then(|select| {
                 Self::resolve_rules(
                     &select,
@@ -283,11 +287,13 @@ impl Combine for LintOptions {
 #[serde(rename_all = "camelCase")]
 struct FormatOptions {
     preview: Option<bool>,
+    backend: Option<FormatBackend>,
 }
 
 impl Combine for FormatOptions {
     fn combine_with(&mut self, other: Self) {
         self.preview.combine_with(other.preview);
+        self.backend.combine_with(other.backend);
     }
 }
 
@@ -443,6 +449,12 @@ pub(crate) trait Combine {
     fn combine_with(&mut self, other: Self);
 }
 
+impl Combine for FormatBackend {
+    fn combine_with(&mut self, other: Self) {
+        *self = other;
+    }
+}
+
 impl<T> Combine for Option<T>
 where
     T: Combine,
@@ -584,6 +596,7 @@ mod tests {
                     format: Some(
                         FormatOptions {
                             preview: None,
+                            backend: None,
                         },
                     ),
                     code_action: Some(
@@ -640,6 +653,7 @@ mod tests {
                         format: Some(
                             FormatOptions {
                                 preview: None,
+                                backend: None,
                             },
                         ),
                         code_action: Some(
@@ -704,6 +718,7 @@ mod tests {
                         format: Some(
                             FormatOptions {
                                 preview: None,
+                                backend: None,
                             },
                         ),
                         code_action: Some(
@@ -782,6 +797,7 @@ mod tests {
                     configuration: None,
                     lint_preview: Some(true),
                     format_preview: None,
+                    format_backend: None,
                     select: Some(vec![
                         RuleSelector::Linter(Linter::Pyflakes),
                         RuleSelector::Linter(Linter::Isort)
@@ -819,6 +835,7 @@ mod tests {
                     configuration: None,
                     lint_preview: Some(false),
                     format_preview: None,
+                    format_backend: None,
                     select: Some(vec![
                         RuleSelector::Linter(Linter::Pyflakes),
                         RuleSelector::Linter(Linter::Isort)
@@ -919,6 +936,7 @@ mod tests {
                     configuration: None,
                     lint_preview: None,
                     format_preview: None,
+                    format_backend: None,
                     select: None,
                     extend_select: None,
                     ignore: Some(vec![RuleSelector::from_str("RUF001").unwrap()]),

crates/ruff_server/src/session/settings.rs

@@ -8,6 +8,7 @@ use ruff_workspace::options::Options;
 
 use crate::{
     ClientOptions,
+    format::FormatBackend,
     session::{
         Client,
         options::{ClientConfiguration, ConfigurationPreference},
@@ -84,6 +85,7 @@ pub(crate) struct EditorSettings {
     pub(super) configuration: Option<ResolvedConfiguration>,
     pub(super) lint_preview: Option<bool>,
     pub(super) format_preview: Option<bool>,
+    pub(super) format_backend: Option<FormatBackend>,
     pub(super) select: Option<Vec<RuleSelector>>,
     pub(super) extend_select: Option<Vec<RuleSelector>>,
     pub(super) ignore: Option<Vec<RuleSelector>>,
@@ -163,3 +165,9 @@ impl ClientSettings {
         &self.editor_settings
     }
 }
+
+impl EditorSettings {
+    pub(crate) fn format_backend(&self) -> FormatBackend {
+        self.format_backend.unwrap_or_default()
+    }
+}