Implement hover menu support for ruff-server; Issue #10595 (#11096)

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## Summary

Add support for hover menu to ruff_server, as requested in
[10595](https://github.com/astral-sh/ruff/issues/10595).
Majority of new code is in hover.rs.
I reused the regex from ruff-lsp's implementation. Also reused the
format_rule_text function from ruff/src/commands/rule.rs
Added capability registration in server.rs, and added the handler to
api.rs.

## Test Plan

Tested in NVIM v0.10.0-dev-2582+g2a8cef6bd, configured with lspconfig
using the default options (other than cmd pointing to my test build,
with options "server" and "--preview"). OS: Ubuntu 24.04, kernel
6.8.0-22.

---------

Co-authored-by: Jane Lewis <me@jane.engineering>

crates/ruff_server/Cargo.toml

@@ -36,6 +36,7 @@ serde = { workspace = true }
 serde_json = { workspace = true }
 tracing = { workspace = true }
 walkdir = { workspace = true }
+regex = { workspace = true }
 
 [dev-dependencies]
 insta = { workspace = true }

crates/ruff_server/src/server.rs

@@ -257,6 +257,7 @@ impl Server {
                     },
                 },
             )),
+            hover_provider: Some(types::HoverProviderCapability::Simple(true)),
             text_document_sync: Some(TextDocumentSyncCapability::Options(
                 TextDocumentSyncOptions {
                     open_close: Some(true),

crates/ruff_server/src/server/api.rs

@@ -49,6 +49,9 @@ pub(super) fn request<'a>(req: server::Request) -> Task<'a> {
         request::FormatRange::METHOD => {
             background_request_task::<request::FormatRange>(req, BackgroundSchedule::Fmt)
         }
+        request::Hover::METHOD => {
+            background_request_task::<request::Hover>(req, BackgroundSchedule::Worker)
+        }
         method => {
             tracing::warn!("Received request {method} which does not have a handler");
             return Task::nothing();

crates/ruff_server/src/server/api/requests.rs

@@ -4,6 +4,7 @@ mod diagnostic;
 mod execute_command;
 mod format;
 mod format_range;
+mod hover;
 
 use super::{
     define_document_url,
@@ -15,5 +16,6 @@ pub(super) use diagnostic::DocumentDiagnostic;
 pub(super) use execute_command::ExecuteCommand;
 pub(super) use format::Format;
 pub(super) use format_range::FormatRange;
+pub(super) use hover::Hover;
 
 type FormatResponse = Option<Vec<lsp_types::TextEdit>>;

crates/ruff_server/src/server/api/requests/hover.rs

@@ -0,0 +1,113 @@
+use crate::server::{client::Notifier, Result};
+use crate::session::DocumentSnapshot;
+use lsp_types::{self as types, request as req};
+use regex::Regex;
+use ruff_diagnostics::FixAvailability;
+use ruff_linter::registry::{Linter, Rule, RuleNamespace};
+use ruff_source_file::OneIndexed;
+
+pub(crate) struct Hover;
+
+impl super::RequestHandler for Hover {
+    type RequestType = req::HoverRequest;
+}
+
+impl super::BackgroundDocumentRequestHandler for Hover {
+    fn document_url(params: &types::HoverParams) -> std::borrow::Cow<lsp_types::Url> {
+        std::borrow::Cow::Borrowed(&params.text_document_position_params.text_document.uri)
+    }
+    fn run_with_snapshot(
+        snapshot: DocumentSnapshot,
+        _notifier: Notifier,
+        params: types::HoverParams,
+    ) -> Result<Option<types::Hover>> {
+        Ok(hover(&snapshot, &params.text_document_position_params))
+    }
+}
+
+pub(crate) fn hover(
+    snapshot: &DocumentSnapshot,
+    position: &types::TextDocumentPositionParams,
+) -> Option<types::Hover> {
+    let document = snapshot.document();
+    let line_number: usize = position
+        .position
+        .line
+        .try_into()
+        .expect("line number should fit within a usize");
+    let line_range = document.index().line_range(
+        OneIndexed::from_zero_indexed(line_number),
+        document.contents(),
+    );
+
+    let line = &document.contents()[line_range];
+
+    // Get the list of codes.
+    let noqa_regex = Regex::new(r"(?i:# (?:(?:ruff|flake8): )?(?P<noqa>noqa))(?::\s?(?P<codes>([A-Z]+[0-9]+(?:[,\s]+)?)+))?").unwrap();
+    let noqa_captures = noqa_regex.captures(line)?;
+    let codes_match = noqa_captures.name("codes")?;
+    let codes_start = codes_match.start();
+    let code_regex = Regex::new(r"[A-Z]+[0-9]+").unwrap();
+    let cursor: usize = position
+        .position
+        .character
+        .try_into()
+        .expect("column number should fit within a usize");
+    let word = code_regex.find_iter(codes_match.as_str()).find(|code| {
+        cursor >= (code.start() + codes_start) && cursor < (code.end() + codes_start)
+    })?;
+
+    // Get rule for the code under the cursor.
+    let rule = Rule::from_code(word.as_str());
+    let output = if let Ok(rule) = rule {
+        format_rule_text(rule)
+    } else {
+        format!("{}: Rule not found", word.as_str())
+    };
+
+    let hover = types::Hover {
+        contents: types::HoverContents::Markup(types::MarkupContent {
+            kind: types::MarkupKind::Markdown,
+            value: output,
+        }),
+        range: None,
+    };
+
+    Some(hover)
+}
+
+fn format_rule_text(rule: Rule) -> String {
+    let mut output = String::new();
+    output.push_str(&format!("# {} ({})", rule.as_ref(), rule.noqa_code()));
+    output.push('\n');
+    output.push('\n');
+
+    let (linter, _) = Linter::parse_code(&rule.noqa_code().to_string()).unwrap();
+    output.push_str(&format!("Derived from the **{}** linter.", linter.name()));
+    output.push('\n');
+    output.push('\n');
+
+    let fix_availability = rule.fixable();
+    if matches!(
+        fix_availability,
+        FixAvailability::Always | FixAvailability::Sometimes
+    ) {
+        output.push_str(&fix_availability.to_string());
+        output.push('\n');
+        output.push('\n');
+    }
+
+    if rule.is_preview() || rule.is_nursery() {
+        output.push_str(r"This rule is in preview and is not stable.");
+        output.push('\n');
+        output.push('\n');
+    }
+
+    if let Some(explanation) = rule.explanation() {
+        output.push_str(explanation.trim());
+    } else {
+        tracing::warn!("Rule {} does not have an explanation", rule.noqa_code());
+        output.push_str("An issue occurred: an explanation for this rule was not found.");
+    }
+    output
+}