Add JSON Lines (NDJSON) message serialization (#5048)

## Summary

This adds `json-lines` (https://jsonlines.org/ or http://ndjson.org/) as
an output format.

I'm sure you already know, but

* JSONL is more greppable (each record is a single line) than the pretty
JSON
* JSONL is faster to ingest piecewise (and/or in parallel) than JSON

## Test Plan

Snapshot test in the new module :)

crates/ruff/src/message/json.rs

@@ -2,7 +2,7 @@ use std::io::Write;
 
 use serde::ser::SerializeSeq;
 use serde::{Serialize, Serializer};
-use serde_json::json;
+use serde_json::{json, Value};
 
 use ruff_diagnostics::Edit;
 use ruff_python_ast::source_code::SourceCode;
@@ -38,37 +38,40 @@ impl Serialize for ExpandedMessages<'_> {
         let mut s = serializer.serialize_seq(Some(self.messages.len()))?;
 
         for message in self.messages {
-            let source_code = message.file.to_source_code();
-
-            let fix = message.fix.as_ref().map(|fix| {
-                json!({
-                    "applicability": fix.applicability(),
-                    "message": message.kind.suggestion.as_deref(),
-                    "edits": &ExpandedEdits { edits: fix.edits(), source_code: &source_code },
-                })
-            });
-
-            let start_location = source_code.source_location(message.start());
-            let end_location = source_code.source_location(message.end());
-            let noqa_location = source_code.source_location(message.noqa_offset);
-
-            let value = json!({
-                "code": message.kind.rule().noqa_code().to_string(),
-                "message": message.kind.body,
-                "fix": fix,
-                "location": start_location,
-                "end_location": end_location,
-                "filename": message.filename(),
-                "noqa_row": noqa_location.row
-            });
-
+            let value = message_to_json_value(message);
             s.serialize_element(&value)?;
         }
 
         s.end()
     }
 }
 
+pub(crate) fn message_to_json_value(message: &Message) -> Value {
+    let source_code = message.file.to_source_code();
+
+    let fix = message.fix.as_ref().map(|fix| {
+        json!({
+            "applicability": fix.applicability(),
+            "message": message.kind.suggestion.as_deref(),
+            "edits": &ExpandedEdits { edits: fix.edits(), source_code: &source_code },
+        })
+    });
+
+    let start_location = source_code.source_location(message.start());
+    let end_location = source_code.source_location(message.end());
+    let noqa_location = source_code.source_location(message.noqa_offset);
+
+    json!({
+        "code": message.kind.rule().noqa_code().to_string(),
+        "message": message.kind.body,
+        "fix": fix,
+        "location": start_location,
+        "end_location": end_location,
+        "filename": message.filename(),
+        "noqa_row": noqa_location.row
+    })
+}
+
 struct ExpandedEdits<'a> {
     edits: &'a [Edit],
     source_code: &'a SourceCode<'a, 'a>,

crates/ruff/src/message/json_lines.rs

@@ -0,0 +1,39 @@
+use std::io::Write;
+
+use crate::message::json::message_to_json_value;
+use crate::message::{Emitter, EmitterContext, Message};
+
+#[derive(Default)]
+pub struct JsonLinesEmitter;
+
+impl Emitter for JsonLinesEmitter {
+    fn emit(
+        &mut self,
+        writer: &mut dyn Write,
+        messages: &[Message],
+        _context: &EmitterContext,
+    ) -> anyhow::Result<()> {
+        let mut w = writer;
+        for message in messages {
+            serde_json::to_writer(&mut w, &message_to_json_value(message))?;
+            w.write_all(b"\n")?;
+        }
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::message::json_lines::JsonLinesEmitter;
+    use insta::assert_snapshot;
+
+    use crate::message::tests::{capture_emitter_output, create_messages};
+
+    #[test]
+    fn output() {
+        let mut emitter = JsonLinesEmitter::default();
+        let content = capture_emitter_output(&mut emitter, &create_messages());
+
+        assert_snapshot!(content);
+    }
+}

crates/ruff/src/message/mod.rs

@@ -12,6 +12,7 @@ pub use github::GithubEmitter;
 pub use gitlab::GitlabEmitter;
 pub use grouped::GroupedEmitter;
 pub use json::JsonEmitter;
+pub use json_lines::JsonLinesEmitter;
 pub use junit::JunitEmitter;
 pub use pylint::PylintEmitter;
 use ruff_diagnostics::{Diagnostic, DiagnosticKind, Fix};
@@ -24,6 +25,7 @@ mod github;
 mod gitlab;
 mod grouped;
 mod json;
+mod json_lines;
 mod junit;
 mod pylint;
 mod text;

crates/ruff/src/message/snapshots/ruff__message__json_lines__tests__output.snap

@@ -0,0 +1,8 @@
+---
+source: crates/ruff/src/message/jsonlines.rs
+expression: content
+---
+{"code":"F401","message":"`os` imported but unused","fix":{"applicability":"Suggested","message":"Remove unused import: `os`","edits":[{"content":"","location":{"row":1,"column":1},"end_location":{"row":2,"column":1}}]},"location":{"row":1,"column":8},"end_location":{"row":1,"column":10},"filename":"fib.py","noqa_row":1}
+{"code":"F841","message":"Local variable `x` is assigned to but never used","fix":{"applicability":"Suggested","message":"Remove assignment to unused variable `x`","edits":[{"content":"","location":{"row":6,"column":5},"end_location":{"row":6,"column":10}}]},"location":{"row":6,"column":5},"end_location":{"row":6,"column":6},"filename":"fib.py","noqa_row":6}
+{"code":"F821","message":"Undefined name `a`","fix":null,"location":{"row":1,"column":4},"end_location":{"row":1,"column":5},"filename":"undef.py","noqa_row":1}
+

crates/ruff/src/settings/types.rs

@@ -214,6 +214,7 @@ impl FromStr for PatternPrefixPair {
 pub enum SerializationFormat {
     Text,
     Json,
+    JsonLines,
     Junit,
     Grouped,
     Github,

crates/ruff_cli/src/printer.rs

@@ -16,7 +16,7 @@ use ruff::linter::FixTable;
 use ruff::logging::LogLevel;
 use ruff::message::{
     AzureEmitter, Emitter, EmitterContext, GithubEmitter, GitlabEmitter, GroupedEmitter,
-    JsonEmitter, JunitEmitter, PylintEmitter, TextEmitter,
+    JsonEmitter, JsonLinesEmitter, JunitEmitter, PylintEmitter, TextEmitter,
 };
 use ruff::notify_user;
 use ruff::registry::{AsRule, Rule};
@@ -184,6 +184,9 @@ impl Printer {
             SerializationFormat::Json => {
                 JsonEmitter::default().emit(writer, &diagnostics.messages, &context)?;
             }
+            SerializationFormat::JsonLines => {
+                JsonLinesEmitter::default().emit(writer, &diagnostics.messages, &context)?;
+            }
             SerializationFormat::Junit => {
                 JunitEmitter::default().emit(writer, &diagnostics.messages, &context)?;
             }

docs/configuration.md

@@ -211,7 +211,7 @@ Options:
       --ignore-noqa
           Ignore any `# noqa` comments
       --format <FORMAT>
-          Output serialization format for violations [env: RUFF_FORMAT=] [possible values: text, json, junit, grouped, github, gitlab, pylint, azure]
+          Output serialization format for violations [env: RUFF_FORMAT=] [possible values: text, json, json-lines, junit, grouped, github, gitlab, pylint, azure]
       --target-version <TARGET_VERSION>
           The minimum Python version that should be supported [possible values: py37, py38, py39, py310, py311]
       --config <CONFIG>