fix(formatter): never format biome.json with trailing commas

crates/biome_cli/tests/cases/biome_json_support.rs

@@ -253,3 +253,41 @@ fn biome_json_is_not_ignored() {
         result,
     ));
 }
+
+#[test]
+fn always_disable_trailing_commas_biome_json() {
+    let mut fs = MemoryFileSystem::default();
+    let mut console = BufferConsole::default();
+
+    let file_path = Path::new("biome.json");
+    let config = r#"{
+    "formatter": {
+        "indentStyle": "space",
+        "indentWidth": 4
+    },
+    "json": {
+        "formatter": {
+            "trailingCommas": "all"
+        }
+    }
+}
+"#;
+    fs.insert(file_path.into(), config);
+
+    let result = run_cli(
+        DynRef::Borrowed(&mut fs),
+        &mut console,
+        Args::from(["check", "--write", "."].as_slice()),
+    );
+
+    assert!(result.is_ok(), "run_cli returned {result:?}");
+
+    assert_file_contents(&fs, file_path, config);
+    assert_cli_snapshot(SnapshotPayload::new(
+        module_path!(),
+        "always_disable_trailing_commas_biome_json",
+        fs,
+        console,
+        result,
+    ));
+}

crates/biome_cli/tests/snapshots/main_cases_biome_json_support/always_disable_trailing_commas_biome_json.snap

@@ -0,0 +1,25 @@
+---
+source: crates/biome_cli/tests/snap_test.rs
+expression: content
+---
+## `biome.json`
+
+```json
+{
+  "formatter": {
+    "indentStyle": "space",
+    "indentWidth": 4
+  },
+  "json": {
+    "formatter": {
+      "trailingCommas": "all"
+    }
+  }
+}
+```
+
+# Emitted Messages
+
+```block
+Checked 1 file in <TIME>. No fixes needed.
+```

crates/biome_service/src/file_handlers/json.rs

@@ -1,3 +1,5 @@
+use std::ffi::OsStr;
+
 use super::{CodeActionsParams, DocumentFileSource, ExtensionHandler, ParseResult};
 use crate::configuration::to_analyzer_rules;
 use crate::file_handlers::DebugCapabilities;
@@ -96,14 +98,22 @@ impl ServiceLanguage for JsonLanguage {
             global.line_ending.unwrap_or_default()
         };
 
+        // ensure it never formats biome.json into a form it can't parse
+        let trailing_commas =
+            if matches!(path.file_name().and_then(OsStr::to_str), Some("biome.json")) {
+                TrailingCommas::None
+            } else {
+                language.trailing_commas.unwrap_or_default()
+            };
+
         overrides.to_override_json_format_options(
             path,
             JsonFormatOptions::new()
                 .with_line_ending(line_ending)
                 .with_indent_style(indent_style)
                 .with_indent_width(indent_width)
                 .with_line_width(line_width)
-                .with_trailing_commas(language.trailing_commas.unwrap_or_default()),
+                .with_trailing_commas(trailing_commas),
         )
     }