fix(ts/formatter): format comment after => in arrow functions (#9527)

Co-authored-by: Emanuele Stoppa <my.burning@gmail.com>

Author: mdm317

.changeset/cuddly-dogs-hide.md

@@ -0,0 +1,5 @@
+---
+"@biomejs/biome": patch
+---
+
+Fixed [#8959](https://github.com/biomejs/biome/issues/8959): Fixed TypeScript arrow function formatting when a comment appears after `=>`.

crates/biome_js_formatter/src/comments.rs

@@ -15,7 +15,7 @@ use biome_js_syntax::{
     JsBlockStatement, JsCallArguments, JsCatchClause, JsEmptyStatement, JsFinallyClause,
     JsFormalParameter, JsFunctionBody, JsIdentifierBinding, JsIdentifierExpression, JsIfStatement,
     JsLanguage, JsNamedImportSpecifiers, JsParameters, JsSyntaxKind, JsSyntaxNode,
-    JsVariableDeclarator, JsWhileStatement, TsInterfaceDeclaration, TsMappedType,
+    JsVariableDeclarator, JsWhileStatement, TsFunctionType, TsInterfaceDeclaration, TsMappedType,
 };
 use biome_rowan::{AstNode, SyntaxNodeOptionExt, SyntaxTriviaPieceComments, TextLen};
 use biome_suppression::{SuppressionKind, parse_suppression_comment};
@@ -180,6 +180,12 @@ fn handle_typecast_comment(comment: DecoratedComment<JsLanguage>) -> CommentPlac
 fn handle_after_arrow_fat_arrow_comment(
     comment: DecoratedComment<JsLanguage>,
 ) -> CommentPlacement<JsLanguage> {
+    if TsFunctionType::can_cast(comment.enclosing_node().kind()) {
+        if let Some(following_node) = comment.following_node() {
+            return CommentPlacement::leading(following_node.clone(), comment);
+        }
+        return CommentPlacement::Default(comment);
+    }
     if JsArrowFunctionExpression::can_cast(comment.enclosing_node().kind()) {
         // input
         // ```javascript

crates/biome_js_formatter/tests/specs/ts/arrow/after_arrow_comment.ts

@@ -0,0 +1,6 @@
+type First = () => 
+  // Comment
+  string
+
+type Second = () => // Comment
+  string

crates/biome_js_formatter/tests/specs/ts/arrow/after_arrow_comment.ts.snap

@@ -0,0 +1,81 @@
+---
+source: crates/biome_formatter_test/src/snapshot_builder.rs
+assertion_line: 212
+info: ts/arrow/after_arrow_comment.ts
+---
+
+# Input
+
+```ts
+type First = () => 
+  // Comment
+  string
+
+type Second = () => // Comment
+  string
+```
+
+
+=============================
+
+# Outputs
+
+## Output 1
+
+-----
+Indent style: Tab
+Indent width: 2
+Line ending: LF
+Line width: 80
+Quote style: Double Quotes
+JSX quote style: Double Quotes
+Quote properties: As needed
+Trailing commas: All
+Semicolons: Always
+Arrow parentheses: Always
+Bracket spacing: true
+Bracket same line: false
+Attribute Position: Auto
+Expand lists: Auto
+Operator linebreak: After
+Trailing newline: true
+-----
+
+```ts
+type First = () => // Comment
+string;
+
+type Second = () => // Comment
+string;
+
+```
+
+## Output 1
+
+-----
+Indent style: Tab
+Indent width: 2
+Line ending: LF
+Line width: 80
+Quote style: Double Quotes
+JSX quote style: Double Quotes
+Quote properties: As needed
+Trailing commas: All
+Semicolons: Always
+Arrow parentheses: As needed
+Bracket spacing: true
+Bracket same line: false
+Attribute Position: Auto
+Expand lists: Auto
+Operator linebreak: After
+Trailing newline: true
+-----
+
+```ts
+type First = () => // Comment
+string;
+
+type Second = () => // Comment
+string;
+
+```