fix: properly assign trailing comments

.changeset/fast-toes-act.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: properly assign trailing comments

packages/svelte/src/compiler/phases/1-parse/acorn.js

@@ -104,18 +104,42 @@ function get_comment_handlers(source) {
 					next();
 
 					if (comments[0]) {
-						const parent = path.at(-1);
+						const parent = /** @type {any} */ (path.at(-1));
+
 						if (parent === undefined || node.end !== parent.end) {
 							const slice = source.slice(node.end, comments[0].start);
-
-							if (/^[,) \t]*$/.test(slice)) {
+							const is_last_in_body =
+								((parent?.type === 'BlockStatement' || parent?.type === 'Program') &&
+									parent.body.indexOf(node) === parent.body.length - 1) ||
+								(parent?.type === 'ArrayExpression' &&
+									parent.elements.indexOf(node) === parent.elements.length - 1) ||
+								(parent?.type === 'ObjectExpression' &&
+									parent.properties.indexOf(node) === parent.properties.length - 1);
+
+							if (is_last_in_body) {
+								// Special case: There can be multiple trailing comments after the last node in a block,
+								// and they can be separated by newlines
+								let end = node.end;
+
+								while (comments.length) {
+									const comment = comments[0];
+									if (parent && comment.start >= parent.end) break;
+
+									(node.trailingComments ||= []).push(comment);
+									comments.shift();
+									end = comment.end;
+								}
+							} else if (node.end <= comments[0].start && /^[,) \t]*$/.test(slice)) {
 								node.trailingComments = [/** @type {CommentWithLocation} */ (comments.shift())];
 							}
 						}
 					}
 				}
 			});
-			if (comments.length > 0) {
+
+			// Special case: Trailing comments after the root node (which can only happen for expression tags or for Program nodes).
+			// Adding them ensures that we can later detect the end of the expression tag correctly.
+			if (comments.length > 0 && (comments[0].start >= ast.end || ast.type === 'Program')) {
 				(ast.trailingComments ||= []).push(...comments.splice(0));
 			}
 		}

packages/svelte/tests/parser-legacy/samples/javascript-comments/input.svelte

@@ -5,15 +5,37 @@
 
 	/** a comment */
 	function asd() {
-
+		foo; // trailing
+		/* leading comment 1 */
+		/* leading comment 2 */
+		/* leading comment 3 */
+		bar;
+		/* trailing comment 1 */
+		/* trailing comment 2 */
+		/* trailing comment 3 */
 	}
+
+	const array = [
+		// leading comment 1
+		// leading comment 2
+		1, // trailing comment 1
+		/* trailing comment 2 */
+	];
+
+	const object = {
+		// leading comment 1
+		// leading comment 2
+		a: 1, // trailing comment 1
+		/* trailing comment 2 */
+	};
 </script>
 
 <button
 	on:click={// comment
 	() => {
 		/* another comment */
-		fn();
+		fn(); // a trailing comment
+		/* trailing block comment */
 	}}
 >
 {/* leading block comment */ a}