fix: properly assign trailing comments

Trailing comments were added even if they started before the node end, which violates the definition of trailing comments.
This fixes that, with the consequence that some comments no longer show up in the AST at all. Previously they were stuffed _somewhere_, but arguably at the wrong positions. For example the last comment in a function body got assigned as a trailing comma for the body, which is wrong, because the body ends _after_ the comma.
The special case is comments at the end of an expression tag - they are added even if there are multiple, and they are added regardless of whether they are separated by newlines or not. This ensures the expression tag end is calculated correctly.

Fixes #12466

Author: dummdidumm

.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

@@ -105,17 +105,22 @@ function get_comment_handlers(source) {
 
 					if (comments[0]) {
 						const parent = path.at(-1);
+
 						if (parent === undefined || node.end !== parent.end) {
 							const slice = source.slice(node.end, comments[0].start);
 
-							if (/^[,) \t]*$/.test(slice)) {
+							if (node.end <= comments[0].start && /^[,) \t]*$/.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) get added
+			// regardless of line breaks between the root node and the comments. This ensures that we can later detect
+			// the end of the expression tag correctly.
+			if (comments.length > 0 && comments[0].start >= ast.end) {
 				(ast.trailingComments ||= []).push(...comments.splice(0));
 			}
 		}

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

@@ -5,15 +5,17 @@
 
 	/** a comment */
 	function asd() {
-
+		foo;
+		/* comment belonging to noone */
 	}
 </script>
 
 <button
 	on:click={// comment
 	() => {
 		/* another comment */
-		fn();
+		fn(); // a trailing comment
+		/* comment belonging to noone */
 	}}
 >
 {/* leading block comment */ a}