fix: prevent invalid `:global` usage (#12474)

* fix: prevent invalid `:global` usage

Error when it's not at the end of a selector

closes #12437

* fix validation

* fix

* fix types

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

packages/svelte/messages/compile-errors/style.md

@@ -22,6 +22,10 @@
 
 > A :global {...} block cannot modify an existing selector
 
+## css_global_block_invalid_placement
+
+> A :global {...} block can only appear at the end of a selector sequence (did you mean to use :global(...) instead?)
+
 ## css_global_invalid_placement
 
 > :global(...) can be at the start or end of a selector sequence, but not in the middle

packages/svelte/src/compiler/errors.js

@@ -470,6 +470,15 @@ export function css_global_block_invalid_modifier(node) {
 	e(node, "css_global_block_invalid_modifier", "A :global {...} block cannot modify an existing selector");
 }
 
+/**
+ * A :global {...} block can only appear at the end of a selector sequence (did you mean to use :global(...) instead?)
+ * @param {null | number | NodeLike} node
+ * @returns {never}
+ */
+export function css_global_block_invalid_placement(node) {
+	e(node, "css_global_block_invalid_placement", "A :global {...} block can only appear at the end of a selector sequence (did you mean to use :global(...) instead?)");
+}
+
 /**
  * :global(...) can be at the start or end of a selector sequence, but not in the middle
  * @param {null | number | NodeLike} node

packages/svelte/src/compiler/phases/2-analyze/css/css-analyze.js

@@ -16,7 +16,11 @@ import { merge } from '../../visitors.js';
  * >} CssVisitors
  */
 
-/** @param {Css.RelativeSelector} relative_selector */
+/**
+ * True if is `:global(...)` or `:global`
+ * @param {Css.RelativeSelector} relative_selector
+ * @returns {relative_selector is Css.RelativeSelector & { selectors: [Css.PseudoClassSelector, ...Array<Css.PseudoClassSelector | Css.PseudoElementSelector>] }}
+ */
 function is_global(relative_selector) {
 	const first = relative_selector.selectors[0];
 
@@ -135,16 +139,26 @@ const validation_visitors = {
 
 		context.next();
 	},
-	ComplexSelector(node, context) {
-		// ensure `:global(...)` is not used in the middle of a selector
+	ComplexSelector(node) {
 		{
-			const a = node.children.findIndex((child) => !is_global(child));
-			const b = node.children.findLastIndex((child) => !is_global(child));
-
-			if (a !== b) {
-				for (let i = a; i <= b; i += 1) {
-					if (is_global(node.children[i])) {
-						e.css_global_invalid_placement(node.children[i].selectors[0]);
+			const global = node.children.find(is_global);
+
+			if (global) {
+				const idx = node.children.indexOf(global);
+
+				if (global.selectors[0].args === null && idx !== node.children.length - 1) {
+					// ensure `:global` is only at the end of a selector
+					e.css_global_block_invalid_placement(global.selectors[0]);
+				} else if (
+					global.selectors[0].args !== null &&
+					idx !== 0 &&
+					idx !== node.children.length - 1
+				) {
+					// ensure `:global(...)` is not used in the middle of a selector (but multiple `global(...)` in sequence are ok)
+					for (let i = idx + 1; i < node.children.length; i++) {
+						if (!is_global(node.children[i])) {
+							e.css_global_invalid_placement(global.selectors[0]);
+						}
 					}
 				}
 			}

packages/svelte/tests/compiler-errors/samples/css-global-block-invalid-selector/_config.js

@@ -0,0 +1,10 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'css_global_block_invalid_placement',
+		message:
+			'A :global {...} block can only appear at the end of a selector sequence (did you mean to use :global(...) instead?)',
+		position: [50, 57]
+	}
+});

packages/svelte/tests/compiler-errors/samples/css-global-block-invalid-selector/main.svelte

@@ -0,0 +1,8 @@
+<style>
+	/* ok */
+	.x :global {
+	}
+	/* not ok */
+	:global .x {
+	}
+</style>