fix: ensure non-matching elements are scoped for :not(...) selector

If the contents of a `:not` selector don't match, then it's actually a match for `:not` because it's inverted. Therefore, we need to scope such elements.

Fixes #13974

.changeset/dirty-parrots-smell.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure non-matching elements are scoped for `:not(...)` selector

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

@@ -515,12 +515,27 @@ function relative_selector_might_apply_to_node(relative_selector, rule, element,
 
 					for (const complex_selector of selector.args.children) {
 						const relative = truncate(complex_selector);
-						if (
-							relative.length === 0 /* is :global(...) */ ||
-							apply_selector(relative, rule, element, stylesheet)
-						) {
+						const is_global = relative.length === 0;
+
+						if (is_global || apply_selector(relative, rule, element, stylesheet)) {
 							complex_selector.metadata.used = true;
 							matched = true;
+						} else if (name === 'not') {
+							// For `:not(...)` we gotta do the inverse: If it did not match, mark the element and possibly
+							// everything above (if the selector is written is a such) as scoped (because they matched by not matching).
+							// The above if condition will ensure the selector itself will be marked as used if it doesn't match at least once,
+							// and therefore having actual usefulness in the CSS output.
+							element.metadata.scoped = true;
+
+							// bar:not(foo bar) means that foo is an ancestor of bar
+							if (complex_selector.children.length > 1) {
+								/** @type {Compiler.AST.RegularElement | Compiler.AST.SvelteElement | null} */
+								let el = get_element_parent(element);
+								while (el) {
+									el.metadata.scoped = true;
+									el = get_element_parent(el);
+								}
+							}
 						} else if (complex_selector.children.length > 1 && (name == 'is' || name == 'where')) {
 							// foo :is(bar baz) can also mean that bar is an ancestor of foo, and baz a descendant.
 							// We can't fully check if that actually matches with our current algorithm, so we just assume it does.

packages/svelte/tests/css/samples/not-selector-global/_config.js

@@ -0,0 +1,20 @@
+import { test } from '../../test';
+
+export default test({
+	warnings: [
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector ":global(.x) :not(.unused)"',
+			start: {
+				line: 17,
+				column: 1,
+				character: 289
+			},
+			end: {
+				line: 17,
+				column: 26,
+				character: 314
+			}
+		}
+	]
+});

packages/svelte/tests/css/samples/not-selector-global/expected.css

@@ -0,0 +1,16 @@
+
+	.svelte-xyz:not(.foo) {
+		color: green;
+	}
+	.svelte-xyz:not(.foo:where(.svelte-xyz)):not(.unused) {
+		color: green;
+	}
+	.x:not(.foo.svelte-xyz) {
+		color: green;
+	}
+	.x:not(.unused.svelte-xyz) {
+		color: red; /* TODO would be nice to prune this one day */
+	}
+	/* (unused) :global(.x) :not(.unused) {
+		color: red;
+	}*/

packages/svelte/tests/css/samples/not-selector-global/expected.html

@@ -0,0 +1 @@
+<p class="foo svelte-xyz">foo</p> <p class="bar svelte-xyz">bar</p>

packages/svelte/tests/css/samples/not-selector-global/input.svelte

@@ -0,0 +1,20 @@
+<p class="foo">foo</p>
+<p class="bar">bar</p>
+
+<style>
+	:not(:global(.foo)) {
+		color: green;
+	}
+	:not(.foo):not(:global(.unused)) {
+		color: green;
+	}
+	:global(.x):not(.foo) {
+		color: green;
+	}
+	:global(.x):not(.unused) {
+		color: red; /* TODO would be nice to prune this one day */
+	}
+	:global(.x) :not(.unused) {
+		color: red;
+	}
+</style>

packages/svelte/tests/css/samples/not-selector/_config.js

@@ -20,42 +20,28 @@ export default test({
 			code: 'css_unused_selector',
 			message: 'Unused CSS selector ":not(.foo):not(.unused)"',
 			start: {
-				line: 18,
+				line: 12,
 				column: 1,
-				character: 221
+				character: 124
 			},
 			end: {
-				line: 18,
+				line: 12,
 				column: 24,
-				character: 244
+				character: 147
 			}
 		},
 		{
 			code: 'css_unused_selector',
 			message: 'Unused CSS selector "p :not(.foo)"',
 			start: {
-				line: 25,
+				line: 19,
 				column: 1,
-				character: 300
+				character: 203
 			},
 			end: {
-				line: 25,
+				line: 19,
 				column: 13,
-				character: 312
-			}
-		},
-		{
-			code: 'css_unused_selector',
-			message: 'Unused CSS selector ":global(.x) :not(.unused)"',
-			start: {
-				line: 34,
-				column: 1,
-				character: 469
-			},
-			end: {
-				line: 34,
-				column: 26,
-				character: 494
+				character: 215
 			}
 		}
 	]

packages/svelte/tests/css/samples/not-selector/expected.css

@@ -5,13 +5,7 @@
 	/* (unused) :not(.unused) {
 		color: red;
 	}*/
-	.svelte-xyz:not(.foo) {
-		color: green;
-	}
 
-	.svelte-xyz:not(.foo:where(.svelte-xyz)):not(.unused) {
-		color: green;
-	}
 	/* (unused) :not(.foo):not(.unused) {
 		color: red;
 	}*/
@@ -22,12 +16,3 @@
 	/* (unused) p :not(.foo) {
 		color: red;
 	}*/
-	.x:not(.foo.svelte-xyz) {
-		color: green;
-	}
-	.x:not(.unused.svelte-xyz) {
-		color: red; /* TODO would be nice to prune this one day */
-	}
-	/* (unused) :global(.x) :not(.unused) {
-		color: red;
-	}*/

packages/svelte/tests/css/samples/not-selector/expected.html

@@ -0,0 +1 @@
+<p class="foo svelte-xyz">foo</p> <p class="bar svelte-xyz">bar</p>

packages/svelte/tests/css/samples/not-selector/input.svelte

@@ -8,13 +8,7 @@
 	:not(.unused) {
 		color: red;
 	}
-	:not(:global(.foo)) {
-		color: green;
-	}
 
-	:not(.foo):not(:global(.unused)) {
-		color: green;
-	}
 	:not(.foo):not(.unused) {
 		color: red;
 	}
@@ -25,13 +19,4 @@
 	p :not(.foo) {
 		color: red;
 	}
-	:global(.x):not(.foo) {
-		color: green;
-	}
-	:global(.x):not(.unused) {
-		color: red; /* TODO would be nice to prune this one day */
-	}
-	:global(.x) :not(.unused) {
-		color: red;
-	}
 </style>