breaking: scope :not(...) selectors

The other part of #13395

This implements scoping for selectors inside `:not(...)`. The approach is almot the same as for `:is/where(...)`.

This is a breaking change because people could've used `:not(.unknown)` with `.unknown` not appearing in the HTML, and so they need to do `:not(:global(.unknown))` instead.

While implementing it I also discovered a few bugs, which are fixed in this PR:
- `foo :is(bar baz)` wasn't properly handled. This selector can mean `foo bar baz` but it can also mean `bar foo baz` (super weird, but it is what it is). Since our current algorithm isn't suited for handling this, we just assume it matches and scope it. Worst case is we missed a prune
- `bar` in `:global(foo):is(bar)` was always marked as unused, even if it matched

Author: dummdidumm

.changeset/nervous-squids-drop.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: scope `:not(...)` selectors

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

@@ -121,16 +121,26 @@ const visitors = {
 };
 
 /**
- * Discard trailing `:global(...)` selectors without a `:has(...)` modifier, these are unused for scoping purposes
+ * Discard trailing `:global(...)` selectors without a `:has/is/where/not(...)` modifier, these are unused for scoping purposes
  * @param {Compiler.Css.ComplexSelector} node
  */
 function truncate(node) {
 	const i = node.children.findLastIndex(({ metadata, selectors }) => {
+		const first = selectors[0];
 		return (
+			// not after a :global selector
 			!metadata.is_global_like &&
+			!(first.type === 'PseudoClassSelector' && first.name === 'global' && first.args === null) &&
+			// not a :global(...) without a :has/is/where/not(...) modifier
 			(!metadata.is_global ||
 				selectors.some(
-					(selector) => selector.type === 'PseudoClassSelector' && selector.name === 'has'
+					(selector) =>
+						selector.type === 'PseudoClassSelector' &&
+						selector.args !== null &&
+						(selector.name === 'has' ||
+							selector.name === 'not' ||
+							selector.name === 'is' ||
+							selector.name === 'where')
 				))
 		);
 	});
@@ -440,17 +450,47 @@ function relative_selector_might_apply_to_node(
 				// We came across a :global, everything beyond it is global and therefore a potential match
 				if (name === 'global' && selector.args === null) return true;
 
-				if ((name === 'is' || name === 'where') && selector.args) {
+				if ((name === 'is' || name === 'where' || name === 'not') && selector.args) {
 					let matched = false;
 
 					for (const complex_selector of selector.args.children) {
-						if (apply_selector(truncate(complex_selector), rule, element, stylesheet, check_has)) {
+						const relative = truncate(complex_selector);
+						if (
+							relative.length === 0 /* is :global(...) */ ||
+							apply_selector(relative, rule, element, stylesheet, check_has)
+						) {
 							complex_selector.metadata.used = true;
 							matched = true;
+						} 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.
+							// The result may not match a real element, so the only drawback is the missing prune.
+							complex_selector.metadata.used = true;
+							matched = true;
+							for (const selector of relative) {
+								selector.metadata.scoped = true;
+							}
 						}
 					}
 
 					if (!matched) {
+						if (
+							relative_selector.metadata.is_global &&
+							!relative_selector.metadata.is_global_like
+						) {
+							// Edge case: `:global(.x):is(.y)` where `.x` is global but `.y` doesn't match.
+							// Since `used` is set to `true` for `:global(.x)` in css-analyze beforehand, and
+							// we have no way of knowing if it's safe to set it back to `false`, we'll mark
+							// the inner selector as used and scoped to prevent it from being pruned, which could
+							// result in a invalid CSS output (e.g. `.x:is(/* unused .y */)`). The result
+							// can't match a real element, so the only drawback is the missing prune.
+							// TODO clean this up some day
+							selector.args.children[0].metadata.used = true;
+							selector.args.children[0].children.forEach((selector) => {
+								selector.metadata.scoped = true;
+							});
+						}
+
 						return false;
 					}
 				}

packages/svelte/src/compiler/phases/3-transform/css/index.js

@@ -311,7 +311,7 @@ const visitors = {
 		context.state.specificity.bumped = before_bumped;
 	},
 	PseudoClassSelector(node, context) {
-		if (node.name === 'is' || node.name === 'where' || node.name === 'has') {
+		if (node.name === 'is' || node.name === 'where' || node.name === 'has' || node.name === 'not') {
 			context.next();
 		}
 	}

packages/svelte/src/compiler/types/css.d.ts

@@ -87,6 +87,8 @@ export namespace Css {
 			/**
 			 * `true` if the whole selector is unscoped, e.g. `:global(...)` or `:global` or `:global.x`.
 			 * Selectors like `:global(...).x` are not considered global, because they still need scoping.
+			 * Selectors like `:global(...):is/where/not/has(...)` are considered global even if they aren't
+			 * strictly speaking (we should consolidate the logic around this at some point).
 			 */
 			is_global: boolean;
 			/** `:root`, `:host`, `::view-transition`, or selectors after a `:global` */

packages/svelte/tests/css/samples/is/_config.js

@@ -4,16 +4,72 @@ export default test({
 	warnings: [
 		{
 			code: 'css_unused_selector',
+			message: 'Unused CSS selector ".unused"',
+			start: {
+				line: 11,
+				column: 10,
+				character: 80
+			},
 			end: {
-				character: 38,
-				column: 11,
-				line: 6
+				line: 11,
+				column: 17,
+				character: 87
+			}
+		},
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector "x :is(.unused)"',
+			start: {
+				line: 14,
+				column: 1,
+				character: 111
 			},
-			message: 'Unused CSS selector "z"',
+			end: {
+				line: 14,
+				column: 15,
+				character: 125
+			}
+		},
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector ".unused"',
 			start: {
-				character: 37,
-				column: 10,
-				line: 6
+				line: 14,
+				column: 7,
+				character: 117
+			},
+			end: {
+				line: 14,
+				column: 14,
+				character: 124
+			}
+		},
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector ":global(.foo) :is(.unused)"',
+			start: {
+				line: 28,
+				column: 1,
+				character: 274
+			},
+			end: {
+				line: 28,
+				column: 27,
+				character: 300
+			}
+		},
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector ".unused"',
+			start: {
+				line: 28,
+				column: 19,
+				character: 292
+			},
+			end: {
+				line: 28,
+				column: 26,
+				character: 299
 			}
 		}
 	]

packages/svelte/tests/css/samples/is/expected.css

@@ -1,4 +1,34 @@
 
-	x.svelte-xyz :is(y:where(.svelte-xyz) /* (unused) z*/) {
-		color: purple;
+	x.svelte-xyz :is(y:where(.svelte-xyz)) {
+		color: green;
+	}
+	x.svelte-xyz :is(y:where(.svelte-xyz) /* (unused) .unused*/) {
+		color: green;
+	}
+	/* (unused) x :is(.unused) {
+		color: red;
+	}*/
+
+	x.svelte-xyz :is(y) {
+		color: green;
+	}
+	x.svelte-xyz :is(.foo) {
+		color: green;
+	}
+
+	.foo :is(x.svelte-xyz) {
+		color: green;
+	}
+	/* (unused) :global(.foo) :is(.unused) {
+		color: red;
+	}*/
+
+	x.svelte-xyz :is(html *) {
+		color: green;
+	}
+	x.svelte-xyz :is(html:where(.svelte-xyz) :where(.svelte-xyz)) {
+		color: red; /* TODO would be nice to prune this one day */
+	}
+	y.svelte-xyz :is(x:where(.svelte-xyz) :where(.svelte-xyz)) {
+		color: green; /* matches z */
 	}

packages/svelte/tests/css/samples/is/input.svelte

@@ -1,9 +1,41 @@
 <x>
-	<y></y>
+	<y>
+		<z></z>
+	</y>
 </x>
 
 <style>
-	x :is(y, z) {
-		color: purple;
+	x :is(y) {
+		color: green;
+	}
+	x :is(y, .unused) {
+		color: green;
+	}
+	x :is(.unused) {
+		color: red;
+	}
+
+	x :is(:global(y)) {
+		color: green;
+	}
+	x :is(:global(.foo)) {
+		color: green;
+	}
+
+	:global(.foo) :is(x) {
+		color: green;
+	}
+	:global(.foo) :is(.unused) {
+		color: red;
+	}
+
+	x :is(:global(html *)) {
+		color: green;
+	}
+	x :is(html *) {
+		color: red; /* TODO would be nice to prune this one day */
+	}
+	y :is(x *) {
+		color: green; /* matches z */
 	}
 </style>

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

@@ -0,0 +1,62 @@
+import { test } from '../../test';
+
+export default test({
+	warnings: [
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector ":not(.unused)"',
+			start: {
+				line: 8,
+				column: 1,
+				character: 89
+			},
+			end: {
+				line: 8,
+				column: 14,
+				character: 102
+			}
+		},
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector ":not(.foo):not(.unused)"',
+			start: {
+				line: 18,
+				column: 1,
+				character: 221
+			},
+			end: {
+				line: 18,
+				column: 24,
+				character: 244
+			}
+		},
+		{
+			code: 'css_unused_selector',
+			message: 'Unused CSS selector "p :not(.foo)"',
+			start: {
+				line: 25,
+				column: 1,
+				character: 300
+			},
+			end: {
+				line: 25,
+				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
+			}
+		}
+	]
+});

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

@@ -1,3 +1,33 @@
+
+	.svelte-xyz:not(.foo:where(.svelte-xyz)) {
+		color: green;
+	}
+	/* (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;
+	}*/
+
+	p.svelte-xyz:not(.foo:where(.svelte-xyz)) {
+		color: green;
 	}
+	/* (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/input.svelte

@@ -3,6 +3,35 @@
 
 <style>
 	:not(.foo) {
+		color: green;
+	}
+	:not(.unused) {
+		color: red;
+	}
+	:not(:global(.foo)) {
+		color: green;
+	}
+
+	:not(.foo):not(:global(.unused)) {
+		color: green;
+	}
+	:not(.foo):not(.unused) {
+		color: red;
+	}
+
+	p:not(.foo) {
+		color: green;
+	}
+	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>