Working CSS nesting implementation

packages/svelte/src/compiler/phases/1-parse/read/style.js

@@ -189,17 +189,17 @@ function read_selector(parser) {
 	while (parser.index < parser.template.length) {
 		const start = parser.index;
 
-		if (parser.eat('&')){
+	if (parser.eat('*')) {
 		children.push({
-				type: 'NestedSelector',
-				name: '&',
+			type: 'TypeSelector',
+			name: '*',
 			start,
 			end: parser.index
 		});
-		} else if (parser.eat('*')) {
+	} else if (parser.eat('&')){
 		children.push({
-				type: 'TypeSelector',
-				name: '*',
+			type: 'NestedSelector',
+			name: '&',
 			start,
 			end: parser.index
 		});
@@ -392,9 +392,9 @@ function read_declaration(parser) {
  * @returns {import('#compiler').Css.Declaration | import('#compiler').Css.Rule}
  */
 function read_declaration_or_rule(parser) {
-	// We will only allow css nesting using & selector
+	// We will only allow css nesting using & selector for now
 	// due to complexities with https://bugs.chromium.org/p/chromium/issues/detail?id=1427259
-	// as most browsers as of 17/11/2023 do not support nesting without & selector prefix
+	// as most browsers as of 17/11/2023 do not support nesting without & selector
 	if (parser.match('&')) {
 		return read_rule(parser)
 	} else {

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

@@ -117,14 +117,13 @@ export default class Selector {
 			}
 		}
 		this.blocks.forEach((block, index) => {
-			console.log(block)
 			if (block.global) {
 				remove_global_pseudo_class(block.selectors[0]);
 			}
 			if (block.should_encapsulate) {
 				encapsulate_block(
 					block,
-					index === this.blocks.length - 1
+					index === this.blocks.filter(block => block.nested !== true).length - 1
 						? attr.repeat(amount_class_specificity_to_increase + 1)
 						: attr
 				);
@@ -314,10 +313,6 @@ function block_might_apply_to_node(block, node) {
 
 		const name = selector.name.replace(regex_backslash_and_following_character, '$1');
 
-		// if(name === "&") {
-		// 	return POSSIBLE_MATCH;
-		// }
-
 		if (selector.type === 'PseudoClassSelector' && (name === 'host' || name === 'root')) {
 			return NO_MATCH;
 		}
@@ -806,6 +801,7 @@ class Block {
 		this.combinator = combinator;
 		this.host = false;
 		this.root = false;
+		this.nested = false;
 		this.selectors = [];
 		this.start = -1;
 		this.end = -1;
@@ -845,6 +841,7 @@ function group_selectors(selector) {
 			block = new Block(child);
 			blocks.push(block);
 		} else {
+			if (child.type === "NestedSelector") block.nested = true;
 			block.add(child);
 		}
 	});

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

@@ -6,6 +6,7 @@ import hash from '../utils/hash.js';
 // import { extract_ignores_above_position } from '../utils/extract_svelte_ignore.js';
 import { push_array } from '../utils/push_array.js';
 import { create_attribute } from '../../nodes.js';
+import assert from 'assert';
 
 const regex_css_browser_prefix = /^-((webkit)|(moz)|(o)|(ms))-/;
 
@@ -61,12 +62,17 @@ class Rule {
 	/** @type {Atrule | Rule | undefined} */
 	parent;
 
+	/** @type {Rule[]} */
+	nested_rules;
+
 	/**
 	 * @param {import('#compiler').Css.Rule} node
-	 * @param {any} stylesheet
+	 * @param {Stylesheet} stylesheet
 	 * @param {Atrule | Rule | undefined} parent
 	 */
 	constructor(node, stylesheet, parent) {
+		// console.log('...............')
+		// console.log(JSON.stringify(node, null, 4))
 		this.node = node;
 		this.parent = parent;
 		this.selectors = node.prelude.children.map((node) => new Selector(node, stylesheet));
@@ -82,9 +88,13 @@ class Rule {
 	/** @param {import('#compiler').RegularElement | import('#compiler').SvelteElement} node */
 	apply(node) {
 		this.selectors.forEach((selector) => selector.apply(node)); // TODO move the logic in here?
+		this.nested_rules.forEach((rule) => rule.apply(node));
 	}
 
-	/** @param {boolean} dev */
+	/**
+	 * @param {boolean} dev
+	 * @returns {boolean}
+	 */
 	is_used(dev) {
 		if (this.parent && this.parent.node.type === 'Atrule' && is_keyframes_node(this.parent.node))
 			return true;
@@ -93,7 +103,7 @@ class Rule {
 		// see them in devtools
 		if (this.declarations.length === 0) return dev;
 
-		return this.selectors.some((s) => s.used);
+		return [this.selectors.some((s) => s.used), this.nested_rules.some(r => r.is_used(dev))].some(Boolean);
 	}
 
 	/**
@@ -120,6 +130,9 @@ class Rule {
 		this.selectors.forEach((selector) => {
 			selector.validate(analysis);
 		});
+		this.nested_rules.forEach((rule) => {
+			rule.validate(analysis);
+		});
 	}
 
 	/** @param {(selector: import('./Selector.js').default) => void} handler */
@@ -127,6 +140,9 @@ class Rule {
 		this.selectors.forEach((selector) => {
 			if (!selector.used) handler(selector);
 		});
+		this.nested_rules.forEach((rule) => {
+			rule.warn_on_unused_selector(handler);
+		});
 	}
 
 	/** @returns number */
@@ -134,6 +150,7 @@ class Rule {
 		return Math.max(
 			...this.selectors.map((selector) => selector.get_amount_class_specificity_increased())
 		);
+		// do we need to check nested rules?
 	}
 
 	/**
@@ -147,7 +164,7 @@ class Rule {
 
 		// keep empty rules in dev, because it's convenient to
 		// see them in devtools
-		if (this.declarations.length === 0) {
+		if (this.declarations.length === 0 && this.nested_rules.length === 0) {
 			if (!dev) {
 				code.prependRight(this.node.start, '/* (empty) ');
 				code.appendLeft(this.node.end, '*/');
@@ -198,6 +215,10 @@ class Rule {
 				code.appendLeft(last, '*/');
 			}
 		}
+
+		this.nested_rules.forEach((rule) => {
+			rule.prune(code, dev);
+		});
 	}
 }
 
@@ -390,9 +411,12 @@ export default class Stylesheet {
 
 		const state = {
 			/** @type {Atrule | undefined} */
-			atrule: undefined
+			atrule: undefined,
 		};
 
+		/** @type {import('#compiler').Css.Node}*/
+		let prev_node;
+
 		walk(/** @type {import('#compiler').Css.Node} */ (ast), state, {
 			Atrule: (node, context) => {
 				const atrule = new Atrule(node);
@@ -429,12 +453,17 @@ export default class Stylesheet {
 			},
 			Rule: (node, context) => {
 				const rule = new Rule(node, this, context.state.atrule);
+
 				if (context.state.atrule) {
 					context.state.atrule.children.push(rule);
 				} else {
 					this.children.push(rule);
 				}
 
+				if (rule.nested_rules.length > 0) {
+					// Skip nested rules as they are instantiated in the Rule constructor
+					return node
+				}
 				context.next();
 			}
 		});

packages/svelte/tests/css/samples/nested-css/expected.css

@@ -10,5 +10,11 @@ button.svelte-xyz {
         color: green;
     }
 
+    & .bar.svelte-xyz {
+        & .hello.svelte-xyz {
+            color: orange;
+        }
+    }
+
     color: black;
 }

packages/svelte/tests/css/samples/nested-css/input.svelte

@@ -1,6 +1,8 @@
 <button>
     <div class="foo"/>
-    <div class="bar"/>
+    <div class="bar">
+        <div class="hello"/>
+    </div>
 </button>
 <style>
 button {
@@ -15,6 +17,12 @@ button {
         color: green;
     }
 
+    & .bar {
+        & .hello {
+            color: orange;
+        }
+    }
+
     color: black;
 }
 </style>