fix :host and :global css scoping

CHANGELOG.md

@@ -1,5 +1,10 @@
 # Svelte changelog
 
+## Unreleased
+
+* Fix scoping of selectors with `:global()` and `~` sibling combinators ([#5499](https://github.com/sveltejs/svelte/issues/5499))
+* Fix removal of `:host` selectors as unused when compiling to a custom element ([#5946](https://github.com/sveltejs/svelte/issues/5946))
+
 ## 3.32.1
 
 * Warn when using `module` variables reactively, and close weird reactivity loophole ([#5847](https://github.com/sveltejs/svelte/pull/5847))

src/compiler/compile/css/Selector.ts

@@ -84,7 +84,7 @@ export default class Selector {
 			while (i--) {
 				const selector = block.selectors[i];
 				if (selector.type === 'PseudoElementSelector' || selector.type === 'PseudoClassSelector') {
-					if (selector.name !== 'root') {
+					if (selector.name !== 'root' && selector.name !== 'host') {
 						if (i === 0) code.prependRight(selector.start, attr);
 					}
 					continue;
@@ -162,7 +162,10 @@ function apply_selector(blocks: Block[], node: Element, to_encapsulate: any[]):
 	if (!block) return false;
 
 	if (!node) {
-		return block.global && blocks.every(block => block.global);
+		return (
+      (block.global && blocks.every(block => block.global)) ||
+      (block.host && blocks.length === 0)
+    );
 	}
 
 	switch (block_might_apply_to_node(block, node)) {
@@ -182,6 +185,11 @@ function apply_selector(blocks: Block[], node: Element, to_encapsulate: any[]):
 					continue;
 				}
 
+				if (ancestor_block.host) {
+					to_encapsulate.push({ node, block });
+					return true;
+				}
+
 				let parent = node;
 				while (parent = get_element_parent(parent)) {
 					if (block_might_apply_to_node(ancestor_block, parent) !== BlockAppliesToNode.NotPossible) {
@@ -211,6 +219,19 @@ function apply_selector(blocks: Block[], node: Element, to_encapsulate: any[]):
 		} else if (block.combinator.name === '+' || block.combinator.name === '~') {
 			const siblings = get_possible_element_siblings(node, block.combinator.name === '+');
 			let has_match = false;
+
+			// NOTE: if we have :global(), we couldn't figure out what is selected within `:global` due to the 
+			// css-tree limitation that does not parse the inner selector of :global
+			// so unless we are sure there will be no sibling to match, we will consider it as matched
+			const has_global = blocks.some(block => block.global);
+			if (has_global) {
+				if (siblings.size === 0 && get_element_parent(node) !== null) {
+					return false;
+				}
+				to_encapsulate.push({ node, block });
+				return true;
+			}
+
 			for (const possible_sibling of siblings.keys()) {
 				if (apply_selector(blocks.slice(), possible_sibling, to_encapsulate)) {
 					to_encapsulate.push({ node, block });
@@ -236,6 +257,10 @@ function block_might_apply_to_node(block: Block, node: Element): BlockAppliesToN
 		const selector = block.selectors[i];
 		const name = typeof selector.name === 'string' && selector.name.replace(/\\(.)/g, '$1');
 
+		if (selector.type === 'PseudoClassSelector' && name === 'host') {
+			return BlockAppliesToNode.NotPossible;
+		}
+
 		if (selector.type === 'PseudoClassSelector' || selector.type === 'PseudoElementSelector') {
 			continue;
 		}
@@ -541,6 +566,7 @@ function loop_child(children: INode[], adjacent_only: boolean) {
 
 class Block {
 	global: boolean;
+	host: boolean;
 	combinator: CssNode;
 	selectors: CssNode[]
 	start: number;
@@ -550,6 +576,7 @@ class Block {
 	constructor(combinator: CssNode) {
 		this.combinator = combinator;
 		this.global = false;
+		this.host = false;
 		this.selectors = [];
 
 		this.start = null;
@@ -562,6 +589,7 @@ class Block {
 		if (this.selectors.length === 0) {
 			this.start = selector.start;
 			this.global = selector.type === 'PseudoClassSelector' && selector.name === 'global';
+			this.host = selector.type === 'PseudoClassSelector' && selector.name === 'host';
 		}
 
 		this.selectors.push(selector);

test/css/samples/host/_config.js

@@ -0,0 +1,27 @@
+export default {
+	warnings: [
+		{
+			code: 'css-unused-selector',
+			message: 'Unused CSS selector ":host > span"',
+			pos: 147,
+			start: {
+				character: 147,
+				column: 1,
+				line: 18
+			},
+			end: {
+				character: 159,
+				column: 13,
+				line: 18
+			},
+			frame: `
+			16:   }
+			17:
+			18:   :host > span {
+			      ^
+			19:     color: red;
+			20:   }
+      `
+		}
+	]
+};

test/css/samples/host/input.svelte

@@ -0,0 +1,27 @@
+<style>
+  :host h1 {
+    color: red;
+  }
+
+  :host > h1 {
+    color: red;
+  }
+
+  :host > * {
+    color: red;
+	}
+
+	:host span {
+		color: red;
+	}
+
+	:host > span {
+		color: red;
+	}
+</style>
+
+<h1>Hello!</h1>
+
+<div>
+  <span>World!</span>
+</div>

test/css/samples/siblings-combinator-global/_config.js

@@ -0,0 +1,50 @@
+export default {
+	warnings: [
+		{
+			code: 'css-unused-selector',
+			message: 'Unused CSS selector ":global(input) + span"',
+			pos: 239,
+			start: {
+				character: 239,
+				column: 2,
+				line: 9
+			},
+			end: {
+				character: 260,
+				column: 23,
+				line: 9
+			},
+			frame: `
+				 7:   :global(input) ~ p { color: red; }
+				 8:
+				 9:   :global(input) + span { color: red; }
+				      ^
+				10:   :global(input) ~ span { color: red; }
+				11: </style>
+			`
+		},
+		{
+			code: 'css-unused-selector',
+			message: 'Unused CSS selector ":global(input) ~ span"',
+			pos: 279,
+			start: {
+				character: 279,
+				column: 2,
+				line: 10
+			},
+			end: {
+				character: 300,
+				column: 23,
+				line: 10
+			},
+			frame: `
+				 8:
+				 9:   :global(input) + span { color: red; }
+				10:   :global(input) ~ span { color: red; }
+				      ^
+				11: </style>
+				12:
+			`
+		}
+	]
+};

test/css/samples/siblings-combinator-global/input.svelte

@@ -0,0 +1,21 @@
+<style>
+  :global(input) + div { color: red; }
+  :global(input) ~ div { color: red; }
+  :global(input) + h1 { color: red; }
+  :global(input) ~ h1 { color: red; }
+  :global(input) + p { color: red; }
+  :global(input) ~ p { color: red; }
+
+  :global(input) + span { color: red; }
+  :global(input) ~ span { color: red; }
+</style>
+
+<h1>Hello!</h1>
+
+<div>
+  <span>World!</span>
+</div>
+
+{#each [] as _}
+  <p />
+{/each}