feat: skip static nodes (#12914)

* step one

* WIP

* more

* fix

* collapse sequential sibling calls

* working

* working but messy

* tidy up

* unused

* tweak

* tweak

* tidy

* tweak

* tweak

* revert

* changeset

* Update packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/fragment.js

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

* revert this bit

* align

* comments

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>

.changeset/ten-trainers-juggle.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: skip over static nodes in compiled client code

packages/svelte/src/compiler/phases/3-transform/client/visitors/shared/fragment.js

@@ -1,6 +1,7 @@
 /** @import { Expression } from 'estree' */
 /** @import { ExpressionTag, SvelteNode, Text } from '#compiler' */
 /** @import { ComponentClientTransformState, ComponentContext } from '../../types' */
+import { is_event_attribute, is_text_attribute } from '../../../../../utils/ast.js';
 import * as b from '../../../../../utils/builders.js';
 import { build_template_literal, build_update } from './utils.js';
 
@@ -9,42 +10,71 @@ import { build_template_literal, build_update } from './utils.js';
  * (e.g. `{a} b {c}`) into a single update function. Along the way it creates
  * corresponding template node references these updates are applied to.
  * @param {SvelteNode[]} nodes
- * @param {(is_text: boolean) => Expression} expression
+ * @param {(is_text: boolean) => Expression} initial
  * @param {boolean} is_element
  * @param {ComponentContext} context
  */
-export function process_children(nodes, expression, is_element, { visit, state }) {
+export function process_children(nodes, initial, is_element, { visit, state }) {
 	const within_bound_contenteditable = state.metadata.bound_contenteditable;
+	let prev = initial;
+	let skipped = 0;
 
 	/** @typedef {Array<Text | ExpressionTag>} Sequence */
-
 	/** @type {Sequence} */
 	let sequence = [];
 
+	/** @param {boolean} is_text */
+	function get_node(is_text) {
+		if (skipped === 0) {
+			return prev(is_text);
+		}
+
+		return b.call(
+			'$.sibling',
+			prev(false),
+			(is_text || skipped !== 1) && b.literal(skipped),
+			is_text && b.true
+		);
+	}
+
+	/**
+	 * @param {boolean} is_text
+	 * @param {string} name
+	 */
+	function flush_node(is_text, name) {
+		const expression = get_node(is_text);
+		let id = expression;
+
+		if (id.type !== 'Identifier') {
+			id = b.id(state.scope.generate(name));
+			state.init.push(b.var(id, expression));
+		}
+
+		prev = () => id;
+		skipped = 1; // the next node is `$.sibling(id)`
+
+		return id;
+	}
+
 	/**
 	 * @param {Sequence} sequence
 	 */
 	function flush_sequence(sequence) {
-		if (sequence.length === 1) {
-			const node = sequence[0];
-
-			if (node.type === 'Text') {
-				let prev = expression;
-				expression = () => b.call('$.sibling', prev(false));
-				state.template.push(node.raw);
-				return;
-			}
+		if (sequence.length === 1 && sequence[0].type === 'Text') {
+			skipped += 1;
+			state.template.push(sequence[0].raw);
+			return;
 		}
 
-		// if this is a standalone `{expression}`, make sure we handle the case where
-		// no text node was created because the expression was empty during SSR
-		const needs_hydration_check = sequence.length === 1;
-		const id = get_node_id(expression(needs_hydration_check), state, 'text');
-
 		state.template.push(' ');
 
 		const { has_state, has_call, value } = build_template_literal(sequence, visit, state);
 
+		// if this is a standalone `{expression}`, make sure we handle the case where
+		// no text node was created because the expression was empty during SSR
+		const is_text = sequence.length === 1;
+		const id = flush_node(is_text, 'text');
+
 		const update = b.stmt(b.call('$.set_text', id, value));
 
 		if (has_call && !within_bound_contenteditable) {
@@ -54,13 +84,9 @@ export function process_children(nodes, expression, is_element, { visit, state }
 		} else {
 			state.init.push(b.stmt(b.assignment('=', b.member(id, 'nodeValue'), value)));
 		}
-
-		expression = (is_text) => b.call('$.sibling', id, is_text && b.true);
 	}
 
-	for (let i = 0; i < nodes.length; i += 1) {
-		const node = nodes[i];
-
+	for (const node of nodes) {
 		if (node.type === 'Text' || node.type === 'ExpressionTag') {
 			sequence.push(node);
 		} else {
@@ -69,60 +95,62 @@ export function process_children(nodes, expression, is_element, { visit, state }
 				sequence = [];
 			}
 
-			if (
-				node.type === 'SvelteHead' ||
-				node.type === 'TitleElement' ||
-				node.type === 'SnippetBlock'
-			) {
-				// These nodes do not contribute to the sibling/child tree
-				// TODO what about e.g. ConstTag and all the other things that
-				// get hoisted inside clean_nodes?
-				visit(node, state);
+			let child_state = state;
+
+			if (is_static_element(node)) {
+				skipped += 1;
+			} else if (node.type === 'EachBlock' && nodes.length === 1 && is_element) {
+				node.metadata.is_controlled = true;
 			} else {
-				if (node.type === 'EachBlock' && nodes.length === 1 && is_element) {
-					node.metadata.is_controlled = true;
-					visit(node, state);
-				} else {
-					const id = get_node_id(
-						expression(false),
-						state,
-						node.type === 'RegularElement' ? node.name : 'node'
-					);
-
-					expression = (is_text) => b.call('$.sibling', id, is_text && b.true);
-
-					visit(node, {
-						...state,
-						node: id
-					});
-				}
+				const id = flush_node(false, node.type === 'RegularElement' ? node.name : 'node');
+				child_state = { ...state, node: id };
 			}
+
+			visit(node, child_state);
 		}
 	}
 
 	if (sequence.length > 0) {
-		// if the final item in a fragment is static text,
-		// we need to force `hydrate_node` to advance
-		if (sequence.length === 1 && sequence[0].type === 'Text' && nodes.length > 1) {
-			state.init.push(b.stmt(b.call('$.next')));
-		}
-
 		flush_sequence(sequence);
 	}
+
+	// if there are trailing static text nodes/elements,
+	// traverse to the last (n - 1) one when hydrating
+	if (skipped > 1) {
+		skipped -= 1;
+		state.init.push(b.stmt(get_node(false)));
+	}
 }
 
 /**
- * @param {Expression} expression
- * @param {ComponentClientTransformState} state
- * @param {string} name
+ *
+ * @param {SvelteNode} node
  */
-function get_node_id(expression, state, name) {
-	let id = expression;
+function is_static_element(node) {
+	if (node.type !== 'RegularElement') return false;
+	if (node.fragment.metadata.dynamic) return false;
 
-	if (id.type !== 'Identifier') {
-		id = b.id(state.scope.generate(name));
+	for (const attribute of node.attributes) {
+		if (attribute.type !== 'Attribute') {
+			return false;
+		}
+
+		if (is_event_attribute(attribute)) {
+			return false;
+		}
+
+		if (attribute.value !== true && !is_text_attribute(attribute)) {
+			return false;
+		}
 
-		state.init.push(b.var(id, expression));
+		if (node.name === 'option' && attribute.name === 'value') {
+			return false;
+		}
+
+		if (node.name.includes('-')) {
+			return false; // we're setting all attributes on custom elements through properties
+		}
 	}
-	return id;
+
+	return true;
 }

packages/svelte/src/internal/client/dom/operations.js

@@ -70,6 +70,7 @@ export function create_text(value = '') {
  * @param {N} node
  * @returns {Node | null}
  */
+/*@__NO_SIDE_EFFECTS__*/
 export function get_first_child(node) {
 	return first_child_getter.call(node);
 }
@@ -79,6 +80,7 @@ export function get_first_child(node) {
  * @param {N} node
  * @returns {Node | null}
  */
+/*@__NO_SIDE_EFFECTS__*/
 export function get_next_sibling(node) {
 	return next_sibling_getter.call(node);
 }
@@ -137,17 +139,21 @@ export function first_child(fragment, is_text) {
 
 /**
  * Don't mark this as side-effect-free, hydration needs to walk all nodes
- * @template {Node} N
- * @param {N} node
+ * @param {TemplateNode} node
+ * @param {number} count
  * @param {boolean} is_text
  * @returns {Node | null}
  */
-export function sibling(node, is_text = false) {
-	if (!hydrating) {
-		return /** @type {TemplateNode} */ (get_next_sibling(node));
+export function sibling(node, count = 1, is_text = false) {
+	let next_sibling = hydrating ? hydrate_node : node;
+
+	while (count--) {
+		next_sibling = /** @type {TemplateNode} */ (get_next_sibling(next_sibling));
 	}
 
-	var next_sibling = /** @type {TemplateNode} */ (get_next_sibling(hydrate_node));
+	if (!hydrating) {
+		return next_sibling;
+	}
 
 	var type = next_sibling.nodeType;
 

packages/svelte/tests/snapshot/samples/dynamic-attributes-casing/_expected/client/main.svelte.js

@@ -9,17 +9,17 @@ export default function Main($$anchor) {
 	let y = () => 'test';
 	var fragment = root();
 	var div = $.first_child(fragment);
-	var svg = $.sibling($.sibling(div));
-	var custom_element = $.sibling($.sibling(svg));
-	var div_1 = $.sibling($.sibling(custom_element));
+	var svg = $.sibling(div, 2);
+	var custom_element = $.sibling(svg, 2);
+	var div_1 = $.sibling(custom_element, 2);
 
 	$.template_effect(() => $.set_attribute(div_1, "foobar", y()));
 
-	var svg_1 = $.sibling($.sibling(div_1));
+	var svg_1 = $.sibling(div_1, 2);
 
 	$.template_effect(() => $.set_attribute(svg_1, "viewBox", y()));
 
-	var custom_element_1 = $.sibling($.sibling(svg_1));
+	var custom_element_1 = $.sibling(svg_1, 2);
 
 	$.template_effect(() => $.set_custom_element_data(custom_element_1, "fooBar", y()));
 

packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js

@@ -13,11 +13,11 @@ export default function Purity($$anchor) {
 
 	p.textContent = Math.max(min, Math.min(max, number));
 
-	var p_1 = $.sibling($.sibling(p));
+	var p_1 = $.sibling(p, 2);
 
 	p_1.textContent = location.href;
 
-	var node = $.sibling($.sibling(p_1));
+	var node = $.sibling(p_1, 2);
 
 	Child(node, { prop: encodeURIComponent(value) });
 	$.append($$anchor, fragment);

packages/svelte/tests/snapshot/samples/skip-static-subtree/_expected/client/index.svelte.js

@@ -1,10 +1,20 @@
 import "svelte/internal/disclose-version";
 import * as $ from "svelte/internal/client";
 
-var root = $.template(`<header><nav><a href="/">Home</a> <a href="/away">Away</a></nav></header>`);
+var root = $.template(`<header><nav><a href="/">Home</a> <a href="/away">Away</a></nav></header> <main><h1> </h1> <div class="static"><p>we don't need to traverse these nodes</p></div> <p>or</p> <p>these</p> <p>ones</p> <!></main>`, 1);
 
-export default function Skip_static_subtree($$anchor) {
-	var header = root();
+export default function Skip_static_subtree($$anchor, $$props) {
+	var fragment = root();
+	var main = $.sibling($.first_child(fragment), 2);
+	var h1 = $.child(main);
+	var text = $.child(h1);
 
-	$.append($$anchor, header);
+	$.reset(h1);
+
+	var node = $.sibling(h1, 10);
+
+	$.html(node, () => $$props.content, false, false);
+	$.reset(main);
+	$.template_effect(() => $.set_text(text, $$props.title));
+	$.append($$anchor, fragment);
 }

packages/svelte/tests/snapshot/samples/skip-static-subtree/_expected/server/index.svelte.js

@@ -1,5 +1,7 @@
 import * as $ from "svelte/internal/server";
 
-export default function Skip_static_subtree($$payload) {
-	$$payload.out += `<header><nav><a href="/">Home</a> <a href="/away">Away</a></nav></header>`;
+export default function Skip_static_subtree($$payload, $$props) {
+	let { title, content } = $$props;
+
+	$$payload.out += `<header><nav><a href="/">Home</a> <a href="/away">Away</a></nav></header> <main><h1>${$.escape(title)}</h1> <div class="static"><p>we don't need to traverse these nodes</p></div> <p>or</p> <p>these</p> <p>ones</p> ${$.html(content)}</main>`;
 }

packages/svelte/tests/snapshot/samples/skip-static-subtree/index.svelte

@@ -1,6 +1,21 @@
+<script>
+	let { title, content } = $props();
+</script>
+
 <header>
 	<nav>
 		<a href="/">Home</a>
 		<a href="/away">Away</a>
 	</nav>
 </header>
+
+<main>
+	<h1>{title}</h1>
+	<div class="static">
+		<p>we don't need to traverse these nodes</p>
+	</div>
+	<p>or</p>
+	<p>these</p>
+	<p>ones</p>
+	{@html content}
+</main>

packages/svelte/tests/snapshot/samples/state-proxy-literal/_expected/client/index.svelte.js

@@ -18,11 +18,11 @@ export default function State_proxy_literal($$anchor) {
 
 	$.remove_input_defaults(input);
 
-	var input_1 = $.sibling($.sibling(input));
+	var input_1 = $.sibling(input, 2);
 
 	$.remove_input_defaults(input_1);
 
-	var button = $.sibling($.sibling(input_1));
+	var button = $.sibling(input_1, 2);
 
 	button.__click = [reset, str, tpl];
 	$.bind_value(input, () => $.get(str), ($$value) => $.set(str, $$value));