feat: add hydrate method, make hydration treeshakeable

Introduces a new `hydrate` method which does hydration. Refactors code so that hydration-related code is treeshaken out when not using that method.
closes #9533
part of #9827

Author: dummdidumm

.changeset/gorgeous-singers-rest.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+feat: add hydrate method, make hydration treeshakeable

packages/svelte/scripts/check-treeshakeability.js

@@ -3,6 +3,33 @@ import path from 'node:path';
 import { rollup } from 'rollup';
 import virtual from '@rollup/plugin-virtual';
 import { nodeResolve } from '@rollup/plugin-node-resolve';
+import { compile } from 'svelte/compiler';
+
+async function bundle_code(entry) {
+	const bundle = await rollup({
+		input: '__entry__',
+		plugins: [
+			virtual({
+				__entry__: entry
+			}),
+			nodeResolve({
+				exportConditions: ['production', 'import', 'browser', 'default']
+			})
+		],
+		onwarn: (warning, handle) => {
+			// if (warning.code !== 'EMPTY_BUNDLE') handle(warning);
+		}
+	});
+
+	const { output } = await bundle.generate({});
+
+	if (output.length > 1) {
+		throw new Error('errr what');
+	}
+
+	const code = output[0].code.trim();
+	return code;
+}
 
 const pkg = JSON.parse(fs.readFileSync('package.json', 'utf8'));
 
@@ -20,31 +47,8 @@ for (const key in pkg.exports) {
 		if (!pkg.exports[key][type]) continue;
 
 		const subpackage = path.join(pkg.name, key);
-
 		const resolved = path.resolve(pkg.exports[key][type]);
-
-		const bundle = await rollup({
-			input: '__entry__',
-			plugins: [
-				virtual({
-					__entry__: `import ${JSON.stringify(resolved)}`
-				}),
-				nodeResolve({
-					exportConditions: ['production', 'import', 'browser', 'default']
-				})
-			],
-			onwarn: (warning, handle) => {
-				// if (warning.code !== 'EMPTY_BUNDLE') handle(warning);
-			}
-		});
-
-		const { output } = await bundle.generate({});
-
-		if (output.length > 1) {
-			throw new Error('errr what');
-		}
-
-		const code = output[0].code.trim();
+		const code = await bundle_code(`import ${JSON.stringify(resolved)}`);
 
 		if (code === '') {
 			// eslint-disable-next-line no-console
@@ -59,6 +63,52 @@ for (const key in pkg.exports) {
 	}
 }
 
+const client_main = path.resolve(pkg.exports['.'].browser);
+const without_hydration = await bundle_code(
+	// Use all features which contain hydration code to ensure it's treeshakeable
+	compile(
+		`
+<script>
+	import { mount } from ${JSON.stringify(client_main)}; mount();
+	let foo;
+</script>
+
+<svelte:head><title>hi</title></svelte:head>
+
+<a href={foo} class={foo}>a</a>
+<a {...foo}>a</a>
+<svelte:component this={foo} />
+<svelte:element this={foo} />
+<C {foo} />
+
+{#if foo}
+{/if}
+{#each foo as bar}
+{/each}
+{#await foo}
+{/await}
+{#key foo}
+{/key}
+{#snippet x()}
+{/snippet}
+
+{@render x()}
+{@html foo}
+	`,
+		{ filename: 'App.svelte' }
+	).js.code
+);
+if (!without_hydration.includes('current_hydration_fragment')) {
+	// eslint-disable-next-line no-console
+	console.error(`✅ Hydration code treeshakeable`);
+} else {
+	// eslint-disable-next-line no-console
+	console.error(without_hydration);
+	// eslint-disable-next-line no-console
+	console.error(`❌ Hydration code not treeshakeable`);
+	failed = true;
+}
+
 // eslint-disable-next-line no-console
 console.groupEnd();
 

packages/svelte/src/internal/client/each.js

@@ -11,6 +11,7 @@ import {
 	current_hydration_fragment,
 	get_hydration_fragment,
 	hydrate_block_anchor,
+	hydrating,
 	set_current_hydration_fragment
 } from './hydration.js';
 import { clear_text_content, empty, map_get, map_set } from './operations.js';
@@ -61,7 +62,10 @@ function each(anchor_node, collection, flags, key_fn, render_fn, fallback_fn, re
 	/** @type {null | import('./types.js').EffectSignal} */
 	let render = null;
 
-	/** Whether or not there was a "rendered fallback but want to render items" (or vice versa) hydration mismatch */
+	/**
+	 * Whether or not there was a "rendered fallback but want to render items" (or vice versa) hydration mismatch.
+	 * Needs to be a `let` or else it isn't treeshaken out
+	 */
 	let mismatch = false;
 
 	block.r =
@@ -107,7 +111,7 @@ function each(anchor_node, collection, flags, key_fn, render_fn, fallback_fn, re
 					// If the each block is controlled, then the anchor node will be the surrounding
 					// element in which the each block is rendered, which requires certain handling
 					// depending on whether we're in hydration mode or not
-					if (current_hydration_fragment === null) {
+					if (!hydrating) {
 						// Create a new anchor on the fly because there's none due to the optimization
 						anchor = empty();
 						block.a.appendChild(anchor);
@@ -153,13 +157,13 @@ function each(anchor_node, collection, flags, key_fn, render_fn, fallback_fn, re
 
 			const length = array.length;
 
-			if (current_hydration_fragment !== null) {
+			if (hydrating) {
 				const is_each_else_comment =
 					/** @type {Comment} */ (current_hydration_fragment?.[0])?.data === 'ssr:each_else';
 				// Check for hydration mismatch which can happen if the server renders the each fallback
 				// but the client has items, or vice versa. If so, remove everything inside the anchor and start fresh.
 				if ((is_each_else_comment && length) || (!is_each_else_comment && !length)) {
-					remove(/** @type {import('./types.js').TemplateNode[]} */ (current_hydration_fragment));
+					remove(current_hydration_fragment);
 					set_current_hydration_fragment(null);
 					mismatch = true;
 				} else if (is_each_else_comment) {
@@ -306,22 +310,22 @@ function reconcile_indexed_array(
 		}
 	} else {
 		var item;
-		var is_hydrating = current_hydration_fragment !== null;
+		/** `true` if there was a hydration mismatch. Needs to be a `let` or else it isn't treeshaken out */
+		let mismatch = false;
 		b_blocks = Array(b);
-		if (is_hydrating) {
+		if (hydrating) {
 			// Hydrate block
 			var hydration_list = /** @type {import('./types.js').TemplateNode[]} */ (
 				current_hydration_fragment
 			);
 			var hydrating_node = hydration_list[0];
 			for (; index < length; index++) {
-				var fragment = /** @type {Array<Text | Comment | Element>} */ (
-					get_hydration_fragment(hydrating_node)
-				);
+				var fragment = get_hydration_fragment(hydrating_node);
 				set_current_hydration_fragment(fragment);
 				if (!fragment) {
 					// If fragment is null, then that means that the server rendered less items than what
 					// the client code specifies -> break out and continue with client-side node creation
+					mismatch = true;
 					break;
 				}
 
@@ -357,7 +361,7 @@ function reconcile_indexed_array(
 			}
 		}
 
-		if (is_hydrating && current_hydration_fragment === null) {
+		if (mismatch) {
 			// Server rendered less nodes than the client -> set empty array so that Svelte continues to operate in hydration mode
 			set_current_hydration_fragment([]);
 		}
@@ -425,23 +429,23 @@ function reconcile_tracked_array(
 		var key;
 		var item;
 		var idx;
-		var is_hydrating = current_hydration_fragment !== null;
+		/** `true` if there was a hydration mismatch. Needs to be a `let` or else it isn't treeshaken out */
+		let mismatch = false;
 		b_blocks = Array(b);
-		if (is_hydrating) {
+		if (hydrating) {
 			// Hydrate block
 			var fragment;
 			var hydration_list = /** @type {import('./types.js').TemplateNode[]} */ (
 				current_hydration_fragment
 			);
 			var hydrating_node = hydration_list[0];
 			while (b > 0) {
-				fragment = /** @type {Array<Text | Comment | Element>} */ (
-					get_hydration_fragment(hydrating_node)
-				);
+				fragment = get_hydration_fragment(hydrating_node);
 				set_current_hydration_fragment(fragment);
 				if (!fragment) {
 					// If fragment is null, then that means that the server rendered less items than what
 					// the client code specifies -> break out and continue with client-side node creation
+					mismatch = true;
 					break;
 				}
 
@@ -594,7 +598,7 @@ function reconcile_tracked_array(
 			}
 		}
 
-		if (is_hydrating && current_hydration_fragment === null) {
+		if (mismatch) {
 			// Server rendered less nodes than the client -> set empty array so that Svelte continues to operate in hydration mode
 			set_current_hydration_fragment([]);
 		}

packages/svelte/src/internal/client/hydration.js

@@ -3,25 +3,37 @@
 import { empty } from './operations.js';
 import { schedule_task } from './runtime.js';
 
-/** @type {null | Array<import('./types.js').TemplateNode>} */
-export let current_hydration_fragment = null;
+/**
+ * Use this variable to guard everything related to hydration code so it can be treeshaken out
+ * if the user doesn't use the `hydrate` method and these code paths are therefore not needed.
+ */
+export let hydrating = false;
+
+/**
+ * Array of nodes to traverse for hydration. This will be null if we're not hydrating, but for
+ * the sake of simplicity we're not going to use `null` checks everywhere and instead rely on
+ * the `hydrating` flag to tell whether or not we're in hydration mode at which point this is set.
+ * @type {import('./types.js').TemplateNode[]}
+ */
+export let current_hydration_fragment = /** @type {any} */ (null);
 
 /**
- * @param {null | Array<import('./types.js').TemplateNode>} fragment
+ * @param {null | import('./types.js').TemplateNode[]} fragment
  * @returns {void}
  */
 export function set_current_hydration_fragment(fragment) {
-	current_hydration_fragment = fragment;
+	hydrating = fragment !== null;
+	current_hydration_fragment = /** @type {import('./types.js').TemplateNode[]} */ (fragment);
 }
 
 /**
  * Returns all nodes between the first `<!--ssr:...-->` comment tag pair encountered.
  * @param {Node | null} node
  * @param {boolean} [insert_text] Whether to insert an empty text node if the fragment is empty
- * @returns {Array<import('./types.js').TemplateNode> | null}
+ * @returns {import('./types.js').TemplateNode[] | null}
  */
 export function get_hydration_fragment(node, insert_text = false) {
-	/** @type {Array<import('./types.js').TemplateNode>} */
+	/** @type {import('./types.js').TemplateNode[]} */
 	const fragment = [];
 
 	/** @type {null | Node} */
@@ -66,9 +78,10 @@ export function get_hydration_fragment(node, insert_text = false) {
  * @returns {void}
  */
 export function hydrate_block_anchor(anchor_node, is_controlled) {
-	/** @type {Node} */
-	let target_node = anchor_node;
-	if (current_hydration_fragment !== null) {
+	if (hydrating) {
+		/** @type {Node} */
+		let target_node = anchor_node;
+
 		if (is_controlled) {
 			target_node = /** @type {Node} */ (target_node.firstChild);
 		}

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

@@ -1,4 +1,4 @@
-import { current_hydration_fragment, get_hydration_fragment } from './hydration.js';
+import { current_hydration_fragment, get_hydration_fragment, hydrating } from './hydration.js';
 import { get_descriptor } from './utils.js';
 
 // We cache the Node and Element prototype methods, so that we can avoid doing
@@ -171,7 +171,7 @@ export function empty() {
 /*#__NO_SIDE_EFFECTS__*/
 export function child(node) {
 	const child = first_child_get.call(node);
-	if (current_hydration_fragment !== null) {
+	if (hydrating) {
 		// Child can be null if we have an element with a single child, like `<p>{text}</p>`, where `text` is empty
 		if (child === null) {
 			const text = empty();
@@ -192,7 +192,7 @@ export function child(node) {
  */
 /*#__NO_SIDE_EFFECTS__*/
 export function child_frag(node, is_text) {
-	if (current_hydration_fragment !== null) {
+	if (hydrating) {
 		const first_node = /** @type {Node[]} */ (node)[0];
 
 		// if an {expression} is empty during SSR, there might be no
@@ -225,7 +225,7 @@ export function child_frag(node, is_text) {
 /*#__NO_SIDE_EFFECTS__*/
 export function sibling(node, is_text = false) {
 	const next_sibling = next_sibling_get.call(node);
-	if (current_hydration_fragment !== null) {
+	if (hydrating) {
 		// if a sibling {expression} is empty during SSR, there might be no
 		// text node to hydrate — we must therefore create one
 		if (is_text && next_sibling?.nodeType !== 3) {
@@ -276,14 +276,15 @@ export function create_element(name) {
 }
 
 /**
+ * Expects to only be called in hydration mode
  * @param {Node} node
  * @returns {Node}
  */
 function capture_fragment_from_node(node) {
 	if (
 		node.nodeType === 8 &&
 		/** @type {Comment} */ (node).data.startsWith('ssr:') &&
-		/** @type {Array<Element | Text | Comment>} */ (current_hydration_fragment).at(-1) !== node
+		current_hydration_fragment.at(-1) !== node
 	) {
 		const fragment = /** @type {Array<Element | Text | Comment>} */ (get_hydration_fragment(node));
 		const last_child = fragment.at(-1) || node;

packages/svelte/src/internal/client/reconciler.js

@@ -1,5 +1,5 @@
 import { append_child } from './operations.js';
-import { current_hydration_fragment, hydrate_block_anchor } from './hydration.js';
+import { current_hydration_fragment, hydrate_block_anchor, hydrating } from './hydration.js';
 import { is_array } from './utils.js';
 
 /** @param {string} html */
@@ -92,7 +92,7 @@ export function remove(current) {
  */
 export function reconcile_html(target, value, svg) {
 	hydrate_block_anchor(target);
-	if (current_hydration_fragment !== null) {
+	if (hydrating) {
 		return current_hydration_fragment;
 	}
 	var html = value + '';