chore: Reduce hydration comment for {:else if}

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

@@ -48,7 +48,9 @@ export function Fragment(node, context) {
 	const is_single_element = trimmed.length === 1 && trimmed[0].type === 'RegularElement';
 	const is_single_child_not_needing_template =
 		trimmed.length === 1 &&
-		(trimmed[0].type === 'SvelteFragment' || trimmed[0].type === 'TitleElement');
+		(trimmed[0].type === 'SvelteFragment' ||
+			trimmed[0].type === 'TitleElement' ||
+			(trimmed[0].type === 'IfBlock' && trimmed[0].elseif));
 
 	const template_name = context.state.scope.root.unique('root'); // TODO infer name from parent
 

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

@@ -1,4 +1,4 @@
-/** @import { BlockStatement, Expression } from 'estree' */
+/** @import { BlockStatement, Expression, Identifier } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types' */
 import * as b from '../../../../utils/builders.js';
@@ -19,28 +19,29 @@ export function IfBlock(node, context) {
 	let alternate_id;
 
 	if (node.alternate) {
-		const alternate = /** @type {BlockStatement} */ (context.visit(node.alternate));
 		alternate_id = context.state.scope.generate('alternate');
-		statements.push(b.var(b.id(alternate_id), b.arrow([b.id('$$anchor')], alternate)));
+		const alternate = /** @type {BlockStatement} */ (context.visit(node.alternate));
+		const nodes = node.alternate.nodes;
+
+		let alternate_args = [b.id('$$anchor')];
+		if (nodes.length === 1 && nodes[0].type === 'IfBlock' && nodes[0].elseif) {
+			alternate_args.push(b.id('$$elseif'));
+		}
+
+		statements.push(b.var(b.id(alternate_id), b.arrow(alternate_args, alternate)));
 	}
 
 	/** @type {Expression[]} */
 	const args = [
-		context.state.node,
+		node.elseif ? b.id('$$anchor') : context.state.node,
 		b.arrow(
 			[b.id('$$render')],
 			b.block([
 				b.if(
 					/** @type {Expression} */ (context.visit(node.test)),
 					b.stmt(b.call(b.id('$$render'), b.id(consequent_id))),
 					alternate_id
-						? b.stmt(
-								b.call(
-									b.id('$$render'),
-									b.id(alternate_id),
-									node.alternate ? b.literal(false) : undefined
-								)
-							)
+						? b.stmt(b.call(b.id('$$render'), b.id(alternate_id), b.literal(false)))
 						: undefined
 				)
 			])
@@ -69,7 +70,7 @@ export function IfBlock(node, context) {
 		// ...even though they're logically equivalent. In the first case, the
 		// transition will only play when `y` changes, but in the second it
 		// should play when `x` or `y` change — both are considered 'local'
-		args.push(b.literal(true));
+		args.push(b.id('$$elseif'));
 	}
 
 	statements.push(b.stmt(b.call('$.if', ...args)));

packages/svelte/src/compiler/phases/3-transform/server/visitors/IfBlock.js

@@ -1,4 +1,4 @@
-/** @import { BlockStatement, Expression } from 'estree' */
+/** @import { BlockStatement, Expression, IfStatement } from 'estree' */
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types.js' */
 import { BLOCK_OPEN_ELSE } from '../../../../../internal/server/hydration.js';
@@ -10,19 +10,29 @@ import { block_close, block_open } from './shared/utils.js';
  * @param {ComponentContext} context
  */
 export function IfBlock(node, context) {
-	const test = /** @type {Expression} */ (context.visit(node.test));
-
 	const consequent = /** @type {BlockStatement} */ (context.visit(node.consequent));
+	consequent.body.unshift(b.stmt(b.assignment('+=', b.id('$$payload.out'), block_open)));
+	let if_statement = b.if(/** @type {Expression} */ (context.visit(node.test)), consequent);
 
-	const alternate = node.alternate
-		? /** @type {BlockStatement} */ (context.visit(node.alternate))
-		: b.block([]);
+	context.state.template.push(if_statement, block_close);
 
-	consequent.body.unshift(b.stmt(b.assignment('+=', b.id('$$payload.out'), block_open)));
+	let index = 1;
+	let alt = node.alternate;
+	while (alt && alt.nodes.length === 1 && alt.nodes[0].type === 'IfBlock' && alt.nodes[0].elseif) {
+		const elseif = alt.nodes[0];
+		const alternate = /** @type {BlockStatement} */ (context.visit(elseif.consequent));
+		alternate.body.unshift(
+			b.stmt(b.assignment('+=', b.id('$$payload.out'), b.literal(`<!--[${index++}-->`)))
+		);
+		if_statement = if_statement.alternate = b.if(
+			/** @type {Expression} */ (context.visit(elseif.test)),
+			alternate
+		);
+		alt = elseif.alternate;
+	}
 
-	alternate.body.unshift(
+	if_statement.alternate = alt ? /** @type {BlockStatement} */ (context.visit(alt)) : b.block([]);
+	if_statement.alternate.body.unshift(
 		b.stmt(b.assignment('+=', b.id('$$payload.out'), b.literal(BLOCK_OPEN_ELSE)))
 	);
-
-	context.state.template.push(b.if(test, consequent, alternate), block_close);
 }

packages/svelte/src/internal/client/dom/blocks/if.js

@@ -9,16 +9,16 @@ import {
 	set_hydrating
 } from '../hydration.js';
 import { block, branch, pause_effect, resume_effect } from '../../reactivity/effects.js';
-import { HYDRATION_START_ELSE, UNINITIALIZED } from '../../../../constants.js';
+import { HYDRATION_START, HYDRATION_START_ELSE, UNINITIALIZED } from '../../../../constants.js';
 
 /**
  * @param {TemplateNode} node
- * @param {(branch: (fn: (anchor: Node) => void, flag?: boolean) => void) => void} fn
- * @param {boolean} [elseif] True if this is an `{:else if ...}` block rather than an `{#if ...}`, as that affects which transitions are considered 'local'
+ * @param {(branch: (fn: (anchor: Node, elseif?: [number,number]) => void, flag?: boolean) => void) => void} fn
+ * @param {[number,number]} [elseif]
  * @returns {void}
  */
-export function if_block(node, fn, elseif = false) {
-	if (hydrating) {
+export function if_block(node, fn, [root_index, hydrate_index] = [0, 0]) {
+	if (hydrating && root_index === 0) {
 		hydrate_next();
 	}
 
@@ -33,26 +33,44 @@ export function if_block(node, fn, elseif = false) {
 	/** @type {UNINITIALIZED | boolean | null} */
 	var condition = UNINITIALIZED;
 
-	var flags = elseif ? EFFECT_TRANSPARENT : 0;
+	var flags = root_index > 0 ? EFFECT_TRANSPARENT : 0;
 
 	var has_branch = false;
 
-	const set_branch = (/** @type {(anchor: Node) => void} */ fn, flag = true) => {
+	const set_branch = (
+		/** @type {(anchor: Node, elseif?: [number,number]) => void} */ fn,
+		flag = true
+	) => {
 		has_branch = true;
 		update_branch(flag, fn);
 	};
 
 	const update_branch = (
 		/** @type {boolean | null} */ new_condition,
-		/** @type {null | ((anchor: Node) => void)} */ fn
+		/** @type {null | ((anchor: Node, elseif?: [number,number]) => void)} */ fn
 	) => {
 		if (condition === (condition = new_condition)) return;
 
 		/** Whether or not there was a hydration mismatch. Needs to be a `let` or else it isn't treeshaken out */
 		let mismatch = false;
 
-		if (hydrating) {
-			const is_else = /** @type {Comment} */ (anchor).data === HYDRATION_START_ELSE;
+		if (hydrating && hydrate_index !== -1) {
+			if (root_index === 0) {
+				const data = /** @type {Comment} */ (anchor).data;
+				if (data === HYDRATION_START) {
+					hydrate_index = 0;
+				} else if (data === HYDRATION_START_ELSE) {
+					hydrate_index = Infinity;
+				} else {
+					hydrate_index = parseInt(data.substring(1));
+					if (hydrate_index !== hydrate_index) {
+						// if hydrate_index is NaN
+						// we set an invalid index to force mismatch
+						hydrate_index = condition ? Infinity : -1;
+					}
+				}
+			}
+			const is_else = hydrate_index > root_index;
 
 			if (!!condition === is_else) {
 				// Hydration mismatch: remove everything inside the anchor and start fresh.
@@ -62,6 +80,7 @@ export function if_block(node, fn, elseif = false) {
 				set_hydrate_node(anchor);
 				set_hydrating(false);
 				mismatch = true;
+				hydrate_index = -1; // ignore hydration in next else if
 			}
 		}
 
@@ -81,7 +100,7 @@ export function if_block(node, fn, elseif = false) {
 			if (alternate_effect) {
 				resume_effect(alternate_effect);
 			} else if (fn) {
-				alternate_effect = branch(() => fn(anchor));
+				alternate_effect = branch(() => fn(anchor, [root_index + 1, hydrate_index]));
 			}
 
 			if (consequent_effect) {