chore: make if blocks tree-shakable (#14549)

* chore: make if blocks dead code eliminable

* chore: make if blocks dead code eliminable

* chore: make if blocks dead code eliminable

* address feedback

* address feedback

* prettier

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/beige-windows-happen.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: make if blocks tree-shakable

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

@@ -9,23 +9,44 @@ import * as b from '../../../../utils/builders.js';
  */
 export function IfBlock(node, context) {
 	context.state.template.push('<!>');
+	const statements = [];
 
 	const consequent = /** @type {BlockStatement} */ (context.visit(node.consequent));
+	const consequent_id = context.state.scope.generate('consequent');
 
+	statements.push(b.var(b.id(consequent_id), b.arrow([b.id('$$anchor')], consequent)));
+
+	let alternate_id;
+
+	if (node.alternate) {
+		const alternate = /** @type {BlockStatement} */ (context.visit(node.alternate));
+		alternate_id = context.state.scope.generate('alternate');
+		statements.push(b.var(b.id(alternate_id), b.arrow([b.id('$$anchor')], alternate)));
+	}
+
+	/** @type {Expression[]} */
 	const args = [
 		context.state.node,
-		b.thunk(/** @type {Expression} */ (context.visit(node.test))),
-		b.arrow([b.id('$$anchor')], consequent)
+		b.arrow(
+			[b.id('$$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
+								)
+							)
+						: undefined
+				)
+			])
+		)
 	];
 
-	if (node.alternate || node.elseif) {
-		args.push(
-			node.alternate
-				? b.arrow([b.id('$$anchor')], /** @type {BlockStatement} */ (context.visit(node.alternate)))
-				: b.literal(null)
-		);
-	}
-
 	if (node.elseif) {
 		// We treat this...
 		//
@@ -51,5 +72,7 @@ export function IfBlock(node, context) {
 		args.push(b.literal(true));
 	}
 
-	context.state.init.push(b.stmt(b.call('$.if', ...args)));
+	statements.push(b.stmt(b.call('$.if', ...args)));
+
+	context.state.init.push(b.block(statements));
 }

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

@@ -13,13 +13,11 @@ import { HYDRATION_START_ELSE } from '../../../../constants.js';
 
 /**
  * @param {TemplateNode} node
- * @param {() => boolean} get_condition
- * @param {(anchor: Node) => void} consequent_fn
- * @param {null | ((anchor: Node) => void)} [alternate_fn]
+ * @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'
  * @returns {void}
  */
-export function if_block(node, get_condition, consequent_fn, alternate_fn = null, elseif = false) {
+export function if_block(node, fn, elseif = false) {
 	if (hydrating) {
 		hydrate_next();
 	}
@@ -37,8 +35,18 @@ export function if_block(node, get_condition, consequent_fn, alternate_fn = null
 
 	var flags = elseif ? EFFECT_TRANSPARENT : 0;
 
-	block(() => {
-		if (condition === (condition = !!get_condition())) return;
+	var has_branch = false;
+
+	const set_branch = (/** @type {(anchor: Node) => 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
+	) => {
+		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;
@@ -60,8 +68,8 @@ export function if_block(node, get_condition, consequent_fn, alternate_fn = null
 		if (condition) {
 			if (consequent_effect) {
 				resume_effect(consequent_effect);
-			} else {
-				consequent_effect = branch(() => consequent_fn(anchor));
+			} else if (fn) {
+				consequent_effect = branch(() => fn(anchor));
 			}
 
 			if (alternate_effect) {
@@ -72,8 +80,8 @@ export function if_block(node, get_condition, consequent_fn, alternate_fn = null
 		} else {
 			if (alternate_effect) {
 				resume_effect(alternate_effect);
-			} else if (alternate_fn) {
-				alternate_effect = branch(() => alternate_fn(anchor));
+			} else if (fn) {
+				alternate_effect = branch(() => fn(anchor));
 			}
 
 			if (consequent_effect) {
@@ -87,6 +95,14 @@ export function if_block(node, get_condition, consequent_fn, alternate_fn = null
 			// continue in hydration mode
 			set_hydrating(true);
 		}
+	};
+
+	block(() => {
+		has_branch = false;
+		fn(set_branch);
+		if (!has_branch) {
+			update_branch(null, null);
+		}
 	}, flags);
 
 	if (hydrating) {