From 249faa55880accbc851877512fa698115e516ae7 Mon Sep 17 00:00:00 2001
From: Rich Harris <rich.harris@vercel.com>
Date: Sat, 25 Oct 2025 10:30:12 -0400
Subject: [PATCH] WIP

---
 .../src/compiler/phases/2-analyze/index.js    |  4 +--
 .../phases/2-analyze/visitors/IfBlock.js      | 19 ----------
 .../3-transform/client/visitors/IfBlock.js    | 22 ++++++------
 .../3-transform/client/visitors/Program.js    | 27 +++++++++-----
 .../svelte/src/compiler/phases/types.d.ts     | 20 ++++-------
 .../svelte/src/compiler/types/template.d.ts   |  3 --
 .../src/internal/client/dom/blocks/async.js   |  6 ++--
 .../src/internal/client/reactivity/async.js   | 36 +++++++++++++------
 .../src/internal/client/reactivity/effects.js |  5 +--
 9 files changed, 70 insertions(+), 72 deletions(-)

diff --git a/packages/svelte/src/compiler/phases/2-analyze/index.js b/packages/svelte/src/compiler/phases/2-analyze/index.js
index 1c92d6f0da..fa10cc6ec0 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/index.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/index.js
@@ -555,8 +555,8 @@ export function analyze_component(root, source, options) {
 		snippets: new Set(),
 		async_deriveds: new Set(),
 		pickled_awaits: new Set(),
-		awaited_declarations: new Map(),
-		awaited_statements: new Map()
+		awaited_statements: new Map(),
+		promise_indexes: new Map()
 	};
 
 	if (!runes) {
diff --git a/packages/svelte/src/compiler/phases/2-analyze/visitors/IfBlock.js b/packages/svelte/src/compiler/phases/2-analyze/visitors/IfBlock.js
index 8471b4d99b..dcdae3587f 100644
--- a/packages/svelte/src/compiler/phases/2-analyze/visitors/IfBlock.js
+++ b/packages/svelte/src/compiler/phases/2-analyze/visitors/IfBlock.js
@@ -22,25 +22,6 @@ export function IfBlock(node, context) {
 		expression: node.metadata.expression
 	});
 
-	// TODO this can be helperised
-	for (const binding of node.metadata.expression.dependencies) {
-		const awaited = context.state.analysis.awaited_declarations.get(binding.node.name);
-
-		if (awaited) {
-			node.metadata.async ??= {
-				declarations: new Set()
-			};
-
-			node.metadata.async.declarations.add(awaited);
-		}
-	}
-
-	if (node.metadata.expression.has_await) {
-		node.metadata.async ??= {
-			declarations: new Set()
-		};
-	}
-
 	context.visit(node.consequent);
 	if (node.alternate) context.visit(node.alternate);
 }
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/IfBlock.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/IfBlock.js
index f96879029c..007f63bbb2 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/IfBlock.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/IfBlock.js
@@ -25,8 +25,16 @@ export function IfBlock(node, context) {
 		statements.push(b.var(alternate_id, b.arrow([b.id('$$anchor')], alternate)));
 	}
 
+	// TODO helperise
+	const promise_index = Array.from(node.metadata.expression.dependencies).reduce(
+		(index, binding) => Math.max(index, context.state.analysis.promise_indexes.get(binding) ?? -1),
+		-1
+	);
+
+	const is_async = promise_index > -1 || node.metadata.expression.has_await;
+
 	const expression = build_expression(context, node.test, node.metadata.expression);
-	const test = node.metadata.async ? b.call('$.get', b.id('$$condition')) : expression;
+	const test = is_async ? b.call('$.get', b.id('$$condition')) : expression;
 
 	/** @type {Expression[]} */
 	const args = [
@@ -70,20 +78,14 @@ export function IfBlock(node, context) {
 
 	statements.push(add_svelte_meta(b.call('$.if', ...args), node, 'if'));
 
-	if (node.metadata.async) {
+	if (is_async) {
 		context.state.init.push(
 			b.stmt(
 				b.call(
 					'$.async',
 					context.state.node,
-					b.array([...node.metadata.async.declarations].map((d) => d.id)),
-					b.array([
-						b.arrow(
-							[...node.metadata.async.declarations].map((d) => d.pattern),
-							expression,
-							node.metadata.expression.has_await
-						)
-					]),
+					promise_index === -1 ? undefined : b.id(`$$promises[${promise_index}]`),
+					b.array([b.thunk(expression, node.metadata.expression.has_await)]),
 					b.arrow([context.state.node, b.id('$$condition')], b.block(statements))
 				)
 			)
diff --git a/packages/svelte/src/compiler/phases/3-transform/client/visitors/Program.js b/packages/svelte/src/compiler/phases/3-transform/client/visitors/Program.js
index 269b353523..6db73808e4 100644
--- a/packages/svelte/src/compiler/phases/3-transform/client/visitors/Program.js
+++ b/packages/svelte/src/compiler/phases/3-transform/client/visitors/Program.js
@@ -166,7 +166,7 @@ function transform_body(program, context) {
 	/** @type {AwaitedStatement[]} */
 	const deriveds = [];
 
-	const { awaited_statements } = context.state.analysis;
+	const { awaited_statements, promise_indexes } = context.state.analysis;
 
 	let awaited = false;
 
@@ -188,14 +188,17 @@ function transform_body(program, context) {
 			return;
 		}
 
-		if (node.type === 'VariableDeclarator') {
-			const rune = get_rune(node.init, context.state.scope);
+		// TODO put deriveds into a separate array, and group them immediately
+		// after their latest dependency. for now, to avoid having to figure
+		// out the intricacies of dependency tracking, just let 'em waterfall
+		// if (node.type === 'VariableDeclarator') {
+		// 	const rune = get_rune(node.init, context.state.scope);
 
-			if (rune === '$derived' || rune === '$derived.by') {
-				deriveds.push(statement);
-				return;
-			}
-		}
+		// 	if (rune === '$derived' || rune === '$derived.by') {
+		// 		deriveds.push(statement);
+		// 		return;
+		// 	}
+		// }
 
 		statements.push(statement);
 	};
@@ -306,5 +309,13 @@ function transform_body(program, context) {
 	// console.log('statements', statements);
 	// console.log('deriveds', deriveds);
 
+	for (let i = 0; i < statements.length; i += 1) {
+		const s = statements[i];
+
+		for (const binding of s.declarations) {
+			promise_indexes.set(binding, i);
+		}
+	}
+
 	return out;
 }
diff --git a/packages/svelte/src/compiler/phases/types.d.ts b/packages/svelte/src/compiler/phases/types.d.ts
index 8d2d526559..efec50cafb 100644
--- a/packages/svelte/src/compiler/phases/types.d.ts
+++ b/packages/svelte/src/compiler/phases/types.d.ts
@@ -129,23 +129,15 @@ export interface ComponentAnalysis extends Analysis {
 	 * Every snippet that is declared locally
 	 */
 	snippets: Set<AST.SnippetBlock>;
-	/**
-	 * A lookup of awaited declarations. If you have something this in `<script>`...
-	 *
-	 *   let a = await get_a();
-	 *   let b = get_b();
-	 *
-	 * ...it will get transformed to something like this...
-	 *
-	 *   let $$0 = $.run([], () => get_a());
-	 *   let $$1 = $.run([$$0], () => get_b());
-	 *
-	 * ...and references to `a` or `b` in the template should be mediated by `$$0` and `$$1`
-	 */
-	awaited_declarations: Map<string, AwaitedDeclaration>;
 	/**
 	 * Information about top-level instance statements that need to be transformed
 	 * so that we can run the template synchronously
 	 */
 	awaited_statements: Map<Statement | ModuleDeclaration | VariableDeclarator, AwaitedStatement>;
+	/**
+	 * A map that tells us which of the `$$promises` needs to be awaited
+	 * before a particular binding can be accessed
+	 * TODO this gets populated during transform, which feels wrong
+	 */
+	promise_indexes: Map<Binding, number>;
 }
diff --git a/packages/svelte/src/compiler/types/template.d.ts b/packages/svelte/src/compiler/types/template.d.ts
index e823b57799..cd4bbf8960 100644
--- a/packages/svelte/src/compiler/types/template.d.ts
+++ b/packages/svelte/src/compiler/types/template.d.ts
@@ -466,9 +466,6 @@ export namespace AST {
 		alternate: Fragment | null;
 		/** @internal */
 		metadata: {
-			async: {
-				declarations: Set<AwaitedDeclaration>;
-			};
 			expression: ExpressionMetadata;
 		};
 	}
diff --git a/packages/svelte/src/internal/client/dom/blocks/async.js b/packages/svelte/src/internal/client/dom/blocks/async.js
index 107649be25..1b4fe57aba 100644
--- a/packages/svelte/src/internal/client/dom/blocks/async.js
+++ b/packages/svelte/src/internal/client/dom/blocks/async.js
@@ -14,11 +14,11 @@ import { get_boundary } from './boundary.js';
 
 /**
  * @param {TemplateNode} node
- * @param {Array<Promise<any>>} dependencies
+ * @param {Promise<void> | undefined} blocker
  * @param {Array<() => Promise<any>>} expressions
  * @param {(anchor: TemplateNode, ...deriveds: Value[]) => void} fn
  */
-export function async(node, dependencies, expressions, fn) {
+export function async(node, blocker, expressions, fn) {
 	var boundary = get_boundary();
 	var batch = /** @type {Batch} */ (current_batch);
 	var blocking = !boundary.is_pending();
@@ -36,7 +36,7 @@ export function async(node, dependencies, expressions, fn) {
 		set_hydrate_node(end);
 	}
 
-	flatten(dependencies, [], expressions, (values) => {
+	flatten(blocker, [], expressions, (values) => {
 		if (was_hydrating) {
 			set_hydrating(true);
 			set_hydrate_node(previous_hydrate_node);
diff --git a/packages/svelte/src/internal/client/reactivity/async.js b/packages/svelte/src/internal/client/reactivity/async.js
index 9540ed7993..c62c0f65fe 100644
--- a/packages/svelte/src/internal/client/reactivity/async.js
+++ b/packages/svelte/src/internal/client/reactivity/async.js
@@ -34,14 +34,15 @@ import {
 	skip_nodes
 } from '../dom/hydration.js';
 import { create_text } from '../dom/operations.js';
+import { noop } from '../../shared/utils.js';
 
 /**
- * @param {Array<Promise<any>>} dependencies
+ * @param {Promise<void> | undefined} blocker
  * @param {Array<() => any>} sync
  * @param {Array<() => Promise<any>>} async
  * @param {(values: Value[]) => any} fn
  */
-export function flatten(dependencies, sync, async, fn) {
+export function flatten(blocker, sync, async, fn) {
 	const d = is_runes() ? derived : derived_safe_equal;
 
 	if (async.length === 0) {
@@ -56,12 +57,10 @@ export function flatten(dependencies, sync, async, fn) {
 
 	var was_hydrating = hydrating;
 
-	Promise.all(dependencies).then((values) => {
+	Promise.resolve(blocker).then((values) => {
 		restore();
 
-		const result = Promise.all(
-			async.map((expression) => async_derived(() => expression(...values)))
-		)
+		const result = Promise.all(async.map((expression) => async_derived(expression)))
 			.then((result) => {
 				restore();
 
@@ -271,10 +270,25 @@ export async function async_body(anchor, fn) {
 }
 
 /**
- * @param {Array<Promise<any>>} deps
- * @param {(...deps: any) => any} fn
+ * @param {Array<() => void | Promise<void>>} thunks
  */
-export function run(deps, fn) {
-	// TODO save/restore context
-	return Promise.all(deps).then(fn);
+export function run(thunks) {
+	const restore = capture();
+
+	let promise = Promise.resolve();
+
+	return thunks.map((fn) => {
+		promise = promise
+			.then(() => {
+				try {
+					restore();
+					return fn();
+				} finally {
+					unset_context();
+				}
+			})
+			.then(noop);
+
+		return promise;
+	});
 }
diff --git a/packages/svelte/src/internal/client/reactivity/effects.js b/packages/svelte/src/internal/client/reactivity/effects.js
index 4235e9cb24..b338b34421 100644
--- a/packages/svelte/src/internal/client/reactivity/effects.js
+++ b/packages/svelte/src/internal/client/reactivity/effects.js
@@ -365,9 +365,10 @@ export function render_effect(fn, flags = 0) {
  * @param {(...expressions: any) => void | (() => void)} fn
  * @param {Array<() => any>} sync
  * @param {Array<() => Promise<any>>} async
+ * @param {Promise<void>} [blocker]
  */
-export function template_effect(fn, sync = [], async = []) {
-	flatten(sync, async, (values) => {
+export function template_effect(fn, sync = [], async = [], blocker) {
+	flatten(blocker, sync, async, (values) => {
 		create_effect(RENDER_EFFECT, () => fn(...values.map(get)), true);
 	});
 }