fix: parallelize async @consts in the template

.changeset/social-taxis-tell.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: ensure async `@const` in boundary hydrates correctly

.changeset/stale-items-know.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: parallelize async `@const`s in the template

packages/svelte/src/compiler/phases/1-parse/utils/create.js

@@ -10,8 +10,7 @@ export function create_fragment(transparent = false) {
 		nodes: [],
 		metadata: {
 			transparent,
-			dynamic: false,
-			has_await: false
+			dynamic: false
 		}
 	};
 }

packages/svelte/src/compiler/phases/2-analyze/visitors/AwaitExpression.js

@@ -26,10 +26,6 @@ export function AwaitExpression(node, context) {
 	if (context.state.expression) {
 		context.state.expression.has_await = true;
 
-		if (context.state.fragment && context.path.some((node) => node.type === 'ConstTag')) {
-			context.state.fragment.metadata.has_await = true;
-		}
-
 		suspend = true;
 	}
 

packages/svelte/src/compiler/phases/3-transform/client/types.d.ts

@@ -51,6 +51,11 @@ export interface ComponentClientTransformState extends ClientTransformState {
 	readonly after_update: Statement[];
 	/** Transformed `{@const }` declarations */
 	readonly consts: Statement[];
+	/** Transformed async `{@const }` declarations (if any) and those coming after them */
+	async_consts?: {
+		id: Identifier;
+		thunks: Expression[];
+	};
 	/** Transformed `let:` directives */
 	readonly let_directives: Statement[];
 	/** Memoized expressions */

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

@@ -24,15 +24,15 @@ export function ConstTag(node, context) {
 			expression = b.call('$.tag', expression, b.literal(declaration.id.name));
 		}
 
-		context.state.consts.push(b.const(declaration.id, expression));
-
 		context.state.transform[declaration.id.name] = { read: get_value };
 
-		// we need to eagerly evaluate the expression in order to hit any
-		// 'Cannot access x before initialization' errors
-		if (dev) {
-			context.state.consts.push(b.stmt(b.call('$.get', declaration.id)));
-		}
+		add_const_declaration(
+			context.state,
+			declaration.id,
+			expression,
+			node.metadata.expression.has_await,
+			context.state.scope.get_bindings(declaration)
+		);
 	} else {
 		const identifiers = extract_identifiers(declaration.id);
 		const tmp = b.id(context.state.scope.generate('computed_const'));
@@ -69,13 +69,13 @@ export function ConstTag(node, context) {
 			expression = b.call('$.tag', expression, b.literal('[@const]'));
 		}
 
-		context.state.consts.push(b.const(tmp, expression));
-
-		// we need to eagerly evaluate the expression in order to hit any
-		// 'Cannot access x before initialization' errors
-		if (dev) {
-			context.state.consts.push(b.stmt(b.call('$.get', tmp)));
-		}
+		add_const_declaration(
+			context.state,
+			tmp,
+			expression,
+			node.metadata.expression.has_await,
+			context.state.scope.get_bindings(declaration)
+		);
 
 		for (const node of identifiers) {
 			context.state.transform[node.name] = {
@@ -84,3 +84,39 @@ export function ConstTag(node, context) {
 		}
 	}
 }
+
+/**
+ * @param {ComponentContext['state']} state
+ * @param {import('estree').Identifier} id
+ * @param {import('estree').Expression} expression
+ * @param {boolean} has_await
+ * @param {import('#compiler').Binding[]} bindings
+ */
+function add_const_declaration(state, id, expression, has_await, bindings) {
+	// we need to eagerly evaluate the expression in order to hit any
+	// 'Cannot access x before initialization' errors
+	const after = dev ? [b.stmt(b.call('$.get', id))] : [];
+
+	if (has_await || state.async_consts) {
+		const run = (state.async_consts ??= {
+			id: b.id(state.scope.generate('promises')),
+			thunks: []
+		});
+
+		state.consts.push(b.let(id));
+
+		const assignment = b.assignment('=', id, expression);
+		const body = after.length === 0 ? assignment : b.block([b.stmt(assignment), ...after]);
+
+		run.thunks.push(b.thunk(body, has_await));
+
+		const blocker = b.member(run.id, b.literal(run.thunks.length - 1), true);
+
+		for (const binding of bindings) {
+			binding.blocker = blocker;
+		}
+	} else {
+		state.consts.push(b.const(id, expression));
+		state.consts.push(...after);
+	}
+}

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

@@ -48,8 +48,6 @@ export function Fragment(node, context) {
 	const is_single_child_not_needing_template =
 		trimmed.length === 1 &&
 		(trimmed[0].type === 'SvelteFragment' || trimmed[0].type === 'TitleElement');
-	const has_await = context.state.init !== null && (node.metadata.has_await || false);
-
 	const template_name = context.state.scope.root.unique('root'); // TODO infer name from parent
 
 	/** @type {Statement[]} */
@@ -72,7 +70,8 @@ export function Fragment(node, context) {
 		metadata: {
 			namespace,
 			bound_contenteditable: context.state.metadata.bound_contenteditable
-		}
+		},
+		async_consts: undefined
 	};
 
 	for (const node of hoisted) {
@@ -153,8 +152,8 @@ export function Fragment(node, context) {
 
 	body.push(...state.let_directives, ...state.consts);
 
-	if (has_await) {
-		body.push(b.if(b.call('$.aborted'), b.return()));
+	if (state.async_consts && state.async_consts.thunks.length > 0) {
+		body.push(b.var(state.async_consts.id, b.call('$.run', b.array(state.async_consts.thunks))));
 	}
 
 	if (is_text_first) {
@@ -177,13 +176,5 @@ export function Fragment(node, context) {
 		body.push(close);
 	}
 
-	if (has_await) {
-		return b.block([
-			b.stmt(
-				b.call('$.async_body', b.id('$$anchor'), b.arrow([b.id('$$anchor')], b.block(body), true))
-			)
-		]);
-	} else {
-		return b.block(body);
-	}
+	return b.block(body);
 }

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

@@ -14,8 +14,6 @@ export function SnippetBlock(node, context) {
 	// TODO hoist where possible
 	/** @type {(Identifier | AssignmentPattern)[]} */
 	const args = [b.id('$$anchor')];
-	const has_await = node.body.metadata.has_await || false;
-
 	/** @type {BlockStatement} */
 	let body;
 
@@ -78,12 +76,8 @@ export function SnippetBlock(node, context) {
 
 	// in dev we use a FunctionExpression (not arrow function) so we can use `arguments`
 	let snippet = dev
-		? b.call(
-				'$.wrap_snippet',
-				b.id(context.state.analysis.name),
-				b.function(null, args, body, has_await)
-			)
-		: b.arrow(args, body, has_await);
+		? b.call('$.wrap_snippet', b.id(context.state.analysis.name), b.function(null, args, body))
+		: b.arrow(args, body);
 
 	const declaration = b.const(node.expression, snippet);
 

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

@@ -48,7 +48,11 @@ export function SvelteBoundary(node, context) {
 		if (child.type === 'ConstTag') {
 			has_const = true;
 			if (!context.state.options.experimental.async) {
-				context.visit(child, { ...context.state, consts: const_tags });
+				context.visit(child, {
+					...context.state,
+					consts: const_tags,
+					scope: context.state.scopes.get(node.fragment) ?? context.state.scope
+				});
 			}
 		}
 	}
@@ -101,7 +105,13 @@ export function SvelteBoundary(node, context) {
 		nodes.push(child);
 	}
 
-	const block = /** @type {BlockStatement} */ (context.visit({ ...node.fragment, nodes }));
+	const block = /** @type {BlockStatement} */ (
+		context.visit(
+			{ ...node.fragment, nodes },
+			// Since we're creating a new fragment the reference in scopes can't match, so we gotta attach the right scope manually
+			{ ...context.state, scope: context.state.scopes.get(node.fragment) ?? context.state.scope }
+		)
+	);
 
 	if (!context.state.options.experimental.async) {
 		block.body.unshift(...const_tags);

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

@@ -105,7 +105,7 @@ export function process_children(nodes, initial, is_element, context) {
 				is_element &&
 				// In case it's wrapped in async the async logic will want to skip sibling nodes up until the end, hence we cannot make this controlled
 				// TODO switch this around and instead optimize for elements with a single block child and not require extra comments (neither for async nor normally)
-				!(node.body.metadata.has_await || node.metadata.expression.is_async())
+				!node.metadata.expression.is_async()
 			) {
 				node.metadata.is_controlled = true;
 			} else {

packages/svelte/src/compiler/phases/3-transform/server/types.d.ts

@@ -1,4 +1,10 @@
-import type { Expression, Statement, ModuleDeclaration, LabeledStatement } from 'estree';
+import type {
+	Expression,
+	Statement,
+	ModuleDeclaration,
+	LabeledStatement,
+	Identifier
+} from 'estree';
 import type { AST, Namespace, ValidatedCompileOptions } from '#compiler';
 import type { TransformState } from '../types.js';
 import type { ComponentAnalysis } from '../../types.js';
@@ -21,6 +27,11 @@ export interface ComponentServerTransformState extends ServerTransformState {
 	readonly namespace: Namespace;
 	readonly preserve_whitespace: boolean;
 	readonly skip_hydration_boundaries: boolean;
+	/** Transformed async `{@const }` declarations (if any) and those coming after them */
+	async_consts?: {
+		id: Identifier;
+		thunks: Expression[];
+	};
 }
 
 export type Context = import('zimmerframe').Context<AST.SvelteNode, ServerTransformState>;

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

@@ -2,6 +2,7 @@
 /** @import { AST } from '#compiler' */
 /** @import { ComponentContext } from '../types.js' */
 import * as b from '#compiler/builders';
+import { extract_identifiers } from '../../../../utils/ast.js';
 
 /**
  * @param {AST.ConstTag} node
@@ -11,6 +12,29 @@ export function ConstTag(node, context) {
 	const declaration = node.declaration.declarations[0];
 	const id = /** @type {Pattern} */ (context.visit(declaration.id));
 	const init = /** @type {Expression} */ (context.visit(declaration.init));
+	const has_await = node.metadata.expression.has_await;
 
-	context.state.init.push(b.const(id, init));
+	if (has_await || context.state.async_consts) {
+		const run = (context.state.async_consts ??= {
+			id: b.id(context.state.scope.generate('promises')),
+			thunks: []
+		});
+
+		const identifiers = extract_identifiers(declaration.id);
+		const bindings = context.state.scope.get_bindings(declaration);
+
+		for (const identifier of identifiers) {
+			context.state.init.push(b.let(identifier.name));
+		}
+
+		const assignment = b.assignment('=', id, init);
+		run.thunks.push(b.thunk(b.block([b.stmt(assignment)]), has_await));
+
+		const blocker = b.member(run.id, b.literal(run.thunks.length - 1), true);
+		for (const binding of bindings) {
+			binding.blocker = blocker;
+		}
+	} else {
+		context.state.init.push(b.const(id, init));
+	}
 }

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

@@ -34,11 +34,7 @@ export function EachBlock(node, context) {
 
 	const new_body = /** @type {BlockStatement} */ (context.visit(node.body)).body;
 
-	if (node.body)
-		each.push(
-			// TODO get rid of fragment.has_await
-			...(node.body.metadata.has_await ? [create_async_block(b.block(new_body))] : new_body)
-		);
+	if (node.body) each.push(...new_body);
 
 	const for_loop = b.for(
 		b.declaration('let', [
@@ -61,7 +57,7 @@ export function EachBlock(node, context) {
 			b.if(
 				b.binary('!==', b.member(array_id, 'length'), b.literal(0)),
 				b.block([open, for_loop]),
-				node.fallback.metadata.has_await ? create_async_block(fallback) : fallback
+				fallback
 			)
 		);
 	} else {

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

@@ -28,7 +28,8 @@ export function Fragment(node, context) {
 		init: [],
 		template: [],
 		namespace,
-		skip_hydration_boundaries: is_standalone
+		skip_hydration_boundaries: is_standalone,
+		async_consts: undefined
 	};
 
 	for (const node of hoisted) {
@@ -42,5 +43,11 @@ export function Fragment(node, context) {
 
 	process_children(trimmed, { ...context, state });
 
+	if (state.async_consts && state.async_consts.thunks.length > 0) {
+		state.init.push(
+			b.var(state.async_consts.id, b.call('$$renderer.run', b.array(state.async_consts.thunks)))
+		);
+	}
+
 	return b.block([...state.init, ...build_template(state.template)]);
 }

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

@@ -25,11 +25,7 @@ export function IfBlock(node, context) {
 
 	const is_async = node.metadata.expression.is_async();
 
-	const has_await =
-		node.metadata.expression.has_await ||
-		// TODO get rid of this stuff
-		node.consequent.metadata.has_await ||
-		node.alternate?.metadata.has_await;
+	const has_await = node.metadata.expression.has_await;
 
 	if (is_async || has_await) {
 		statement = create_async_block(

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

@@ -3,7 +3,6 @@
 /** @import { ComponentContext } from '../types.js' */
 import { dev } from '../../../../state.js';
 import * as b from '#compiler/builders';
-import { create_async_block } from './shared/utils.js';
 
 /**
  * @param {AST.SnippetBlock} node
@@ -16,10 +15,6 @@ export function SnippetBlock(node, context) {
 		/** @type {BlockStatement} */ (context.visit(node.body))
 	);
 
-	if (node.body.metadata.has_await) {
-		fn.body = b.block([create_async_block(fn.body)]);
-	}
-
 	// @ts-expect-error - TODO remove this hack once $$render_inner for legacy bindings is gone
 	fn.___snippet = true;