fix: reset context after waiting on blockers of `@const` expressions (#18100)

Regression from #18039 - we need to have each await expression (and
waiting on blockers is one) in its own entry of `(renderer.)run`. Else
context is not restored correctly and if the synchronous expression
afterwards requires it stuff breaks.

Fixes #18098

.changeset/soft-moons-wear.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: reset context after waiting on blockers of `@const` expressions

packages/svelte/src/compiler/phases/2-analyze/types.d.ts

@@ -38,7 +38,7 @@ export interface AnalysisState {
 	/** Collected info about async `{@const }` declarations */
 	async_consts?: {
 		id: Identifier;
-		/** How many `@const` declarations there are (already) in this scope */
+		/** How many `$.run(...)` entries are already allocated in this scope */
 		declaration_count: number;
 	};
 }

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

@@ -61,7 +61,8 @@ export function ConstTag(node, context) {
 		// keep the counter in sync with the number of thunks pushed in ConstTag in transform
 		// TODO 6.0 once non-async and non-runes mode is gone investigate making this more robust
 		// via something like the approach in https://github.com/sveltejs/svelte/pull/18032
-		const length = run.declaration_count++;
+		const length = run.declaration_count + (blockers.length > 0 ? 1 : 0);
+		run.declaration_count += blockers.length > 0 ? 2 : 1;
 		const blocker = b.member(run.id, b.literal(length), true);
 		for (const binding of bindings) {
 			binding.blocker = blocker;

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

@@ -96,22 +96,15 @@ function add_const_declaration(state, id, expression, metadata) {
 
 		state.consts.push(b.let(id));
 
-		/** @type {Statement | undefined} */
-		let promise_stmt;
-
 		if (blockers.length === 1) {
-			promise_stmt = b.stmt(b.await(b.member(/** @type {Expression} */ (blockers[0]), 'promise')));
+			run.thunks.push(b.thunk(b.member(/** @type {Expression} */ (blockers[0]), 'promise')));
 		} else if (blockers.length > 0) {
-			promise_stmt = b.stmt(b.await(b.call('$.wait', b.array(blockers))));
+			run.thunks.push(b.thunk(b.call('$.wait', b.array(blockers))));
 		}
 
 		// keep the number of thunks pushed in sync with ConstTag in analysis phase
 		const assignment = b.assignment('=', id, expression);
-		if (promise_stmt) {
-			run.thunks.push(b.thunk(b.block([promise_stmt, b.stmt(assignment)]), true));
-		} else {
-			run.thunks.push(b.thunk(assignment, metadata.expression.has_await));
-		}
+		run.thunks.push(b.thunk(assignment, metadata.expression.has_await));
 	} else {
 		state.consts.push(b.const(id, expression));
 		state.consts.push(...after);

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

@@ -28,22 +28,15 @@ export function ConstTag(node, context) {
 			context.state.init.push(b.let(identifier.name));
 		}
 
-		/** @type {Statement | undefined} */
-		let promise_stmt;
-
 		if (blockers.length === 1) {
-			promise_stmt = b.stmt(b.await(/** @type {Expression} */ (blockers[0])));
+			run.thunks.push(b.thunk(/** @type {Expression} */ (blockers[0])));
 		} else if (blockers.length > 0) {
-			promise_stmt = b.stmt(b.await(b.call('Promise.all', b.array(blockers))));
+			run.thunks.push(b.thunk(b.call('Promise.all', b.array(blockers))));
 		}
 
 		// keep the number of thunks pushed in sync with ConstTag in analysis phase
 		const assignment = b.assignment('=', id, init);
-		if (promise_stmt) {
-			run.thunks.push(b.thunk(b.block([promise_stmt, b.stmt(assignment)]), true));
-		} else {
-			run.thunks.push(b.thunk(assignment, node.metadata.expression.has_await));
-		}
+		run.thunks.push(b.thunk(assignment, node.metadata.expression.has_await));
 	} else {
 		context.state.init.push(b.const(id, init));
 	}

packages/svelte/tests/runtime-runes/samples/async-context-after-await-const/_config.js

@@ -0,0 +1,11 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+// Tests that context is restored after await (const has to wait on a blocker), so that getContext etc work correctly
+export default test({
+	mode: ['hydrate', 'async-server', 'client'],
+	async test({ assert, target }) {
+		await tick();
+		assert.htmlEqual(target.innerHTML, 'hi');
+	}
+});

packages/svelte/tests/runtime-runes/samples/async-context-after-await-const/main.svelte

@@ -0,0 +1,11 @@
+<script>
+	import { getContext } from 'svelte';
+	const _unused = $derived(await 1);
+	//This must be created after any $derived(await...)
+	const bar = $derived(getContext('') ?? 'hi');
+</script>
+
+{#if true}
+	{@const foo = bar}
+    {foo}
+{/if}

packages/svelte/tests/snapshot/samples/async-in-derived/_expected/client/index.svelte.js

@@ -2,6 +2,8 @@ import 'svelte/internal/disclose-version';
 import 'svelte/internal/flags/async';
 import * as $ from 'svelte/internal/client';
 
+var root = $.from_html(`<!> <!>`, 1);
+
 export default function Async_in_derived($$anchor, $$props) {
 	$.push($$props, true);
 
@@ -21,7 +23,7 @@ export default function Async_in_derived($$anchor, $$props) {
 		}
 	]);
 
-	var fragment = $.comment();
+	var fragment = root();
 	var node = $.first_child(fragment);
 
 	{
@@ -49,6 +51,23 @@ export default function Async_in_derived($$anchor, $$props) {
 		});
 	}
 
+	var node_1 = $.sibling(node, 2);
+
+	{
+		var consequent_1 = ($$anchor) => {
+			let x;
+
+			var promises_1 = $.run([
+				() => $$promises[2].promise,
+				() => x = $.derived(() => $.get(no2))
+			]);
+		};
+
+		$.if(node_1, ($$render) => {
+			if (true) $$render(consequent_1);
+		});
+	}
+
 	$.append($$anchor, fragment);
 	$.pop();
 }

packages/svelte/tests/snapshot/samples/async-in-derived/_expected/server/index.svelte.js

@@ -42,6 +42,17 @@ export default function Async_in_derived($$renderer, $$props) {
 			$$renderer.push('<!--[-1-->');
 		}
 
+		$$renderer.push(`<!--]--> `);
+
+		if (true) {
+			$$renderer.push('<!--[0-->');
+
+			let x;
+			var promises_1 = $$renderer.run([() => $$promises[2], () => x = no2()]);
+		} else {
+			$$renderer.push('<!--[-1-->');
+		}
+
 		$$renderer.push(`<!--]-->`);
 	});
 }

packages/svelte/tests/snapshot/samples/async-in-derived/index.svelte

@@ -19,3 +19,7 @@
 		return await 1;
 	})()}
 {/if}
+
+{#if true}
+	{@const x = no2}
+{/if}