only dewaterfall when necessary

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -131,9 +131,6 @@ const visitors = {
 		ignore_map.set(node, structuredClone(ignore_stack));
 
 		const scope = state.scopes.get(node);
-		if (node.type === 'Fragment') {
-			node.metadata.hoisted_promises.name = state.scope.generate('promises');
-		}
 		next(scope !== undefined && scope !== state.scope ? { ...state, scope } : state);
 
 		if (ignores.length > 0) {

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

@@ -7,4 +7,18 @@
  */
 export function Fragment(node, context) {
 	context.next({ ...context.state, fragment: node });
+
+	// TODO this indicates whether the fragment contains an `await` expression (not inside
+	// a child fragment), which is necessary for ensuring that a `SnippetBlock` creates an
+	// async function in SSR. It feels like this is probably duplicative, but it's late
+	// and it works, so for now I'm doing it like this
+	node.metadata.is_async = node.metadata.hoisted_promises.promises.length > 0;
+
+	if (node.metadata.hoisted_promises.promises.length > 1) {
+		node.metadata.hoisted_promises.name = context.state.scope.generate('promises');
+	} else {
+		// if there's only one promise in this fragment, we don't need to de-waterfall it
+		context.state.analysis.hoisted_promises.delete(node.metadata.hoisted_promises.promises[0]);
+		node.metadata.hoisted_promises.promises.length = 0;
+	}
 }

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

@@ -12,7 +12,8 @@ export function SnippetBlock(node, context) {
 	let fn = b.function_declaration(
 		node.expression,
 		[b.id('$$payload'), ...node.parameters],
-		/** @type {BlockStatement} */ (context.visit(node.body))
+		/** @type {BlockStatement} */ (context.visit(node.body)),
+		node.body.metadata.is_async
 	);
 
 	// @ts-expect-error - TODO remove this hack once $$render_inner for legacy bindings is gone

packages/svelte/tests/snapshot/samples/async-each-fallback-hoisting/_expected/server/index.svelte.js

@@ -2,32 +2,21 @@ import * as $ from 'svelte/internal/server';
 
 export default function Async_each_fallback_hoisting($$payload) {
 	$$payload.child(async ($$payload) => {
-		const promises = [Promise.resolve([])];
+		const each_array = $.ensure_array_like(await Promise.resolve([]));
-		const each_array = $.ensure_array_like(await promises[0]);
 
-		$$payload.child(async ($$payload) => {
 		if (each_array.length !== 0) {
 			$$payload.push('<!--[-->');
 
 			for (let $$index = 0, $$length = each_array.length; $$index < $$length; $$index++) {
 				let item = each_array[$$index];
-					const promises_1 = [Promise.reject('This should never be reached')];
 
-					$$payload.child(async ($$payload) => {
+				$$payload.push(`<!---->${$.escape(await Promise.reject('This should never be reached'))}`);
-						$$payload.push(`<!---->${$.escape(await promises_1[0])}`);
-					});
 			}
 		} else {
 			$$payload.push('<!--[!-->');
-
+			$$payload.push(`<!---->${$.escape(await Promise.resolve(4))}`);
-				const promises_2 = [Promise.resolve(4)];
-
-				$$payload.child(async ($$payload) => {
-					$$payload.push(`<!---->${$.escape(await promises_2[0])}`);
-				});
 		}
 
 		$$payload.push(`<!--]-->`);
 	});
-	});
 }

packages/svelte/tests/snapshot/samples/async-each-hoisting/_expected/server/index.svelte.js

@@ -5,22 +5,16 @@ export default function Async_each_hoisting($$payload) {
 		const first = Promise.resolve(1);
 		const second = Promise.resolve(2);
 		const third = Promise.resolve(3);
-		const promises = [Promise.resolve([first, second, third])];
+		const each_array = $.ensure_array_like(await Promise.resolve([first, second, third]));
-		const each_array = $.ensure_array_like(await promises[0]);
 
-		$$payload.child(async ($$payload) => {
 		$$payload.push(`<!--[-->`);
 
 		for (let $$index = 0, $$length = each_array.length; $$index < $$length; $$index++) {
 			let item = each_array[$$index];
-				const promises_1 = [item];
 
-				$$payload.child(async ($$payload) => {
+			$$payload.push(`<!---->${$.escape(await item)}`);
-					$$payload.push(`<!---->${$.escape(await promises_1[0])}`);
-				});
 		}
 
 		$$payload.push(`<!--]-->`);
 	});
-	});
 }

packages/svelte/tests/snapshot/samples/async-if-alternate-hoisting/_expected/server/index.svelte.js

@@ -2,28 +2,14 @@ import * as $ from 'svelte/internal/server';
 
 export default function Async_if_alternate_hoisting($$payload) {
 	$$payload.child(async ($$payload) => {
-		const promises = [Promise.resolve(false)];
+		if (await Promise.resolve(false)) {
-
-		$$payload.child(async ($$payload) => {
-			if (await promises[0]) {
 			$$payload.push('<!--[-->');
-
+			$$payload.push(`${$.escape(await Promise.reject('no no no'))}`);
-				const promises_1 = [Promise.reject('no no no')];
-
-				$$payload.child(async ($$payload) => {
-					$$payload.push(`${$.escape(await promises_1[0])}`);
-				});
 		} else {
 			$$payload.push('<!--[!-->');
-
+			$$payload.push(`${$.escape(await Promise.resolve('yes yes yes'))}`);
-				const promises_2 = [Promise.resolve('yes yes yes')];
-
-				$$payload.child(async ($$payload) => {
-					$$payload.push(`${$.escape(await promises_2[0])}`);
-				});
 		}
 
 		$$payload.push(`<!--]-->`);
 	});
-	});
 }

packages/svelte/tests/snapshot/samples/async-if-hoisting/_expected/server/index.svelte.js

@@ -2,28 +2,14 @@ import * as $ from 'svelte/internal/server';
 
 export default function Async_if_hoisting($$payload) {
 	$$payload.child(async ($$payload) => {
-		const promises = [Promise.resolve(true)];
+		if (await Promise.resolve(true)) {
-
-		$$payload.child(async ($$payload) => {
-			if (await promises[0]) {
 			$$payload.push('<!--[-->');
-
+			$$payload.push(`${$.escape(await Promise.resolve('yes yes yes'))}`);
-				const promises_1 = [Promise.resolve('yes yes yes')];
-
-				$$payload.child(async ($$payload) => {
-					$$payload.push(`${$.escape(await promises_1[0])}`);
-				});
 		} else {
 			$$payload.push('<!--[!-->');
-
+			$$payload.push(`${$.escape(await Promise.reject('no no no'))}`);
-				const promises_2 = [Promise.reject('no no no')];
-
-				$$payload.child(async ($$payload) => {
-					$$payload.push(`${$.escape(await promises_2[0])}`);
-				});
 		}
 
 		$$payload.push(`<!--]-->`);
 	});
-	});
 }