feat: better code generation for `let:` directives in SSR mode (#12611)

* better code generation for slot props in SSR

* simplify

* remove getters mechanism from server compiler

* changeset

* no need to use getters in SSR mode

* fix comment

.changeset/stupid-bottles-lay.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: better code generation for `let:` directives in SSR mode

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

@@ -3,7 +3,8 @@ import type {
 	Statement,
 	LabeledStatement,
 	Identifier,
-	PrivateIdentifier
+	PrivateIdentifier,
+	Expression
 } from 'estree';
 import type { Namespace, SvelteNode, ValidatedCompileOptions } from '#compiler';
 import type { TransformState } from '../types.js';
@@ -22,6 +23,11 @@ export interface ClientTransformState extends TransformState {
 
 	/** The $: calls, which will be ordered in the end */
 	readonly legacy_reactive_statements: Map<LabeledStatement, Statement>;
+	/**
+	 * A map of `[name, node]` pairs, where `Identifier` nodes matching `name`
+	 * will be replaced with `node` (e.g. `x` -> `$.get(x)`)
+	 */
+	readonly getters: Record<string, Expression | ((id: Identifier) => Expression)>;
 }
 
 export interface ComponentClientTransformState extends ClientTransformState {

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

@@ -23,7 +23,6 @@ import { Identifier } from './visitors/Identifier.js';
 import { IfBlock } from './visitors/IfBlock.js';
 import { KeyBlock } from './visitors/KeyBlock.js';
 import { LabeledStatementLegacy } from './visitors/LabeledStatement.js';
-import { LetDirective } from './visitors/LetDirective.js';
 import { MemberExpressionRunes } from './visitors/MemberExpression.js';
 import { PropertyDefinitionRunes } from './visitors/PropertyDefinition.js';
 import { RegularElement } from './visitors/RegularElement.js';
@@ -79,7 +78,6 @@ const template_visitors = {
 	HtmlTag,
 	IfBlock,
 	KeyBlock,
-	LetDirective,
 	RegularElement,
 	RenderTag,
 	SlotElement,
@@ -113,7 +111,6 @@ export function server_component(analysis, options) {
 		namespace: options.namespace,
 		preserve_whitespace: options.preserveWhitespace,
 		private_derived: new Map(),
-		getters: {},
 		skip_hydration_boundaries: false
 	};
 
@@ -422,8 +419,7 @@ export function server_module(analysis, options) {
 		// to be present for `javascript_visitors_legacy` and so is included in module
 		// transform state as well as component transform state
 		legacy_reactive_statements: new Map(),
-		private_derived: new Map(),
+		private_derived: new Map()
-		getters: {}
 	};
 
 	const module = /** @type {Program} */ (

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

@@ -1,40 +0,0 @@
-/** @import { LetDirective } from '#compiler' */
-/** @import { ComponentContext } from '../types.js' */
-import * as b from '../../../../utils/builders.js';
-
-/**
- * @param {LetDirective} node
- * @param {ComponentContext} context
- */
-export function LetDirective(node, context) {
-	if (node.expression === null || node.expression.type === 'Identifier') {
-		const name = node.expression === null ? node.name : node.expression.name;
-		return b.const(name, b.member(b.id('$$slotProps'), b.id(node.name)));
-	}
-
-	const name = context.state.scope.generate(node.name);
-	const bindings = context.state.scope.get_bindings(node);
-
-	for (const binding of bindings) {
-		context.state.getters[binding.node.name] = b.member(b.id(name), b.id(binding.node.name));
-	}
-
-	return b.const(
-		name,
-		b.call(
-			b.thunk(
-				b.block([
-					b.let(
-						node.expression.type === 'ObjectExpression'
-							? // @ts-expect-error types don't match, but it can't contain spread elements and the structure is otherwise fine
-								b.object_pattern(node.expression.properties)
-							: // @ts-expect-error types don't match, but it can't contain spread elements and the structure is otherwise fine
-								b.array_pattern(node.expression.elements),
-						b.member(b.id('$$slotProps'), b.id(node.name))
-					),
-					b.return(b.object(bindings.map((binding) => b.init(binding.node.name, binding.node))))
-				])
-			)
-		)
-	);
-}

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

@@ -19,7 +19,6 @@ export function RegularElement(node, context) {
 	/** @type {ComponentServerTransformState} */
 	const state = {
 		...context.state,
-		getters: { ...context.state.getters },
 		namespace,
 		preserve_whitespace:
 			context.state.preserve_whitespace ||

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

@@ -15,9 +15,6 @@ export function SlotElement(node, context) {
 	/** @type {Expression[]} */
 	const spreads = [];
 
-	/** @type {ExpressionStatement[]} */
-	const lets = [];
-
 	/** @type {Expression} */
 	let expression = b.call('$.default_slot', b.id('$$props'));
 
@@ -30,20 +27,11 @@ export function SlotElement(node, context) {
 			if (attribute.name === 'name') {
 				expression = b.member(b.member_id('$$props.$$slots'), value, true, true);
 			} else if (attribute.name !== 'slot') {
-				if (attribute.metadata.dynamic) {
-					props.push(b.get(attribute.name, [b.return(value)]));
-				} else {
 				props.push(b.init(attribute.name, value));
 			}
 		}
-		} else if (attribute.type === 'LetDirective') {
-			lets.push(/** @type {ExpressionStatement} */ (context.visit(attribute)));
-		}
 	}
 
-	// Let bindings first, they can be used on attributes
-	context.state.init.push(...lets);
-
 	const props_expression =
 		spreads.length === 0
 			? b.object(props)

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

@@ -27,7 +27,6 @@ export function SvelteElement(node, context) {
 
 	const state = {
 		...context.state,
-		getteres: { ...context.state.getters },
 		namespace: determine_namespace_for_children(node, context.state.namespace),
 		template: [],
 		init: []

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

@@ -1,4 +1,4 @@
-/** @import { BlockStatement, ExpressionStatement } from 'estree' */
+/** @import { BlockStatement } from 'estree' */
 /** @import { SvelteFragment } from '#compiler' */
 /** @import { ComponentContext } from '../types' */
 
@@ -7,20 +7,5 @@
  * @param {ComponentContext} context
  */
 export function SvelteFragment(node, context) {
-	const child_state = {
+	context.state.template.push(/** @type {BlockStatement} */ (context.visit(node.fragment)));
-		...context.state,
-		getters: { ...context.state.getters }
-	};
-
-	for (const attribute of node.attributes) {
-		if (attribute.type === 'LetDirective') {
-			context.state.template.push(
-				/** @type {ExpressionStatement} */ (context.visit(attribute, child_state))
-			);
-		}
-	}
-
-	const block = /** @type {BlockStatement} */ (context.visit(node.fragment, child_state));
-
-	context.state.template.push(block);
 }

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

@@ -1,5 +1,5 @@
-/** @import { BlockStatement, Expression, ExpressionStatement, Property, Statement } from 'estree' */
+/** @import { BlockStatement, Expression, Pattern, Property, Statement } from 'estree' */
-/** @import { Attribute, Component, SvelteComponent, SvelteSelf, TemplateNode, Text } from '#compiler' */
+/** @import { Attribute, Component, LetDirective, SvelteComponent, SvelteSelf, TemplateNode, Text } from '#compiler' */
 /** @import { ComponentContext } from '../../types.js' */
 import { empty_comment, serialize_attribute_value } from './utils.js';
 import * as b from '../../../../../utils/builders.js';
@@ -17,8 +17,8 @@ export function serialize_inline_component(node, expression, context) {
 	/** @type {Property[]} */
 	const custom_css_props = [];
 
-	/** @type {ExpressionStatement[]} */
+	/** @type {Record<string, LetDirective[]>} */
-	const lets = [];
+	const lets = { default: [] };
 
 	/** @type {Record<string, TemplateNode[]>} */
 	const children = {};
@@ -27,7 +27,9 @@ export function serialize_inline_component(node, expression, context) {
 	 * If this component has a slot property, it is a named slot within another component. In this case
 	 * the slot scope applies to the component itself, too, and not just its children.
 	 */
-	let slot_scope_applies_to_itself = false;
+	const slot_scope_applies_to_itself = node.attributes.some(
+		(node) => node.type === 'Attribute' && node.name === 'slot'
+	);
 
 	/**
 	 * Components may have a children prop and also have child nodes. In this case, we assume
@@ -50,7 +52,9 @@ export function serialize_inline_component(node, expression, context) {
 	}
 	for (const attribute of node.attributes) {
 		if (attribute.type === 'LetDirective') {
-			lets.push(/** @type {ExpressionStatement} */ (context.visit(attribute)));
+			if (!slot_scope_applies_to_itself) {
+				lets.default.push(attribute);
+			}
 		} else if (attribute.type === 'SpreadAttribute') {
 			props_and_spreads.push(/** @type {Expression} */ (context.visit(attribute)));
 		} else if (attribute.type === 'Attribute') {
@@ -60,10 +64,6 @@ export function serialize_inline_component(node, expression, context) {
 				continue;
 			}
 
-			if (attribute.name === 'slot') {
-				slot_scope_applies_to_itself = true;
-			}
-
 			if (attribute.name === 'children') {
 				has_children_prop = true;
 			}
@@ -90,10 +90,6 @@ export function serialize_inline_component(node, expression, context) {
 		}
 	}
 
-	if (slot_scope_applies_to_itself) {
-		context.state.init.push(...lets);
-	}
-
 	/** @type {Statement[]} */
 	const snippet_declarations = [];
 
@@ -115,13 +111,20 @@ export function serialize_inline_component(node, expression, context) {
 
 		let slot_name = 'default';
 		if (is_element_node(child)) {
-			const attribute = /** @type {Attribute | undefined} */ (
+			const slot = /** @type {Attribute | undefined} */ (
 				child.attributes.find(
 					(attribute) => attribute.type === 'Attribute' && attribute.name === 'slot'
 				)
 			);
-			if (attribute !== undefined) {
+
-				slot_name = /** @type {Text[]} */ (attribute.value)[0].data;
+			if (slot !== undefined) {
+				slot_name = /** @type {Text[]} */ (slot.value)[0].data;
+
+				lets[slot_name] = child.attributes.filter((attribute) => attribute.type === 'LetDirective');
+			} else if (child.type === 'SvelteFragment') {
+				lets.default.push(
+					...child.attributes.filter((attribute) => attribute.type === 'LetDirective')
+				);
 			}
 		}
 
@@ -152,16 +155,40 @@ export function serialize_inline_component(node, expression, context) {
 
 		if (block.body.length === 0) continue;
 
-		const slot_fn = b.arrow(
+		/** @type {Pattern[]} */
-			[b.id('$$payload'), b.id('$$slotProps')],
+		const params = [b.id('$$payload')];
-			b.block([
+
-				...(slot_name === 'default' && !slot_scope_applies_to_itself ? lets : []),
+		if (lets[slot_name].length > 0) {
-				...block.body
+			const pattern = b.object_pattern(
-			])
+				lets[slot_name].map((node) => {
+					if (node.expression === null) {
+						return b.init(node.name, b.id(node.name));
+					}
+
+					if (node.expression.type === 'ObjectExpression') {
+						// @ts-expect-error it gets parsed as an `ObjectExpression` but is really an `ObjectPattern`
+						return b.init(node.name, b.object_pattern(node.expression.properties));
+					}
+
+					if (node.expression.type === 'ArrayExpression') {
+						// @ts-expect-error it gets parsed as an `ArrayExpression` but is really an `ArrayPattern`
+						return b.init(node.name, b.array_pattern(node.expression.elements));
+					}
+
+					return b.init(node.name, node.expression);
+				})
 			);
 
+			params.push(pattern);
+		}
+
+		const slot_fn = b.arrow(params, b.block(block.body));
+
 		if (slot_name === 'default' && !has_children_prop) {
-			if (lets.length === 0 && children.default.every((node) => node.type !== 'SvelteFragment')) {
+			if (
+				lets.default.length === 0 &&
+				children.default.every((node) => node.type !== 'SvelteFragment')
+			) {
 				// create `children` prop...
 				push_prop(b.prop('init', b.id('children'), slot_fn));
 

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

@@ -38,9 +38,6 @@ export function serialize_element_attributes(node, context) {
 	/** @type {StyleDirective[]} */
 	const style_directives = [];
 
-	/** @type {ExpressionStatement[]} */
-	const lets = [];
-
 	/** @type {Expression | null} */
 	let content = null;
 
@@ -185,7 +182,7 @@ export function serialize_element_attributes(node, context) {
 		} else if (attribute.type === 'StyleDirective') {
 			style_directives.push(attribute);
 		} else if (attribute.type === 'LetDirective') {
-			lets.push(/** @type {ExpressionStatement} */ (context.visit(attribute)));
+			// do nothing, these are handled inside `serialize_inline_component`
 		} else {
 			context.visit(attribute);
 		}
@@ -212,9 +209,6 @@ export function serialize_element_attributes(node, context) {
 		}
 	}
 
-	// Let bindings first, they can be used on attributes
-	context.state.init.push(...lets);
-
 	if (has_spread) {
 		serialize_element_spread_attributes(
 			node,

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

@@ -225,11 +225,6 @@ export function serialize_get_binding(node, state) {
 		);
 	}
 
-	if (Object.hasOwn(state.getters, node.name)) {
-		const getter = state.getters[node.name];
-		return typeof getter === 'function' ? getter(node) : getter;
-	}
-
 	return node;
 }
 

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

@@ -1,16 +1,10 @@
 import type { Scope } from '../scope.js';
 import type { SvelteNode, ValidatedModuleCompileOptions } from '#compiler';
 import type { Analysis } from '../types.js';
-import type { Expression, Identifier } from 'estree';
 
 export interface TransformState {
 	readonly analysis: Analysis;
 	readonly options: ValidatedModuleCompileOptions;
 	readonly scope: Scope;
 	readonly scopes: Map<SvelteNode, Scope>;
-	/**
-	 * A map of `[name, node]` pairs, where `Identifier` nodes matching `name`
-	 * will be replaced with `node` (e.g. `x` -> `$.get(x)`)
-	 */
-	readonly getters: Record<string, Expression | ((id: Identifier) => Expression)>;
 }

packages/svelte/src/compiler/utils/builders.js

@@ -320,10 +320,11 @@ export function object(properties) {
 }
 
 /**
- * @param {Array<ESTree.RestElement | ESTree.AssignmentProperty>} properties
+ * @param {Array<ESTree.RestElement | ESTree.AssignmentProperty | ESTree.Property>} properties
  * @returns {ESTree.ObjectPattern}
  */
 export function object_pattern(properties) {
+	// @ts-expect-error the types appear to be wrong
 	return { type: 'ObjectPattern', properties };
 }
 

packages/svelte/src/internal/server/index.js

@@ -428,12 +428,10 @@ export async function value_or_fallback_async(value, fallback) {
  * @returns {void}
  */
 export function slot(payload, slot_fn, slot_props, fallback_fn) {
-	if (slot_fn === undefined) {
+	if (slot_fn !== undefined) {
-		if (fallback_fn !== null) {
-			fallback_fn();
-		}
-	} else {
 		slot_fn(payload, slot_props);
+	} else {
+		fallback_fn?.();
 	}
 }
 

packages/svelte/tests/snapshot/samples/function-prop-no-getter/_expected/server/index.svelte.js

@@ -13,7 +13,7 @@ export default function Function_prop_no_getter($$payload) {
 		onmousedown: () => count += 1,
 		onmouseup,
 		onmouseenter: () => count = plusOne(count),
-		children: ($$payload, $$slotProps) => {
+		children: ($$payload) => {
 			$$payload.out += `<!---->clicks: ${$.escape(count)}`;
 		},
 		$$slots: { default: true }