fix private fields

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

@@ -546,7 +546,6 @@ export function analyze_component(root, source, options) {
 				component_slots: new Set(),
 				expression: null,
 				render_tag: null,
-				private_derived_state: [],
 				function_depth: scope.function_depth,
 				instance_scope: instance.scope,
 				reactive_statement: null,
@@ -618,7 +617,6 @@ export function analyze_component(root, source, options) {
 				component_slots: new Set(),
 				expression: null,
 				render_tag: null,
-				private_derived_state: [],
 				function_depth: scope.function_depth
 			};
 

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

@@ -17,7 +17,6 @@ export interface AnalysisState {
 	expression: ExpressionMetadata | null;
 	/** The current {@render ...} tag, if any */
 	render_tag: null | RenderTag;
-	private_derived_state: string[];
 	function_depth: number;
 
 	// legacy stuff

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

@@ -1,27 +1,10 @@
 /** @import { ClassBody } from 'estree' */
 /** @import { Context } from '../types' */
-import { get_rune } from '../../scope.js';
 
 /**
  * @param {ClassBody} node
  * @param {Context} context
  */
 export function ClassBody(node, context) {
-	/** @type {string[]} */
-	const private_derived_state = [];
-
-	for (const definition of node.body) {
-		if (
-			definition.type === 'PropertyDefinition' &&
-			definition.key.type === 'PrivateIdentifier' &&
-			definition.value?.type === 'CallExpression'
-		) {
-			const rune = get_rune(definition.value, context.state.scope);
-			if (rune === '$derived' || rune === '$derived.by') {
-				private_derived_state.push(definition.key.name);
-			}
-		}
-	}
-
-	context.next({ ...context.state, private_derived_state });
+	context.next({ ...context.state });
 }

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

@@ -51,12 +51,6 @@ export function validate_assignment(node, argument, state) {
 		property = object.property;
 		object = object.object;
 	}
-
-	if (object.type === 'ThisExpression' && property?.type === 'PrivateIdentifier') {
-		if (state.private_derived_state.includes(property.name)) {
-			e.constant_assignment(node, 'derived state');
-		}
-	}
 }
 
 /**

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

@@ -46,7 +46,7 @@ function build_assignment(operator, left, right, context) {
 				if (should_proxy(value, context.state.scope)) {
 					transformed = true;
 					value =
-						private_state.kind === 'raw_state'
+						private_state.kind === 'raw_state' || private_state.kind === 'derived'
 							? value
 							: build_proxy_reassignment(value, private_state.id);
 				}

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

@@ -24,6 +24,23 @@ export function AssignmentExpression(node, context) {
 function build_assignment(operator, left, right, context) {
 	let object = left;
 
+	if (
+		context.state.analysis.runes &&
+		left.type === 'MemberExpression' &&
+		left.object.type === 'ThisExpression' &&
+		left.property.type === 'PrivateIdentifier'
+	) {
+		const field = context.state.private_derived.get(left.property.name);
+
+		if (field) {
+			return b.assignment(
+				operator,
+				left,
+				b.call('$.once', b.thunk(/** @type {Expression} */ (context.visit(right))))
+			);
+		}
+	}
+
 	while (object.type === 'MemberExpression') {
 		// @ts-expect-error
 		object = object.object;

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

@@ -97,17 +97,6 @@ export function ClassBody(node, context) {
 
 					// get foo() { return this.#foo; }
 					body.push(b.method('get', definition.key, [], [b.return(b.call(member))]));
-
-					if (dev && (field.kind === 'derived' || field.kind === 'derived_by')) {
-						body.push(
-							b.method(
-								'set',
-								definition.key,
-								[b.id('_')],
-								[b.throw_error(`Cannot update a derived property ('${name}')`)]
-							)
-						);
-					}
 				}
 
 				continue;

packages/svelte/tests/compiler-errors/samples/runes-no-derived-state-field-assignment/_config.js

@@ -1,8 +0,0 @@
-import { test } from '../../test';
-
-export default test({
-	error: {
-		code: 'constant_assignment',
-		message: 'Cannot assign to derived state'
-	}
-});

packages/svelte/tests/compiler-errors/samples/runes-no-derived-state-field-assignment/main.svelte

@@ -1,10 +0,0 @@
-<script>
-	class Counter {
-		count = $state();
-		#doubled = $derived(this.count * 2);
-
-		nope() {
-			this.#doubled += 1;
-		}
-	}
-</script>

packages/svelte/tests/compiler-errors/samples/runes-no-derived-state-field-update/_config.js

@@ -1,8 +0,0 @@
-import { test } from '../../test';
-
-export default test({
-	error: {
-		code: 'constant_assignment',
-		message: 'Cannot assign to derived state'
-	}
-});

packages/svelte/tests/compiler-errors/samples/runes-no-derived-state-field-update/main.svelte

@@ -1,10 +0,0 @@
-<script>
-	class Counter {
-		count = $state();
-		#doubled = $derived(this.count * 2);
-
-		nope() {
-			this.#doubled++;
-		}
-	}
-</script>