fix: allow mutation of private derived state (#15228)

Also disallow reassignment of private derived state

Fixes #15227

.changeset/rare-carpets-wave.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: allow mutation of private derived state

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

@@ -605,7 +605,7 @@ export function analyze_component(root, source, options) {
 				has_props_rune: false,
 				component_slots: new Set(),
 				expression: null,
-				private_derived_state: [],
+				derived_state: [],
 				function_depth: scope.function_depth,
 				instance_scope: instance.scope,
 				reactive_statement: null,
@@ -676,7 +676,7 @@ export function analyze_component(root, source, options) {
 				reactive_statements: analysis.reactive_statements,
 				component_slots: new Set(),
 				expression: null,
-				private_derived_state: [],
+				derived_state: [],
 				function_depth: scope.function_depth
 			};
 

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

@@ -19,7 +19,7 @@ export interface AnalysisState {
 	component_slots: Set<string>;
 	/** Information about the current expression/directive/block value */
 	expression: ExpressionMetadata | null;
-	private_derived_state: string[];
+	derived_state: string[];
 	function_depth: number;
 
 	// legacy stuff

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

@@ -8,20 +8,20 @@ import { get_rune } from '../../scope.js';
  */
 export function ClassBody(node, context) {
 	/** @type {string[]} */
-	const private_derived_state = [];
+	const derived_state = [];
 
 	for (const definition of node.body) {
 		if (
 			definition.type === 'PropertyDefinition' &&
-			definition.key.type === 'PrivateIdentifier' &&
+			(definition.key.type === 'PrivateIdentifier' || definition.key.type === 'Identifier') &&
 			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);
+				derived_state.push(definition.key.name);
 			}
 		}
 	}
 
-	context.next({ ...context.state, private_derived_state });
+	context.next({ ...context.state, derived_state });
 }

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

@@ -35,20 +35,17 @@ export function validate_assignment(node, argument, state) {
 		}
 	}
 
-	let object = /** @type {Expression | Super} */ (argument);
-
-	/** @type {Expression | PrivateIdentifier | null} */
-	let property = null;
-
-	while (object.type === 'MemberExpression') {
-		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');
-		}
+	if (
+		argument.type === 'MemberExpression' &&
+		argument.object.type === 'ThisExpression' &&
+		(((argument.property.type === 'PrivateIdentifier' || argument.property.type === 'Identifier') &&
+			state.derived_state.includes(argument.property.name)) ||
+			(argument.property.type === 'Literal' &&
+				argument.property.value &&
+				typeof argument.property.value === 'string' &&
+				state.derived_state.includes(argument.property.value)))
+	) {
+		e.constant_assignment(node, 'derived state');
 	}
 }
 

packages/svelte/tests/validator/samples/mutate-derived-private-field/errors.json

@@ -0,0 +1 @@
+[]

packages/svelte/tests/validator/samples/mutate-derived-private-field/input.svelte

@@ -0,0 +1,9 @@
+<script>
+	class Test{
+		#der = $derived({test: 0});
+
+		set test(v){
+			this.#der.test = 45;
+		}
+	}
+</script>

packages/svelte/tests/validator/samples/reassign-derived-literal/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "constant_assignment",
+		"message": "Cannot assign to derived state",
+		"start": {
+			"column": 3,
+			"line": 6
+		},
+		"end": {
+			"column": 29,
+			"line": 6
+		}
+	}
+]

packages/svelte/tests/validator/samples/reassign-derived-literal/input.svelte

@@ -0,0 +1,9 @@
+<script>
+	class Test{
+		der = $derived({ test: 0 });
+
+		set test(v){
+			this["der"] = { test: 45 };
+		}
+	}
+</script>

packages/svelte/tests/validator/samples/reassign-derived-private-field/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "constant_assignment",
+		"message": "Cannot assign to derived state",
+		"start": {
+			"column": 3,
+			"line": 6
+		},
+		"end": {
+			"column": 27,
+			"line": 6
+		}
+	}
+]

packages/svelte/tests/validator/samples/reassign-derived-private-field/input.svelte

@@ -0,0 +1,9 @@
+<script>
+	class Test{
+		#der = $derived({ test: 0 });
+
+		set test(v){
+			this.#der = { test: 45 };
+		}
+	}
+</script>

packages/svelte/tests/validator/samples/reassign-derived-public-field/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "constant_assignment",
+		"message": "Cannot assign to derived state",
+		"start": {
+			"column": 3,
+			"line": 6
+		},
+		"end": {
+			"column": 26,
+			"line": 6
+		}
+	}
+]

packages/svelte/tests/validator/samples/reassign-derived-public-field/input.svelte

@@ -0,0 +1,9 @@
+<script>
+	class Test{
+		der = $derived({ test: 0 });
+
+		set test(v){
+			this.der = { test: 45 };
+		}
+	}
+</script>