relax const requirement, validate assignments instead

Author: Rich-Harris

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

@@ -1,5 +1,5 @@
 /** @import { ArrowFunctionExpression, CallExpression, Expression, FunctionDeclaration, FunctionExpression, Identifier, VariableDeclarator } from 'estree' */
-/** @import { AST } from '#compiler' */
+/** @import { AST, Binding } from '#compiler' */
 /** @import { Context } from '../types' */
 import { get_rune } from '../../scope.js';
 import * as e from '../../../errors.js';
@@ -86,8 +86,7 @@ export function CallExpression(node, context) {
 				parent.id.type !== 'Identifier' ||
 				context.state.ast_type !== 'instance' ||
 				context.state.scope !== context.state.analysis.instance.scope ||
-				grand_parent.type !== 'VariableDeclaration' ||
-				grand_parent.kind !== 'const'
+				grand_parent.type !== 'VariableDeclaration'
 			) {
 				e.props_id_invalid_placement(node);
 			}

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

@@ -25,6 +25,10 @@ export function validate_assignment(node, argument, state) {
 				e.constant_assignment(node, 'derived state');
 			}
 
+			if (binding?.node === state.analysis.props_id) {
+				e.constant_assignment(node, '$props.id()');
+			}
+
 			if (binding?.kind === 'each') {
 				e.each_item_invalid_assignment(node);
 			}