tighten up validation etc

Author: Rich-Harris

packages/svelte/src/compiler/errors.js

@@ -171,10 +171,9 @@ const runes = {
 		`$props() assignment must not contain nested properties or computed keys`,
 	'invalid-props-location': () =>
 		`$props() can only be used at the top level of components as a variable declaration initializer`,
-	'invalid-derived-location': () =>
-		`$derived() can only be used as a variable declaration initializer or a class field`,
-	'invalid-state-location': () =>
-		`$state() can only be used as a variable declaration initializer or a class field`,
+	/** @param {string} rune */
+	'invalid-state-location': (rune) =>
+		`${rune}(...) can only be used as a variable declaration initializer or a class field`,
 	'invalid-effect-location': () => `$effect() can only be used as an expression statement`,
 	/**
 	 * @param {boolean} is_binding

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

@@ -718,7 +718,7 @@ function validate_call_expression(node, scope, path) {
 	if (rune === '$state' || rune === '$derived' || rune === '$derived.call') {
 		if (parent.type === 'VariableDeclarator') return;
 		if (parent.type === 'PropertyDefinition' && !parent.static && !parent.computed) return;
-		error(node, rune === '$derived' ? 'invalid-derived-location' : 'invalid-state-location');
+		error(node, 'invalid-state-location', rune);
 	}
 
 	if (rune === '$effect' || rune === '$effect.pre') {
@@ -786,10 +786,10 @@ export const validation_runes_js = {
 
 		const args = /** @type {import('estree').CallExpression} */ (init).arguments;
 
-		if (rune === '$derived' && args.length !== 1) {
-			error(node, 'invalid-rune-args-length', '$derived', [1]);
+		if ((rune === '$derived' || rune === '$derived.call') && args.length !== 1) {
+			error(node, 'invalid-rune-args-length', rune, [1]);
 		} else if (rune === '$state' && args.length > 1) {
-			error(node, 'invalid-rune-args-length', '$state', [0, 1]);
+			error(node, 'invalid-rune-args-length', rune, [0, 1]);
 		} else if (rune === '$props') {
 			error(node, 'invalid-props-location');
 		}
@@ -811,7 +811,7 @@ export const validation_runes_js = {
 				definition.value?.type === 'CallExpression'
 			) {
 				const rune = get_rune(definition.value, context.state.scope);
-				if (rune === '$derived') {
+				if (rune === '$derived' || rune === '$derived.call') {
 					private_derived_state.push(definition.key.name);
 				}
 			}
@@ -938,25 +938,33 @@ export const validation_runes = merge(validation, a11y_validators, {
 			context.type === 'Identifier' &&
 			(context.name === '$state' || context.name === '$derived')
 		) {
-			error(
-				node,
-				context.name === '$derived' ? 'invalid-derived-location' : 'invalid-state-location'
-			);
+			error(node, 'invalid-state-location', context.name);
 		}
 		next({ ...state });
 	},
-	VariableDeclarator(node, { state }) {
+	VariableDeclarator(node, { state, path }) {
 		const init = unwrap_ts_expression(node.init);
 		const rune = get_rune(init, state.scope);
 
 		if (rune === null) return;
 
 		const args = /** @type {import('estree').CallExpression} */ (init).arguments;
 
-		if (rune === '$derived' && args.length !== 1) {
-			error(node, 'invalid-rune-args-length', '$derived', [1]);
+		if (rune === '$derived') {
+			const arg = args[0];
+			if (
+				arg.type === 'CallExpression' &&
+				(arg.callee.type === 'ArrowFunctionExpression' || arg.callee.type === 'FunctionExpression')
+			) {
+				warn(state.analysis.warnings, node, path, 'derived-iife');
+			}
+		}
+
+		// TODO some of this is duplicated with above, seems off
+		if ((rune === '$derived' || rune === '$derived.call') && args.length !== 1) {
+			error(node, 'invalid-rune-args-length', rune, [1]);
 		} else if (rune === '$state' && args.length > 1) {
-			error(node, 'invalid-rune-args-length', '$state', [0, 1]);
+			error(node, 'invalid-rune-args-length', rune, [0, 1]);
 		} else if (rune === '$props') {
 			if (state.has_props_rune) {
 				error(node, 'duplicate-props-rune');

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

@@ -395,6 +395,7 @@ export function thunk(expression) {
 		expression.type === 'CallExpression' &&
 		expression.callee.type !== 'Super' &&
 		expression.callee.type !== 'MemberExpression' &&
+		expression.callee.type !== 'CallExpression' &&
 		expression.arguments.length === 0
 	) {
 		return expression.callee;

packages/svelte/src/compiler/warnings.js

@@ -23,7 +23,9 @@ const runes = {
 		`Referencing a local variable with a $ prefix will create a store subscription. Please rename ${name} to avoid the ambiguity.`,
 	/** @param {string} name */
 	'non-state-reference': (name) =>
-		`${name} is updated, but is not declared with $state(...). Changing its value will not correctly trigger updates.`
+		`${name} is updated, but is not declared with $state(...). Changing its value will not correctly trigger updates.`,
+	'derived-iife': () =>
+		`Use \`$derived.call(() => {...})\` instead of \`$derived((() => {...})());\``
 };
 
 /** @satisfies {Warnings} */