Warn on referenced mutated nonstate

.changeset/friendly-lies-camp.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+feat: warn on references to mutated non-state in template

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

@@ -218,7 +218,7 @@ export function analyze_module(ast, options) {
 	for (const [, scope] of scopes) {
 		for (const [name, binding] of scope.declarations) {
 			if (binding.kind === 'state' && !binding.mutated) {
-				warn(warnings, binding.node, [], 'state-rune-not-mutated', name);
+				warn(warnings, binding.node, [], 'state-not-mutated', name);
 			}
 		}
 	}
@@ -377,7 +377,7 @@ export function analyze_component(root, options) {
 		for (const [, scope] of instance.scopes) {
 			for (const [name, binding] of scope.declarations) {
 				if (binding.kind === 'state' && !binding.mutated) {
-					warn(warnings, binding.node, [], 'state-rune-not-mutated', name);
+					warn(warnings, binding.node, [], 'state-not-mutated', name);
 				}
 			}
 		}
@@ -414,6 +414,30 @@ export function analyze_component(root, options) {
 		analysis.reactive_statements = order_reactive_statements(analysis.reactive_statements);
 	}
 
+	// warn on any nonstate declarations that are a) mutated and b) referenced in the template
+	for (const scope of [module.scope, instance.scope]) {
+		outer: for (const [name, binding] of scope.declarations) {
+			if (binding.kind === 'normal' && binding.mutated) {
+				for (const { path } of binding.references) {
+					if (path[0].type !== 'Fragment') continue;
+					for (let i = 1; i < path.length; i += 1) {
+						const type = path[i].type;
+						if (
+							type === 'FunctionDeclaration' ||
+							type === 'FunctionExpression' ||
+							type === 'ArrowFunctionExpression'
+						) {
+							continue;
+						}
+					}
+
+					warn(warnings, binding.node, [], 'non-state-reference', name);
+					continue outer;
+				}
+			}
+		}
+	}
+
 	analysis.stylesheet.validate(analysis);
 
 	for (const element of analysis.elements) {

packages/svelte/src/compiler/phases/scope.js

@@ -175,13 +175,13 @@ export class Scope {
 
 		references.push({ node, path });
 
-		const declaration = this.declarations.get(node.name);
-		if (declaration) {
-			declaration.references.push({ node, path });
+		const binding = this.declarations.get(node.name);
+		if (binding) {
+			binding.references.push({ node, path });
 		} else if (this.#parent) {
 			this.#parent.reference(node, path);
 		} else {
-			// no declaration was found, and this is the top level scope,
+			// no binding was found, and this is the top level scope,
 			// which means this is a global
 			this.root.conflicts.add(node.name);
 		}

packages/svelte/src/compiler/warnings.js

@@ -22,8 +22,11 @@ const runes = {
 		`It looks like you're using the $${name} rune, but there is a local binding called ${name}. ` +
 		`Referencing a local variable with a $ prefix will create a store subscription. Please rename ${name} to avoid the ambiguity.`,
 	/** @param {string} name */
-	'state-rune-not-mutated': (name) =>
-		`${name} is declared with $state(...) but is never updated. Did you mean to create a function that changes its value?`
+	'state-not-mutated': (name) =>
+		`${name} is declared with $state(...) but is never updated. Did you mean to create a function that changes its value?`,
+	/** @param {string} name */
+	'non-state-reference': (name) =>
+		`${name} is updated, but is not declared with $state(...). Changing its value will not correctly trigger updates.`
 };
 
 /** @satisfies {Warnings} */

packages/svelte/tests/validator/samples/runes-referenced-nonstate/_config.js

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

packages/svelte/tests/validator/samples/runes-referenced-nonstate/input.svelte

@@ -0,0 +1,10 @@
+<script>
+	let a = $state(1);
+	let b = 2;
+	let c = 3;
+</script>
+
+<button onclick={() => a += 1}>a += 1</button>
+<button onclick={() => b += 1}>b += 1</button>
+<button onclick={() => c += 1}>c += 1</button>
+<p>{a} + {b} + {c} = {a + b + c}</p>

packages/svelte/tests/validator/samples/runes-referenced-nonstate/warnings.json

@@ -0,0 +1,26 @@
+[
+	{
+		"code": "non-state-reference",
+		"message": "b is updated, but is not declared with $state(...). Changing its value will not correctly trigger updates.",
+		"start": {
+			"column": 5,
+			"line": 3
+		},
+		"end": {
+			"column": 6,
+			"line": 3
+		}
+	},
+	{
+		"code": "non-state-reference",
+		"message": "c is updated, but is not declared with $state(...). Changing its value will not correctly trigger updates.",
+		"start": {
+			"column": 5,
+			"line": 4
+		},
+		"end": {
+			"column": 6,
+			"line": 4
+		}
+	}
+]

packages/svelte/tests/validator/samples/runes-state-rune-not-mutated/warnings.json

@@ -1,6 +1,6 @@
 [
 	{
-		"code": "state-rune-not-mutated",
+		"code": "state-not-mutated",
 		"end": {
 			"column": 11,
 			"line": 3