chore: add inline new class warning

Author: trueadm

.changeset/cold-birds-own.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: add inline new class warning

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

@@ -579,6 +579,13 @@ export const validation_runes_js = {
 			...context.state,
 			private_derived_state
 		});
+	},
+	NewExpression(node, context) {
+		const callee = node.callee;
+
+		if (callee.type === 'ClassExpression' && context.state.scope.function_depth !== 0) {
+			warn(context.state.analysis.warnings, node, context.path, 'inline-new-class');
+		}
 	}
 };
 
@@ -721,5 +728,6 @@ export const validation_runes = merge(validation, a11y_validators, {
 			}
 		}
 	},
-	ClassBody: validation_runes_js.ClassBody
+	ClassBody: validation_runes_js.ClassBody,
+	NewExpression: validation_runes_js.NewExpression
 });

packages/svelte/src/compiler/warnings.js

@@ -191,12 +191,20 @@ const state = {
 		`State referenced in its own scope will never update. Did you mean to reference it inside a closure?`
 };
 
+/** @satisfies {Warnings} */
+const performance = {
+	'inline-new-class': () =>
+		`Creating inline classes will likely cause performance issues. ` +
+		`Instead, declare the class at the module-level and create new instances from the class reference.`
+};
+
 /** @satisfies {Warnings} */
 const warnings = {
 	...css,
 	...attributes,
 	...runes,
 	...a11y,
+	...performance,
 	...state
 };
 

packages/svelte/tests/validator/samples/inline-new-class/input.svelte

@@ -0,0 +1,5 @@
+<script>
+	const a = new class {
+		foo = $state(0)
+	}
+</script>

packages/svelte/tests/validator/samples/inline-new-class/warnings.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "inline-new-class",
+		"message": "Creating inline classes will likely cause performance issues. Instead, declare the class at the module-level and create new instances from the class reference.",
+		"start": {
+			"column": 11,
+			"line": 2
+		},
+		"end": {
+			"column": 2,
+			"line": 4
+		}
+	}
+]