feat: warn on obvious legacy component instantiation

packages/svelte/messages/compile-warnings/script.md

@@ -6,6 +6,10 @@
 
 > Component has unused export property '%name%'. If it is for external reference only, please consider using `export const %name%`
 
+## legacy_component_creation
+
+> Svelte 5 components are no longer classes. Instantiate them using `mount` or `hydrate` (imported from 'svelte') instead.
+
 ## non_reactive_update
 
 > `%name%` is updated, but is not declared with `$state(...)`. Changing its value will not correctly trigger updates

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

@@ -1,4 +1,4 @@
-/** @import { AssignmentExpression, CallExpression, Expression, Identifier, Node, Pattern, PrivateIdentifier, Super, UpdateExpression, VariableDeclarator } from 'estree' */
+/** @import { AssignmentExpression, CallExpression, Expression, ImportDeclaration, Identifier, Node, Pattern, PrivateIdentifier, Super, UpdateExpression, VariableDeclarator } from 'estree' */
 /** @import { Attribute, Component, ElementLike, Fragment, RegularElement, SvelteComponent, SvelteElement, SvelteNode, SvelteSelf, TransitionDirective } from '#compiler' */
 /** @import { NodeLike } from '../../errors.js' */
 /** @import { AnalysisState, Context, Visitors } from './types.js' */
@@ -362,6 +362,32 @@ function validate_block_not_empty(node, context) {
  * @type {Visitors}
  */
 const validation = {
+	ExpressionStatement(node, { state }) {
+		if (
+			node.expression.type === 'NewExpression' &&
+			node.expression.callee.type === 'Identifier' &&
+			node.expression.arguments.length === 1 &&
+			node.expression.arguments[0].type === 'ObjectExpression' &&
+			node.expression.arguments[0].properties.some(
+				(p) => p.type === 'Property' && p.key.type === 'Identifier' && p.key.name === 'target'
+			)
+		) {
+			const binding = state.scope.get(node.expression.callee.name);
+			if (binding?.kind === 'normal' && binding.declaration_kind === 'import') {
+				const declaration = /** @type {ImportDeclaration} */ (binding.initial);
+
+				// Theoretically someone could import a class from a `.svelte.js` module, but that's too rare to worry about
+				if (
+					/** @type {string} */ (declaration.source.value)?.endsWith('.svelte') &&
+					declaration.specifiers.find(
+						(s) => s.local.name === binding.node.name && s.type === 'ImportDefaultSpecifier'
+					)
+				) {
+					w.legacy_component_creation(node.expression);
+				}
+			}
+		}
+	},
 	MemberExpression(node, context) {
 		if (node.object.type === 'Identifier' && node.property.type === 'Identifier') {
 			const binding = context.state.scope.get(node.object.name);

packages/svelte/src/compiler/warnings.js

@@ -96,6 +96,7 @@ export const codes = [
 	"options_renamed_ssr_dom",
 	"derived_iife",
 	"export_let_unused",
+	"legacy_component_creation",
 	"non_reactive_update",
 	"perf_avoid_inline_class",
 	"perf_avoid_nested_class",
@@ -586,6 +587,14 @@ export function export_let_unused(node, name) {
 	w(node, "export_let_unused", `Component has unused export property '${name}'. If it is for external reference only, please consider using \`export const ${name}\``);
 }
 
+/**
+ * Svelte 5 components are no longer classes. Instantiate them using `mount` or `hydrate` (imported from 'svelte') instead.
+ * @param {null | NodeLike} node
+ */
+export function legacy_component_creation(node) {
+	w(node, "legacy_component_creation", "Svelte 5 components are no longer classes. Instantiate them using `mount` or `hydrate` (imported from 'svelte') instead.");
+}
+
 /**
  * `%name%` is updated, but is not declared with `$state(...)`. Changing its value will not correctly trigger updates
  * @param {null | NodeLike} node

packages/svelte/tests/validator/samples/component-legacy-instantiation/input.svelte

@@ -0,0 +1,13 @@
+<script>
+	import Foo, { Bar } from './Somewhere.svelte';
+	import Baz from './somewhereelse';
+
+	let Buzz;
+
+	new Foo({ target: null });
+	new Foo({}); // also a false negative to be really sure we don't get false positives
+	new Foo();
+	new Bar();
+	new Baz();
+	new Buzz();
+</script>

packages/svelte/tests/validator/samples/component-legacy-instantiation/warnings.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "legacy_component_creation",
+		"message": "Svelte 5 components are no longer classes. Instantiate them using `mount` or `hydrate` (imported from 'svelte') instead.",
+		"start": {
+			"column": 1,
+			"line": 7
+		},
+		"end": {
+			"column": 26,
+			"line": 7
+		}
+	}
+]