breaking: warn/error on old syntax in runes mode

- warn on slots and event handlers in runes mode
- error on `<slot>` + `{@render ...}` tag usage in same component

closes #9416

Author: dummdidumm

.changeset/four-pugs-listen.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+breaking: warn on slots and event handlers in runes mode, error on `<slot>` + `{@render ...}` tag usage in same component

packages/svelte/src/compiler/errors.js

@@ -163,7 +163,9 @@ const special_elements = {
 	 * @param {string | null} match
 	 */
 	'invalid-svelte-tag': (tags, match) =>
-		`Valid <svelte:...> tag names are ${list(tags)}${match ? ' (did you mean ' + match + '?)' : ''}`
+		`Valid <svelte:...> tag names are ${list(tags)}${match ? ' (did you mean ' + match + '?)' : ''}`,
+	'conflicting-slot-usage': () =>
+		`Cannot use <slot> syntax and {@render ...} tags in the same component. Migrate towards {@render ...} tags completely.`
 };
 
 /** @satisfies {Errors} */

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

@@ -379,6 +379,7 @@ export function analyze_component(root, source, options) {
 		uses_rest_props: false,
 		uses_slots: false,
 		uses_component_bindings: false,
+		uses_render_tags: false,
 		custom_element: options.customElementOptions ?? options.customElement,
 		inject_styles: options.css === 'injected' || options.customElement,
 		accessors: options.customElement
@@ -388,7 +389,7 @@ export function analyze_component(root, source, options) {
 				!!options.legacy?.componentApi,
 		reactive_statements: new Map(),
 		binding_groups: new Map(),
-		slot_names: new Set(),
+		slot_names: new Map(),
 		warnings,
 		css: {
 			ast: root.css,
@@ -455,6 +456,10 @@ export function analyze_component(root, source, options) {
 				}
 			}
 		}
+
+		if (analysis.uses_render_tags && (analysis.uses_slots || analysis.slot_names.size > 0)) {
+			error(analysis.slot_names.values().next().value, 'conflicting-slot-usage');
+		}
 	} else {
 		instance.scope.declare(b.id('$$props'), 'bindable_prop', 'synthetic');
 		instance.scope.declare(b.id('$$restProps'), 'rest_prop', 'synthetic');
@@ -1087,7 +1092,7 @@ const common_visitors = {
 				break;
 			}
 		}
-		context.state.analysis.slot_names.add(name);
+		context.state.analysis.slot_names.set(name, node);
 	},
 	StyleDirective(node, context) {
 		if (node.value === true) {

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

@@ -1183,6 +1183,21 @@ export const validation_runes = merge(validation, a11y_validators, {
 			warn(state.analysis.warnings, node, path, 'invalid-bindable-declaration');
 		}
 	},
+	SlotElement(node, { state, path }) {
+		if (!state.analysis.custom_element) {
+			warn(state.analysis.warnings, node, path, 'deprecated-slot-element');
+		}
+	},
+	OnDirective(node, { state, path }) {
+		const parent_type = path.at(-1)?.type;
+		// Don't warn on component events; these might not be under the author's control so the warning would be unactionable
+		if (parent_type === 'RegularElement' || parent_type === 'SvelteElement') {
+			warn(state.analysis.warnings, node, path, 'deprecated-event-handler', node.name);
+		}
+	},
+	RenderTag(_, { state }) {
+		state.analysis.uses_render_tags = true;
+	},
 	// TODO this is a code smell. need to refactor this stuff
 	ClassBody: validation_runes_js.ClassBody,
 	ClassDeclaration: validation_runes_js.ClassDeclaration,

packages/svelte/src/compiler/phases/types.d.ts

@@ -3,6 +3,7 @@ import type {
 	Css,
 	Fragment,
 	RegularElement,
+	SlotElement,
 	SvelteElement,
 	SvelteNode,
 	SvelteOptions
@@ -61,13 +62,14 @@ export interface ComponentAnalysis extends Analysis {
 	/** Whether the component uses `$$slots` */
 	uses_slots: boolean;
 	uses_component_bindings: boolean;
+	uses_render_tags: boolean;
 	custom_element: boolean | SvelteOptions['customElement'];
 	/** If `true`, should append styles through JavaScript */
 	inject_styles: boolean;
 	reactive_statements: Map<LabeledStatement, ReactiveStatement>;
 	/** Identifiers that make up the `bind:group` expression -> internal group binding name */
 	binding_groups: Map<[key: string, bindings: Array<Binding | null>], Identifier>;
-	slot_names: Set<string>;
+	slot_names: Map<string, SlotElement>;
 	css: {
 		ast: Css.StyleSheet | null;
 		hash: string;

packages/svelte/src/compiler/warnings.js

@@ -233,7 +233,12 @@ const legacy = {
 		'All dependencies of the reactive declaration are declared in a module script and will not be reactive',
 	/** @param {string} name */
 	'unused-export-let': (name) =>
-		`Component has unused export property '${name}'. If it is for external reference only, please consider using \`export const ${name}\``
+		`Component has unused export property '${name}'. If it is for external reference only, please consider using \`export const ${name}\``,
+	'deprecated-slot-element': () =>
+		`Using <slot> to render parent content is deprecated. Use {@render ...} tags instead.`,
+	/** @param {string} name */
+	'deprecated-event-handler': (name) =>
+		`Using on:${name} to listen to the ${name} event is is deprecated. Use the event attribute on${name} instead.`
 };
 
 const block = {

packages/svelte/tests/compiler-errors/samples/slot-conflicting-with-render-tag/_config.js

@@ -0,0 +1,10 @@
+import { test } from '../../test';
+
+export default test({
+	error: {
+		code: 'conflicting-slot-usage',
+		message:
+			'Cannot use <slot> syntax and {@render ...} tags in the same component. Migrate towards {@render ...} tags completely.',
+		position: [71, 84]
+	}
+});

packages/svelte/tests/compiler-errors/samples/slot-conflicting-with-render-tag/main.svelte

@@ -0,0 +1,6 @@
+<script>
+	let { children } = $props();
+</script>
+
+{@render children()}
+<slot></slot>

packages/svelte/tests/validator/samples/runes-legacy-syntax-warnings/input.svelte

@@ -0,0 +1,13 @@
+<script>
+	let { foo } = $props();
+</script>
+
+<!-- ok -->
+<button onclick={foo}>click me</button>
+<Button onclick={foo}>click me</Button>
+<Button on:click={foo}>click me</Button>
+
+<!-- warn -->
+<slot></slot>
+<slot name="foo"></slot>
+<button on:click={foo}>click me</button>

packages/svelte/tests/validator/samples/runes-legacy-syntax-warnings/warnings.json

@@ -0,0 +1,38 @@
+[
+	{
+		"code": "deprecated-slot-element",
+		"end": {
+			"column": 13,
+			"line": 11
+		},
+		"message": "Using <slot> to render parent content is deprecated. Use {@render ...} tags instead.",
+		"start": {
+			"column": 0,
+			"line": 11
+		}
+	},
+	{
+		"code": "deprecated-slot-element",
+		"end": {
+			"column": 24,
+			"line": 12
+		},
+		"message": "Using <slot> to render parent content is deprecated. Use {@render ...} tags instead.",
+		"start": {
+			"column": 0,
+			"line": 12
+		}
+	},
+	{
+		"code": "deprecated-event-handler",
+		"end": {
+			"column": 22,
+			"line": 13
+		},
+		"message": "Using on:click to listen to the click event is is deprecated. Use the event attribute onclick instead.",
+		"start": {
+			"column": 8,
+			"line": 13
+		}
+	}
+]