feat: raising a warning if the code contains both old and new event handling syntaxes

Author: caiquetorres

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

@@ -374,6 +374,8 @@ export function analyze_component(root, source, options) {
 		uses_slots: false,
 		uses_component_bindings: false,
 		uses_render_tags: false,
+		event_directive_node: null,
+		uses_event_attributes: false,
 		custom_element: options.customElementOptions ?? options.customElement,
 		inject_styles: options.css === 'injected' || options.customElement,
 		accessors: options.customElement
@@ -1126,6 +1128,8 @@ const common_visitors = {
 		});
 
 		if (is_event_attribute(node)) {
+			context.state.analysis.uses_event_attributes = true;
+
 			const expression = node.value[0].expression;
 
 			const delegated_event = get_delegated_event(node.name.slice(2), expression, context);
@@ -1243,6 +1247,11 @@ const common_visitors = {
 
 		context.next();
 	},
+	OnDirective(node, context) {
+		context.state.analysis.event_directive_node = node;
+
+		context.next();
+	},
 	BindDirective(node, context) {
 		let i = context.path.length;
 		while (i--) {

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

@@ -7,6 +7,7 @@ import * as e from '../../errors.js';
 import {
 	extract_identifiers,
 	get_parent,
+	is_event_attribute,
 	is_expression_attribute,
 	is_text_attribute,
 	object,
@@ -102,6 +103,17 @@ function validate_element(node, context) {
 
 	for (const attribute of node.attributes) {
 		if (attribute.type === 'Attribute') {
+			const parent_type = attribute.parent?.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') &&
+				is_event_attribute(attribute) &&
+				context.state.analysis.event_directive_node
+			) {
+				w.mixing_old_and_new_event_handler_syntaxes(node, node.name);
+			}
+
 			const is_expression = is_expression_attribute(attribute);
 
 			if (context.state.analysis.runes && is_expression) {
@@ -1181,10 +1193,13 @@ export const validation_runes = merge(validation, a11y_validators, {
 			w.deprecated_slot_element(node);
 		}
 	},
-	OnDirective(node, { path }) {
+	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') {
+			if (state.analysis.uses_event_attributes) {
+				w.mixing_old_and_new_event_handler_syntaxes(node, node.name);
+			}
 			w.deprecated_event_handler(node, node.name);
 		}
 	},

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

@@ -9,7 +9,7 @@ import type {
 	SvelteOptions,
 	Warning
 } from '#compiler';
-import type { Identifier, LabeledStatement, Program } from 'estree';
+import type { BaseNode, Identifier, LabeledStatement, Program } from 'estree';
 import type { Scope, ScopeRoot } from './scope.js';
 
 export interface Js {
@@ -57,6 +57,8 @@ export interface ComponentAnalysis extends Analysis {
 	uses_slots: boolean;
 	uses_component_bindings: boolean;
 	uses_render_tags: boolean;
+	event_directive_node: BaseNode | null;
+	uses_event_attributes: boolean;
 	custom_element: boolean | SvelteOptions['customElement'];
 	/** If `true`, should append styles through JavaScript */
 	inject_styles: boolean;

packages/svelte/src/compiler/warnings.js

@@ -535,6 +535,15 @@ export function deprecated_event_handler(node, name) {
 	w(node, "deprecated_event_handler", `Using on:${name} to listen to the ${name} event is is deprecated. Use the event attribute on${name} instead.`);
 }
 
+/**
+ * Mixing old (on:%name%) and new syntaxes for event handling is not recommended. Use only the on%name% syntax.
+ * @param {null | NodeLike} node
+ * @param {string} name
+ */
+export function mixing_old_and_new_event_handler_syntaxes(node, name) {
+	w(node, "mixing-events-handling-syntax", `Mixing old (on:${name}) and new syntaxes for event handling is not recommended. Use only the on${name} syntax.`);
+}
+
 /**
  * Self-closing HTML tags for non-void elements are ambiguous — use <%name% ...></%name%> rather than <%name% ... />
  * @param {null | NodeLike} node
@@ -665,4 +674,4 @@ export function static_state_reference(node) {
  */
 export function invalid_rest_eachblock_binding(node, name) {
 	w(node, "invalid_rest_eachblock_binding", `The rest operator (...) will create a new object and binding '${name}' with the original object will not work`);
-}
+}

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

@@ -23,6 +23,18 @@
 			"line": 12
 		}
 	},
+	{
+		"code": "mixing-events-handling-syntax",
+		"end": {
+			"column": 22,
+			"line": 13
+		},
+		"message": "Mixing old (on:click) and new syntaxes for event handling is not recommended. Use only the onclick syntax.",
+		"start": {
+			"column": 8,
+			"line": 13
+		}
+	},
 	{
 		"code": "deprecated_event_handler",
 		"end": {