fix: add `$set` and `$on` methods in legacy compat mode

People could've done bind:this and called instance methods on the instance - a rare case, but not impossible. This shims $set and $on when in legacy compat mode. $destroy is never shimmed because you shouldn't manually destroy a component, ever, and there's no way to make that work in the new world.
closes #10420

.changeset/tough-radios-punch.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: add `$set` and `$on` methods in legacy compat mode

packages/svelte/src/compiler/phases/3-transform/client/transform-client.js

@@ -245,7 +245,11 @@ export function client_component(source, analysis, options) {
 		return b.get(alias ?? name, [b.return(expression)]);
 	});
 
-	if (analysis.accessors) {
+	if (
+		analysis.accessors ||
+		// because $set method needs accessors
+		options.legacy.componentApi
+	) {
 		for (const [name, binding] of analysis.instance.scope.declarations) {
 			if (binding.kind !== 'prop' || name.startsWith('$$')) continue;
 
@@ -258,6 +262,60 @@ export function client_component(source, analysis, options) {
 		}
 	}
 
+	if (options.legacy.componentApi) {
+		properties.push(
+			b.init('$set', b.id('$.update_legacy_props')),
+			b.init(
+				'$on',
+				b.arrow(
+					[b.id('$$event_name'), b.id('$$event_cb')],
+					b.call(
+						'$.add_legacy_event_listener',
+						b.id('$$props'),
+						b.id('$$event_name'),
+						b.id('$$event_cb')
+					)
+				)
+			)
+		);
+	} else if (options.dev) {
+		properties.push(
+			b.init(
+				'$set',
+				b.thunk(
+					b.block([
+						b.throw_error(
+							`The component shape you get when doing bind:this changed. Updating its properties via $set is no longer valid in Svelte 5. ` +
+								'See https://svelte-5-preview.vercel.app/docs/breaking-changes#components-are-no-longer-classes for more information'
+						)
+					])
+				)
+			),
+			b.init(
+				'$on',
+				b.thunk(
+					b.block([
+						b.throw_error(
+							`The component shape you get when doing bind:this changed. Listening to events via $on is no longer valid in Svelte 5. ` +
+								'See https://svelte-5-preview.vercel.app/docs/breaking-changes#components-are-no-longer-classes for more information'
+						)
+					])
+				)
+			),
+			b.init(
+				'$destroy',
+				b.thunk(
+					b.block([
+						b.throw_error(
+							`The component shape you get when doing bind:this changed. Destroying such a component via $destroy is no longer valid in Svelte 5. ` +
+								'See https://svelte-5-preview.vercel.app/docs/breaking-changes#components-are-no-longer-classes for more information'
+						)
+					])
+				)
+			)
+		);
+	}
+
 	const push_args = [b.id('$$props'), b.literal(analysis.runes)];
 	if (options.dev) push_args.push(b.id(analysis.name));
 

packages/svelte/src/internal/client/render.js

@@ -2929,3 +2929,30 @@ export function bubble_event($$props, event) {
 		fn.call(this, event);
 	}
 }
+
+/**
+ * Used to simulate `$on` on a component instance when `legacy.componentApi` is `true`
+ * @param {Record<string, any>} $$props
+ * @param {string} event_name
+ * @param {Function} event_callback
+ */
+export function add_legacy_event_listener($$props, event_name, event_callback) {
+	$$props.$$events ||= {};
+	$$props.$$events[event_name] ||= [];
+	$$props.$$events[event_name].push(event_callback);
+}
+
+/**
+ * Used to simulate `$set` on a component instance when `legacy.componentApi` is `true`.
+ * Needs component accessors so that it can call the setter of the prop. Therefore doesn't
+ * work for updating props in `$$props` or `$$restProps`.
+ * @this {Record<string, any>}
+ * @param {Record<string, any>} $$new_props
+ */
+export function update_legacy_props($$new_props) {
+	for (const key in $$new_props) {
+		if (key in this) {
+			this[key] = $$new_props[key];
+		}
+	}
+}

packages/svelte/tests/runtime-legacy/samples/binding-this-legacy-component-api/_config.js

@@ -0,0 +1,17 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	compileOptions: {
+		legacy: {
+			componentApi: true
+		}
+	},
+	html: '<button>0</button>',
+	async test({ assert, target }) {
+		const button = target.querySelector('button');
+		await button?.click();
+		await tick();
+		assert.htmlEqual(target.innerHTML, '<button>1</button>');
+	}
+});

packages/svelte/tests/runtime-legacy/samples/binding-this-legacy-component-api/main.svelte

@@ -0,0 +1,16 @@
+<script>
+	import Sub from './sub.svelte';
+	import { onMount } from 'svelte';
+
+	let count = 0;
+	let component;
+
+	onMount(() => {
+		component.$on('increment', (e) => {
+			count += e.detail;
+			component.$set({ count });
+		});
+	});
+</script>
+
+<Sub bind:this={component} />

packages/svelte/tests/runtime-legacy/samples/binding-this-legacy-component-api/sub.svelte

@@ -0,0 +1,8 @@
+<script>
+    import { createEventDispatcher } from 'svelte';
+
+    export let count = 0;
+    const dispatch = createEventDispatcher();
+</script>
+
+<button on:click={() => dispatch('increment', 1)}>{count}</button>

sites/svelte-5-preview/src/routes/docs/content/03-appendix/02-breaking-changes.md

@@ -70,7 +70,7 @@ import App from './App.svelte'
 export default app;
 ```
 
-If this component is not under your control, you can use the `legacy.componentApi` compiler option for auto-applied backwards compatibility (note that this adds a bit of overhead to each component).
+If this component is not under your control, you can use the `legacy.componentApi` compiler option for auto-applied backwards compatibility (note that this adds a bit of overhead to each component). This will also add `$set` and `$on` methods for all component instances you get through `bind:this`.
 
 ### Server API changes