breaking: make `$props()` rune non-generic (#10694)

* breaking: make `$props()` rune non-generic

* explain why type argument was removed

---------

Co-authored-by: Rich Harris <rich.harris@vercel.com>

.changeset/fair-spies-repeat.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+breaking: make `$props()` rune non-generic

packages/svelte/elements.d.ts

@@ -67,7 +67,7 @@ export type ToggleEventHandler<T extends EventTarget> = EventHandler<ToggleEvent
 
 export interface DOMAttributes<T extends EventTarget> {
 	// Implicit children prop every element has
-	// Add this here so that libraries doing `$props<HTMLButtonAttributes>()` don't need a separate interface
+	// Add this here so that libraries doing `let { ...props }: HTMLButtonAttributes = $props()` don't need a separate interface
 	children?: import('svelte').Snippet;
 
 	// Clipboard Events

packages/svelte/src/main/ambient.d.ts

@@ -172,12 +172,12 @@ declare namespace $effect {
  * Declares the props that a component accepts. Example:
  *
  * ```ts
- * let { optionalProp = 42, requiredProp } = $props<{ optionalProp?: number; requiredProps: string}>();
+ * let { optionalProp = 42, requiredProp }: { optionalProp?: number; requiredProps: string } = $props();
  * ```
  *
  * https://svelte-5-preview.vercel.app/docs/runes#$props
  */
-declare function $props<T>(): T;
+declare function $props(): any;
 
 /**
  * Inspects one or more values whenever they, or the properties they contain, change. Example:

packages/svelte/src/main/public.d.ts

@@ -186,7 +186,7 @@ declare const SnippetReturn: unique symbol;
 /**
  * The type of a `#snippet` block. You can use it to (for example) express that your component expects a snippet of a certain type:
  * ```ts
- * let { banner } = $props<{ banner: Snippet<{ text: string }> }>();
+ * let { banner }: { banner: Snippet<{ text: string }> } = $props();
  * ```
  * You can only call a snippet through the `{@render ...}` tag.
  */

packages/svelte/types/index.d.ts

@@ -187,7 +187,7 @@ declare module 'svelte' {
 	/**
 	 * The type of a `#snippet` block. You can use it to (for example) express that your component expects a snippet of a certain type:
 	 * ```ts
-	 * let { banner } = $props<{ banner: Snippet<{ text: string }> }>();
+	 * let { banner }: { banner: Snippet<{ text: string }> } = $props();
 	 * ```
 	 * You can only call a snippet through the `{@render ...}` tag.
 	 */
@@ -1888,7 +1888,7 @@ declare module 'svelte/legacy' {
 	/**
 	 * The type of a `#snippet` block. You can use it to (for example) express that your component expects a snippet of a certain type:
 	 * ```ts
-	 * let { banner } = $props<{ banner: Snippet<{ text: string }> }>();
+	 * let { banner }: { banner: Snippet<{ text: string }> } = $props();
 	 * ```
 	 * You can only call a snippet through the `{@render ...}` tag.
 	 */
@@ -2624,12 +2624,12 @@ declare namespace $effect {
  * Declares the props that a component accepts. Example:
  *
  * ```ts
- * let { optionalProp = 42, requiredProp } = $props<{ optionalProp?: number; requiredProps: string}>();
+ * let { optionalProp = 42, requiredProp }: { optionalProp?: number; requiredProps: string } = $props();
  * ```
  *
  * https://svelte-5-preview.vercel.app/docs/runes#$props
  */
-declare function $props<T>(): T;
+declare function $props(): any;
 
 /**
  * Inspects one or more values whenever they, or the properties they contain, change. Example:

sites/svelte-5-preview/src/routes/docs/content/01-api/02-runes.md

@@ -454,14 +454,23 @@ To get all properties, use rest syntax:
 let { a, b, c, ...everythingElse } = $props();
 ```
 
-If you're using TypeScript, you can use type arguments:
+If you're using TypeScript, you can declare the prop types:
 
 ```ts
 type MyProps = any;
 // ---cut---
-let { a, b, c, ...everythingElse } = $props<MyProps>();
+let { a, b, c, ...everythingElse }: MyProps = $props();
 ```
 
+> In an earlier preview, `$props()` took a type argument. This caused bugs, since in a case like this...
+>
+> ```ts
+> // @errors: 2558
+> let { x = 42 } = $props<{ x: string }>();
+> ```
+>
+> ...TypeScript [widens the type](https://www.typescriptlang.org/play?#code/CYUwxgNghgTiAEAzArgOzAFwJYHtXwBIAHGHIgZwB4AVeAXnilQE8A+ACgEoAueagbgBQgiCAzwA3vAAe9eABYATPAC+c4qQqUp03uQwwsqAOaqOnIfCsB6a-AB6AfiA) of `x` to be `string | number`, instead of erroring.
+
 Props cannot be mutated, unless the parent component uses `bind:`. During development, attempts to mutate props will result in an error.
 
 ### What this replaces

sites/svelte-5-preview/src/routes/docs/content/01-api/03-snippets.md

@@ -229,11 +229,11 @@ Snippets implement the `Snippet` interface imported from `'svelte'`:
 +<script lang="ts">
 +	import type { Snippet } from 'svelte';
 +
-+	let { data, children, row } = $props<{
++	let { data, children, row }: {
 +		data: any[];
 +		children: Snippet;
 +		row: Snippet<[any]>;
-+	}>();
++	} = $props();
 </script>
 ```
 
@@ -246,13 +246,13 @@ We can tighten things up further by declaring a generic, so that `data` and `row
 +<script lang="ts" generics="T">
 	import type { Snippet } from 'svelte';
 
-	let { data, children, row } = $props<{
+	let { data, children, row }: {
 -		data: any[];
 +		data: T[];
 		children: Snippet;
 -		row: Snippet<[any]>;
 +		row: Snippet<[T]>;
-	}>();
+	} = $props();
 </script>
 ```
 

sites/svelte-5-preview/src/routes/docs/render.js

@@ -166,7 +166,7 @@ const render_content = (filename, body) =>
 		twoslashBanner: (filename, source) => {
 			const injected = [
 				`// @filename: runes.d.ts`,
-				`declare function $props<T>(): T`,
+				`declare function $props(): any`,
 				`declare function $state<T>(initial: T): T`,
 				`declare function $derived<T>(value: T): T`,
 				`declare const $effect: ((callback: () => void | (() => void)) => void) & { pre: (callback: () => void | (() => void)) => void };`