feat: improve type arguments for Snippet and $bindable (#12197)

* Improve type arguments for snippet and bindable

* generate:types

* tweak

* changeset

---------

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

.changeset/tricky-laws-bathe.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+feat: improve type arguments for Snippet and $bindable

packages/svelte/src/ambient.d.ts

@@ -306,7 +306,7 @@ declare function $props(): any;
  *
  * https://svelte-5-preview.vercel.app/docs/runes#$bindable
  */
-declare function $bindable<T>(t?: T): T;
+declare function $bindable<T>(fallback?: T): T;
 
 /**
  * Inspects one or more values whenever they, or the properties they contain, change. Example:

packages/svelte/src/index.d.ts

@@ -259,20 +259,24 @@ 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 }: { banner: Snippet<{ text: string }> } = $props();
+ * let { banner }: { banner: Snippet<[{ text: string }]> } = $props();
  * ```
  * You can only call a snippet through the `{@render ...}` tag.
+ *
+ * https://svelte-5-preview.vercel.app/docs/snippets
+ *
+ * @template Parameters the parameters that the snippet expects (if any) as a tuple.
  */
-export type Snippet<T extends unknown[] = []> =
+export type Snippet<Parameters extends unknown[] = []> =
 	// this conditional allows tuples but not arrays. Arrays would indicate a
 	// rest parameter type, which is not supported. If rest parameters are added
 	// in the future, the condition can be removed.
-	number extends T['length']
+	number extends Parameters['length']
 		? never
 		: {
 				(
 					this: void,
-					...args: T
+					...args: Parameters
 				): typeof SnippetReturn & {
 					_: 'functions passed to {@render ...} tags must use the `Snippet` type imported from "svelte"';
 				};

packages/svelte/types/index.d.ts

@@ -256,20 +256,24 @@ 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 }: { banner: Snippet<{ text: string }> } = $props();
+	 * let { banner }: { banner: Snippet<[{ text: string }]> } = $props();
 	 * ```
 	 * You can only call a snippet through the `{@render ...}` tag.
+	 *
+	 * https://svelte-5-preview.vercel.app/docs/snippets
+	 *
+	 * @template Parameters the parameters that the snippet expects (if any) as a tuple.
 	 */
-	type Snippet<T extends unknown[] = []> =
+	type Snippet<Parameters extends unknown[] = []> =
 		// this conditional allows tuples but not arrays. Arrays would indicate a
 		// rest parameter type, which is not supported. If rest parameters are added
 		// in the future, the condition can be removed.
-		number extends T['length']
+		number extends Parameters['length']
 			? never
 			: {
 					(
 						this: void,
-						...args: T
+						...args: Parameters
 					): typeof SnippetReturn & {
 						_: 'functions passed to {@render ...} tags must use the `Snippet` type imported from "svelte"';
 					};
@@ -3015,7 +3019,7 @@ declare function $props(): any;
  *
  * https://svelte-5-preview.vercel.app/docs/runes#$bindable
  */
-declare function $bindable<T>(t?: T): T;
+declare function $bindable<T>(fallback?: T): T;
 
 /**
  * Inspects one or more values whenever they, or the properties they contain, change. Example: