feat: add no-unused-props rule

.changeset/twelve-beers-talk.md

@@ -0,0 +1,5 @@
+---
+'eslint-plugin-svelte': minor
+---
+
+feat: add `no-unused-props` rule

README.md

@@ -362,6 +362,7 @@ These rules relate to better ways of doing things to help you avoid problems:
 | [svelte/no-reactive-literals](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-reactive-literals/) | don't assign literal values in reactive statements | :star::bulb: |
 | [svelte/no-svelte-internal](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-svelte-internal/) | svelte/internal will be removed in Svelte 6. | :star: |
 | [svelte/no-unused-class-name](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-unused-class-name/) | disallow the use of a class in the template without a corresponding style |  |
+| [svelte/no-unused-props](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-unused-props/) | Warns about defined Props properties that are unused | :star: |
 | [svelte/no-unused-svelte-ignore](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-unused-svelte-ignore/) | disallow unused svelte-ignore comments | :star: |
 | [svelte/no-useless-children-snippet](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-useless-children-snippet/) | disallow explicit children snippet where it's not needed | :star: |
 | [svelte/no-useless-mustaches](https://sveltejs.github.io/eslint-plugin-svelte/rules/no-useless-mustaches/) | disallow unnecessary mustache interpolations | :star::wrench: |

docs/rules.md

@@ -59,6 +59,7 @@ These rules relate to better ways of doing things to help you avoid problems:
 | [svelte/no-reactive-literals](./rules/no-reactive-literals.md)                           | don't assign literal values in reactive statements                                                                                        | :star::bulb:   |
 | [svelte/no-svelte-internal](./rules/no-svelte-internal.md)                               | svelte/internal will be removed in Svelte 6.                                                                                              | :star:         |
 | [svelte/no-unused-class-name](./rules/no-unused-class-name.md)                           | disallow the use of a class in the template without a corresponding style                                                                 |                |
+| [svelte/no-unused-props](./rules/no-unused-props.md)                                     | Warns about defined Props properties that are unused                                                                                      | :star:         |
 | [svelte/no-unused-svelte-ignore](./rules/no-unused-svelte-ignore.md)                     | disallow unused svelte-ignore comments                                                                                                    | :star:         |
 | [svelte/no-useless-children-snippet](./rules/no-useless-children-snippet.md)             | disallow explicit children snippet where it's not needed                                                                                  | :star:         |
 | [svelte/no-useless-mustaches](./rules/no-useless-mustaches.md)                           | disallow unnecessary mustache interpolations                                                                                              | :star::wrench: |

docs/rules/no-unused-props.md

@@ -0,0 +1,208 @@
+---
+pageClass: 'rule-details'
+sidebarDepth: 0
+title: 'svelte/no-unused-props'
+description: 'Warns about defined Props properties that are unused'
+---
+
+# svelte/no-unused-props
+
+> Warns about defined Props properties that are unused
+
+- :exclamation: <badge text="This rule has not been released yet." vertical="middle" type="error"> **_This rule has not been released yet._** </badge>
+- :gear: This rule is included in `"plugin:svelte/recommended"`.
+
+## :book: Rule Details
+
+This rule reports properties that are defined in Props but never used in the component code.  
+It helps to detect dead code and improve component clarity by ensuring that every declared prop is utilized.
+
+This rule checks various usage patterns of props:
+
+- Direct property access
+- Destructuring assignment
+- Method calls
+- Computed property access
+- Object spread
+- Constructor calls (new expressions)
+- Assignment to other variables
+- Index signatures (e.g. `[key: string]: unknown`)
+
+Additionally, this rule checks if index signatures are properly used. When an index signature is defined but not captured using the rest operator (`...`), the rule will suggest using it.
+
+Note: Properties of class types are not checked for usage, as they might be used in other parts of the application.
+
+:warning: This rule requires `@typescript-eslint/parser` to work. Make sure you have installed `@typescript-eslint/parser` and configured it in your ESLint configuration. Therefore, the rule violations cannot be seen in the examples on this page because this documentation does not use `@typescript-eslint/parser`.
+
+<!--eslint-skip-->
+
+```svelte
+<!-- ✓ Good Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: "error" */
+  // Direct property access
+  const props: { value: string } = $props();
+  console.log(props.value);
+</script>
+```
+
+```svelte
+<!-- ✓ Good Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: "error" */
+  // Destructuring assignment
+  const { width, height }: { width: number; height: number } = $props();
+  console.log(width, height);
+</script>
+```
+
+```svelte
+<!-- ✓ Good Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: "error" */
+  // Class properties are not checked
+  class User {
+    constructor(
+      public name: string,
+      public age: number
+    ) {}
+  }
+  type Props = {
+    user: User;
+  };
+  const props: Props = $props();
+  console.log(props.user.name); // age is not reported as unused
+</script>
+```
+
+```svelte
+<!-- ✓ Good Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: "error" */
+  // Method calls
+  const props2: { callback: () => void } = $props();
+  props2.callback();
+</script>
+```
+
+```svelte
+<!-- ✓ Good Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: "error" */
+  // Computed property access
+  const props3: { 'data-value': string } = $props();
+  const value = props3['data-value'];
+</script>
+```
+
+```svelte
+<!-- ✓ Good Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: "error" */
+  // Constructor calls
+  const props4: { config: { new (): any } } = $props();
+  new props4.config();
+</script>
+```
+
+```svelte
+<!-- ✓ Good Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: "error" */
+  // Using index signature with rest operator
+  interface Props {
+    a: number;
+    [key: string]: unknown;
+  }
+  let { a, ...rest }: Props = $props();
+  console.log(rest);
+</script>
+```
+
+```svelte
+<!-- ✗ Bad Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: "error" */
+  // Unused property 'b'
+  const props: { a: string; b: number } = $props();
+  console.log(props.a);
+</script>
+```
+
+```svelte
+<!-- ✗ Bad Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: "error" */
+  // Unused property in destructuring
+  const { x }: { x: number; y: number } = $props();
+  console.log(x);
+</script>
+```
+
+```svelte
+<!-- ✗ Bad Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: "error" */
+  // Unused index signature
+  interface Props {
+    a: number;
+    [key: string]: unknown; // This will be reported
+  }
+  let { a }: Props = $props();
+</script>
+```
+
+## :wrench: Options
+
+```js
+{
+  "svelte/no-unused-props": ["error", {
+    // Whether to check properties from imported types
+    "checkImportedTypes": false,
+    // Patterns to ignore when checking for unused props
+    "ignorePatterns": []
+  }]
+}
+```
+
+- `checkImportedTypes` ... Controls whether to check properties from imported types. Default is `false`.
+- `ignorePatterns` ... Patterns to ignore when checking for unused props. Default is an empty array.
+
+Examples:
+
+```svelte
+<!-- ✓ Good Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: ["error", { "checkImportedTypes": true }] */
+  // Check properties from imported types
+  import type { BaseProps } from './types';
+  interface Props extends BaseProps {
+    age: number;
+  }
+  let { name, age }: Props = $props();
+  console.log(name, age);
+</script>
+```
+
+```svelte
+<!-- ✓ Good Examples -->
+<script lang="ts">
+  /* eslint svelte/no-unused-props: ["error", { "ignorePatterns": ["^_"] }] */
+  // Ignore properties starting with underscore
+  interface Props {
+    _internal: string;
+    value: number;
+  }
+  let { value }: Props = $props();
+  console.log(value);
+</script>
+```
+
+## :gear: Required Configuration
+
+This rule requires `@typescript-eslint/parser` to work. Please refer to the [User Guide](../user-guide.md) for more information.
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/packages/eslint-plugin-svelte/src/rules/no-unused-props.ts)
+- [Test source](https://github.com/sveltejs/eslint-plugin-svelte/blob/main/packages/eslint-plugin-svelte/tests/src/rules/no-unused-props.ts)

packages/eslint-plugin-svelte/src/configs/flat/recommended.ts

@@ -32,6 +32,7 @@ const config: Linter.Config[] = [
 			'svelte/no-store-async': 'error',
 			'svelte/no-svelte-internal': 'error',
 			'svelte/no-unknown-style-directive-property': 'error',
+			'svelte/no-unused-props': 'error',
 			'svelte/no-unused-svelte-ignore': 'error',
 			'svelte/no-useless-children-snippet': 'error',
 			'svelte/no-useless-mustaches': 'error',

packages/eslint-plugin-svelte/src/rule-types.ts

@@ -261,6 +261,11 @@ export interface RuleOptions {
    * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/no-unused-class-name/
    */
   'svelte/no-unused-class-name'?: Linter.RuleEntry<SvelteNoUnusedClassName>
+  /**
+   * Warns about defined Props properties that are unused
+   * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/no-unused-props/
+   */
+  'svelte/no-unused-props'?: Linter.RuleEntry<SvelteNoUnusedProps>
   /**
    * disallow unused svelte-ignore comments
    * @see https://sveltejs.github.io/eslint-plugin-svelte/rules/no-unused-svelte-ignore/
@@ -512,6 +517,11 @@ type SvelteNoUnknownStyleDirectiveProperty = []|[{
 type SvelteNoUnusedClassName = []|[{
   allowedClassNames?: string[]
 }]
+// ----- svelte/no-unused-props -----
+type SvelteNoUnusedProps = []|[{
+  checkImportedTypes?: boolean
+  ignorePatterns?: string[]
+}]
 // ----- svelte/no-useless-mustaches -----
 type SvelteNoUselessMustaches = []|[{
   ignoreIncludesComment?: boolean