feat: Added the experimental-require-slot-types rule (#368)

Co-authored-by: ota-meshi <otameshiyo23@gmail.com>

.changeset/strong-wombats-worry.md

@@ -0,0 +1,5 @@
+---
+"eslint-plugin-svelte": minor
+---
+
+feat: added the `svelte/experimental-require-slot-types` rule

README.md

@@ -379,6 +379,7 @@ These rules extend the rules provided by ESLint itself, or other plugins to work
 
 | Rule ID | Description |    |
 |:--------|:------------|:---|
+| [svelte/experimental-require-slot-types](https://ota-meshi.github.io/eslint-plugin-svelte/rules/experimental-require-slot-types/) | require slot type declaration using the `$$Slots` interface |  |
 | [svelte/experimental-require-strict-events](https://ota-meshi.github.io/eslint-plugin-svelte/rules/experimental-require-strict-events/) | require the strictEvents attribute on `<script>` tags |  |
 
 ## System

docs/rules.md

@@ -98,6 +98,7 @@ These rules extend the rules provided by ESLint itself, or other plugins to work
 
 | Rule ID | Description |    |
 |:--------|:------------|:---|
+| [svelte/experimental-require-slot-types](./rules/experimental-require-slot-types.md) | require slot type declaration using the `$$Slots` interface |  |
 | [svelte/experimental-require-strict-events](./rules/experimental-require-strict-events.md) | require the strictEvents attribute on `<script>` tags |  |
 
 ## System

docs/rules/experimental-require-slot-types.md

@@ -0,0 +1,113 @@
+---
+pageClass: "rule-details"
+sidebarDepth: 0
+title: "svelte/experimental-require-slot-types"
+description: "require slot type declaration using the `$$Slots` interface"
+---
+
+# svelte/experimental-require-slot-types
+
+> require slot type declaration using the `$$Slots` interface
+
+- :exclamation: <badge text="This rule has not been released yet." vertical="middle" type="error"> **_This rule has not been released yet._** </badge>
+
+## :book: Rule Details
+
+This rule enforces the presence of the `$$Slots` interface if any slots are present in the component. This interface declares all of the used slots and their props and enables typechecking both in the component itself as well as all components that include it.
+The  `$$Slots` interface is experimental and is documented in [svelte RFC #38](https://github.com/dummdidumm/rfcs/blob/ts-typedefs-within-svelte-components/text/ts-typing-props-slots-events.md#typing-slots).
+
+<ESLintCodeBlock>
+
+<!--eslint-skip-->
+
+```svelte
+<!-- ✓ GOOD -->
+<script>
+  /* eslint svelte/experimental-require-slot-types: "error" */
+</script>
+
+<b>No slots here!</b>
+```
+
+</ESLintCodeBlock>
+
+<ESLintCodeBlock>
+
+<!--eslint-skip-->
+
+```svelte
+<!-- ✓ GOOD -->
+<script>
+  /* eslint svelte/experimental-require-slot-types: "error" */
+
+  interface $$Slots {
+    default: Record<string, never>;
+  }
+</script>
+
+<slot />
+```
+
+</ESLintCodeBlock>
+
+<ESLintCodeBlock>
+
+<!--eslint-skip-->
+
+```svelte
+<!-- ✓ GOOD -->
+<script lang="ts">
+  /* eslint svelte/experimental-require-slot-types: "error" */
+
+  interface $$Slots {
+    default: { prop: boolean; };
+  }
+</script>
+
+<slot prop={true} />
+```
+
+</ESLintCodeBlock>
+
+<ESLintCodeBlock>
+
+<!--eslint-skip-->
+
+```svelte
+<!-- ✓ GOOD -->
+<script lang="ts">
+  /* eslint svelte/experimental-require-slot-types: "error" */
+
+  interface $$Slots {
+    named: Record<string, never>;
+  }
+</script>
+
+<slot name = "named" />
+```
+
+</ESLintCodeBlock>
+
+<ESLintCodeBlock>
+
+<!--eslint-skip-->
+
+```svelte
+<!-- ✗ BAD -->
+<script>
+  /* eslint svelte/experimental-require-slot-types: "error" */
+</script>
+
+<slot />
+```
+
+</ESLintCodeBlock>
+
+## :wrench: Options
+
+Nothing.
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/ota-meshi/eslint-plugin-svelte/blob/main/src/rules/experimental-require-slot-types.ts)
+- [Test source](https://github.com/ota-meshi/eslint-plugin-svelte/blob/main/tests/src/rules/experimental-require-slot-types.ts)

src/rules/experimental-require-slot-types.ts

@@ -0,0 +1,50 @@
+import { createRule } from "../utils"
+import { getLangValue } from "../utils/ast-utils"
+
+export default createRule("experimental-require-slot-types", {
+  meta: {
+    docs: {
+      description:
+        "require slot type declaration using the `$$Slots` interface",
+      category: "Experimental",
+      recommended: false,
+    },
+    schema: [],
+    messages: {
+      missingSlotsInterface: `The component must define the $$Slots interface.`,
+    },
+    type: "suggestion",
+  },
+  create(context) {
+    let isTs = false
+    let hasSlot = false
+    let hasInterface = false
+    return {
+      SvelteScriptElement(node) {
+        const lang = getLangValue(node)?.toLowerCase()
+        isTs = lang === "ts" || lang === "typescript"
+      },
+      SvelteElement(node) {
+        if (node.name.type === "SvelteName" && node.name.name === "slot") {
+          hasSlot = true
+        }
+      },
+      TSInterfaceDeclaration(node) {
+        if (node.id.name === "$$Slots") {
+          hasInterface = true
+        }
+      },
+      "Program:exit"() {
+        if (isTs && hasSlot && !hasInterface) {
+          context.report({
+            loc: {
+              line: 1,
+              column: 1,
+            },
+            messageId: "missingSlotsInterface",
+          })
+        }
+      },
+    }
+  },
+})

src/utils/rules.ts

@@ -3,6 +3,7 @@ import typescriptEslintNoUnnecessaryCondition from "../rules/@typescript-eslint/
 import buttonHasType from "../rules/button-has-type"
 import commentDirective from "../rules/comment-directive"
 import derivedHasSameInputsOutputs from "../rules/derived-has-same-inputs-outputs"
+import experimentalRequireSlotTypes from "../rules/experimental-require-slot-types"
 import experimentalRequireStrictEvents from "../rules/experimental-require-strict-events"
 import firstAttributeLinebreak from "../rules/first-attribute-linebreak"
 import htmlClosingBracketSpacing from "../rules/html-closing-bracket-spacing"
@@ -56,6 +57,7 @@ export const rules = [
   buttonHasType,
   commentDirective,
   derivedHasSameInputsOutputs,
+  experimentalRequireSlotTypes,
   experimentalRequireStrictEvents,
   firstAttributeLinebreak,
   htmlClosingBracketSpacing,

tests/fixtures/rules/experimental-require-slot-types/invalid/no-slot-types01-errors.yaml

@@ -0,0 +1,4 @@
+- message: The component must define the $$Slots interface.
+  line: 1
+  column: 2
+  suggestions: null

tests/fixtures/rules/experimental-require-slot-types/invalid/no-slot-types01-input.svelte

@@ -0,0 +1,4 @@
+<script lang="ts">
+</script>
+
+<slot />

tests/fixtures/rules/experimental-require-slot-types/valid/has-slot-types01-input.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+  interface $$Slots {
+    defalt: Record<string, never>
+  }
+</script>
+
+<slot />

tests/fixtures/rules/experimental-require-slot-types/valid/named-slot01-input.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+  interface $$Slots {
+    named: Record<string, never>
+  }
+</script>
+
+<slot name="named" />

tests/fixtures/rules/experimental-require-slot-types/valid/no-slots01-input.svelte

@@ -0,0 +1,4 @@
+<script lang="ts">
+</script>
+
+content

tests/fixtures/rules/experimental-require-slot-types/valid/no-typescript01-input.svelte

@@ -0,0 +1,4 @@
+<script>
+</script>
+
+<slot />

tests/src/rules/experimental-require-slot-types.ts

@@ -0,0 +1,16 @@
+import { RuleTester } from "eslint"
+import rule from "../../../src/rules/experimental-require-slot-types"
+import { loadTestCases } from "../../utils/utils"
+
+const tester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 2020,
+    sourceType: "module",
+  },
+})
+
+tester.run(
+  "experimental-require-slot-types",
+  rule as any,
+  loadTestCases("experimental-require-slot-types"),
+)