feat: Add prefer-interface rule.

README.md

@@ -74,4 +74,5 @@ The package includes the following rules:
 | [`no-internal`](https://github.com/cartant/eslint-plugin-etc/blob/main/docs/rules/no-internal.md) | Forbids the use of internal APIs. | Yes |
 | [`no-misused-generics`](https://github.com/cartant/eslint-plugin-etc/blob/main/docs/rules/no-misused-generics.md) | Forbids type parameters without inference sites and type parameters that don't add type safety to declarations. This is an ESLint port of [Wotan's `no-misused-generics` rule](https://github.com/fimbullinter/wotan/blob/11368a193ba90a9e79b9f6ab530be1b434b122de/packages/mimir/docs/no-misused-generics.md). See also ["The Golden Rule of Generics"](https://effectivetypescript.com/2020/08/12/generics-golden-rule/). | Yes |
 | [`no-t`](https://github.com/cartant/eslint-plugin-etc/blob/main/docs/rules/no-t.md) | Forbids single-character type parameters. | No |
+| [`prefer-interface`](https://github.com/cartant/eslint-plugin-etc/blob/main/docs/rules/prefer-interface.md) | Forbids type aliases where interfaces can be used. | No |
 | [`throw-error`](https://github.com/cartant/eslint-plugin-etc/blob/main/docs/rules/throw-error.md) | Forbids throwing - or rejecting with - non-`Error` values. | No |

docs/rules/prefer-interface.md

@@ -0,0 +1,37 @@
+# Use interfaces instead of type aliases (`prefer-interface`)
+
+This rule effects failures for type alias declarations that can be declared as interfaces.
+
+<blockquote class="twitter-tweet"><p lang="en" dir="ltr">Honestly, my take is that it should really just be interfaces for anything that they can model. There is no benefit to type aliases when there are so many issues around display/perf.</p>&mdash; Daniel Rosenwasser (@drosenwasser) <a href="https://twitter.com/drosenwasser/status/1319205169918144513?ref_src=twsrc%5Etfw">October 22, 2020</a></blockquote> <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>
+
+## Rule details
+
+Examples of **incorrect** code for this rule:
+
+```ts
+type Person = {
+  age: number;
+  name: string;
+};
+```
+
+Examples of **correct** code for this rule:
+
+```ts
+interface Person {
+  age: number;
+  name: string;
+}
+```
+
+```ts
+type Worker = Person | Robot;
+```
+
+## Options
+
+This rule has no options.
+
+## Further reading
+
+- The [Twitter thread](https://twitter.com/robpalmer2/status/1319188885197422594) from which the above quote was taken.

source/rules/prefer-interface.ts

@@ -0,0 +1,39 @@
+/**
+ * @license Use of this source code is governed by an MIT-style license that
+ * can be found in the LICENSE file at https://github.com/cartant/eslint-plugin-etc
+ */
+
+import { TSESTree as es } from "@typescript-eslint/experimental-utils";
+import { getParent } from "eslint-etc";
+import { ruleCreator } from "../utils";
+
+const rule = ruleCreator({
+  defaultOptions: [],
+  meta: {
+    docs: {
+      category: "Best Practices",
+      description: "Forbids type aliases where interfaces can be used.",
+      recommended: false,
+    },
+    fixable: undefined,
+    messages: {
+      forbidden: "Use an interface instead of a type alias.",
+    },
+    schema: [],
+    type: "problem",
+  },
+  name: "prefer-interface",
+  create: (context) => {
+    return {
+      "TSTypeAliasDeclaration > TSTypeLiteral": (node: es.Node) => {
+        const parent = getParent(node) as es.TSTypeAliasDeclaration;
+        context.report({
+          messageId: "forbidden",
+          node: parent.id,
+        });
+      },
+    };
+  },
+});
+
+export = rule;

tests/rules/prefer-interface.ts

@@ -0,0 +1,38 @@
+/**
+ * @license Use of this source code is governed by an MIT-style license that
+ * can be found in the LICENSE file at https://github.com/cartant/eslint-plugin-etc
+ */
+
+import { stripIndent } from "common-tags";
+import { fromFixture } from "eslint-etc";
+import rule = require("../../source/rules/prefer-interface");
+import { ruleTester } from "../utils";
+
+ruleTester({ types: true }).run("prefer-interface", rule, {
+  valid: [
+    `type T = string;`,
+    `type T = string | number;`,
+    `type T = "zero";`,
+    `type T = "zero" | "one";`,
+    `type T = { length: number; } | { width: number; };`,
+    `type T = { length: number; } & { width: number; };`,
+    `type T = Set<string>;`,
+  ],
+  invalid: [
+    fromFixture(
+      stripIndent`
+        type T = { length: number; };
+             ~ [forbidden]
+      `
+    ),
+    fromFixture(
+      stripIndent`
+        type T = {
+             ~ [forbidden]
+          length: number;
+          width: number;
+        };
+      `
+    ),
+  ],
+});