feat(eslint-plugin): add no-unnecessary-type-constraint rule

packages/eslint-plugin/README.md

@@ -142,6 +142,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
 | [`@typescript-eslint/no-unnecessary-qualifier`](./docs/rules/no-unnecessary-qualifier.md)                             | Warns when a namespace qualifier is unnecessary                                                                         |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/no-unnecessary-type-arguments`](./docs/rules/no-unnecessary-type-arguments.md)                   | Enforces that type arguments will not be used if not required                                                           |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/no-unnecessary-type-assertion`](./docs/rules/no-unnecessary-type-assertion.md)                   | Warns if a type assertion does not change the type of an expression                                                     | :heavy_check_mark: | :wrench: | :thought_balloon: |
+| [`@typescript-eslint/no-unnecessary-type-constraint`](./docs/rules/no-unnecessary-type-constraint.md)                 | Disallows unnecessary constraints on generic types                                                                      |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/no-unsafe-assignment`](./docs/rules/no-unsafe-assignment.md)                                     | Disallows assigning any to variables and properties                                                                     | :heavy_check_mark: |          | :thought_balloon: |
 | [`@typescript-eslint/no-unsafe-call`](./docs/rules/no-unsafe-call.md)                                                 | Disallows calling an any type value                                                                                     | :heavy_check_mark: |          | :thought_balloon: |
 | [`@typescript-eslint/no-unsafe-member-access`](./docs/rules/no-unsafe-member-access.md)                               | Disallows member access on any typed variables                                                                          | :heavy_check_mark: |          | :thought_balloon: |

packages/eslint-plugin/docs/rules/no-unnecessary-type-constraint.md

@@ -0,0 +1,55 @@
+# Disallows unnecessary constraints on generic types (`no-unnecessary-type-constraint`)
+
+## Rule Details
+
+Type parameters (`<T>`) may be "constrained" with an `extends` keyword ([docs](https://www.typescriptlang.org/docs/handbook/generics.html#generic-constraints)).
+When not provided, type parameters happen to default to:
+
+- As of TypeScript 3.9: `unknown` ([docs](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-3-9.html#type-parameters-that-extend-any-no-longer-act-as-any))
+- Before that, as of 3.5: `any` ([docs](https://devblogs.microsoft.com/typescript/announcing-typescript-3-5/#breaking-changes))
+
+It is therefore redundant to `extend` from these types in later versions of TypeScript.
+
+Examples of **incorrect** code for this rule:
+
+```ts
+interface FooAny<T extends any> {}
+interface FooUnknown<T extends unknown> {}
+
+type BarAny<T extends any> = {};
+type BarUnknown<T extends unknown> = {};
+
+class BazAny<T extends any> {
+  quxUnknown<U extends unknown>() {}
+}
+
+class BazUnknown<T extends unknown> {
+  quxUnknown<U extends unknown>() {}
+}
+
+const QuuxAny = <T extends any>() => {};
+const QuuxUnknown = <T extends unknown>() => {};
+
+function QuuzAny<T extends any>() {}
+function QuuzUnknown<T extends unknown>() {}
+```
+
+Examples of **correct** code for this rule:
+
+```ts
+interface Foo<T> {}
+
+type Bar<T> = {};
+
+class Baz<T> {
+    qux<U> { }
+}
+
+const Quux = <T>() => {};
+
+function Quuz<T>() {}
+```
+
+## When Not To Use It
+
+If you don't care about the specific styles of your type constraints, or never use them in the first place, then you will not need this rule.

packages/eslint-plugin/src/configs/all.ts

@@ -87,6 +87,7 @@ export = {
     '@typescript-eslint/no-type-alias': 'error',
     '@typescript-eslint/no-unnecessary-boolean-literal-compare': 'error',
     '@typescript-eslint/no-unnecessary-condition': 'error',
+    '@typescript-eslint/no-unnecessary-type-constraint': 'error',
     '@typescript-eslint/no-unnecessary-qualifier': 'error',
     '@typescript-eslint/no-unnecessary-type-arguments': 'error',
     '@typescript-eslint/no-unnecessary-type-assertion': 'error',

packages/eslint-plugin/src/rules/index.ts

@@ -64,6 +64,7 @@ import noUnnecessaryCondition from './no-unnecessary-condition';
 import noUnnecessaryQualifier from './no-unnecessary-qualifier';
 import noUnnecessaryTypeArguments from './no-unnecessary-type-arguments';
 import noUnnecessaryTypeAssertion from './no-unnecessary-type-assertion';
+import noUnnecessaryTypeConstraint from './no-unnecessary-type-constraint';
 import noUnsafeAssignment from './no-unsafe-assignment';
 import noUnsafeCall from './no-unsafe-call';
 import noUnsafeMemberAccess from './no-unsafe-member-access';
@@ -170,6 +171,7 @@ export default {
   'no-unnecessary-qualifier': noUnnecessaryQualifier,
   'no-unnecessary-type-arguments': noUnnecessaryTypeArguments,
   'no-unnecessary-type-assertion': noUnnecessaryTypeAssertion,
+  'no-unnecessary-type-constraint': noUnnecessaryTypeConstraint,
   'no-unsafe-assignment': noUnsafeAssignment,
   'no-unsafe-call': noUnsafeCall,
   'no-unsafe-member-access': noUnsafeMemberAccess,

packages/eslint-plugin/src/rules/no-unnecessary-type-constraint.ts

@@ -0,0 +1,116 @@
+import { TSESTree } from '@typescript-eslint/experimental-utils';
+import * as semver from 'semver';
+import * as ts from 'typescript';
+import * as util from '../util';
+
+type MakeRequired<Base, Key extends keyof Base> = Omit<Base, Key> &
+  Required<Pick<Base, Key>>;
+
+type TypeParameterWithConstraint = MakeRequired<
+  TSESTree.TSTypeParameter,
+  'constraint'
+>;
+
+type KeywordFilter = (type: ts.Type) => boolean;
+
+const is3dot5 = semver.satisfies(
+  ts.version,
+  `>= 3.5.0 || >= 3.5.1-rc || >= 3.5.0-beta`,
+  {
+    includePrerelease: true,
+  },
+);
+
+const is3dot9 =
+  is3dot5 &&
+  semver.satisfies(ts.version, `>= 3.9.0 || >= 3.9.1-rc || >= 3.9.0-beta`, {
+    includePrerelease: true,
+  });
+
+export default util.createRule({
+  name: 'no-unnecessary-type-constraint',
+  meta: {
+    docs: {
+      category: 'Best Practices',
+      description: 'Disallows unnecessary constraints on generic types',
+      recommended: false,
+      requiresTypeChecking: true,
+      suggestion: true,
+    },
+    fixable: 'code',
+    messages: {
+      unnecessaryConstraint:
+        'Constraining a generic type to {{constraint}} does nothing and is unnecessary.',
+    },
+    schema: [],
+    type: 'suggestion',
+  },
+  defaultOptions: [],
+  create(context) {
+    const parserServices = util.getParserServices(context);
+    const checker = parserServices.program.getTypeChecker();
+
+    const keywordFilters: [KeywordFilter, string][] = [];
+    if (is3dot5) {
+      keywordFilters.push([util.isTypeUnknownType, 'unknown']);
+
+      if (is3dot9) {
+        keywordFilters.push([util.isTypeAnyType, 'any']);
+      }
+    }
+
+    if (!keywordFilters.length) {
+      return {};
+    }
+
+    const inJsx = context.getFilename().toLowerCase().endsWith('tsx');
+
+    const report = (
+      node: TypeParameterWithConstraint,
+      constraint: string,
+      inArrowFunction: boolean,
+    ): void => {
+      context.report({
+        data: { constraint },
+        fix(fixer) {
+          return fixer.replaceTextRange(
+            [node.name.range[1], node.constraint.range[1]],
+            inArrowFunction && inJsx ? ',' : '',
+          );
+        },
+        messageId: 'unnecessaryConstraint',
+        node,
+      });
+    };
+
+    const checkNode = (
+      node: TypeParameterWithConstraint,
+      inArrowFunction: boolean,
+    ): void => {
+      const constraint = parserServices.esTreeNodeToTSNodeMap.get(
+        node.constraint,
+      );
+      const constraintType = checker.getTypeAtLocation(constraint);
+
+      for (const [filter, type] of keywordFilters) {
+        if (filter(constraintType)) {
+          report(node, type, inArrowFunction);
+          return;
+        }
+      }
+    };
+
+    return {
+      ':not(ArrowFunctionExpression) > TSTypeParameterDeclaration > TSTypeParameter[constraint]'(
+        node: TypeParameterWithConstraint,
+      ): void {
+        checkNode(node, false);
+      },
+      'ArrowFunctionExpression > TSTypeParameterDeclaration > TSTypeParameter[constraint]'(
+        node: TypeParameterWithConstraint,
+      ): void {
+        checkNode(node, true);
+      },
+    };
+  },
+});