Add no-mixed rule

.README/README.md

@@ -53,6 +53,7 @@ npm install eslint babel-eslint eslint-plugin-flowtype --save-dev
       2,
       "never"
     ],
+    "flowtype/no-mixed": 2,
     "flowtype/no-primitive-constructor-types": 2,
     "flowtype/no-types-missing-file-annotation": 2,
     "flowtype/no-weak-types": 2,
@@ -161,6 +162,7 @@ When `true`, only checks files with a [`@flow` annotation](http://flowtype.org/d
 {"gitdown": "include", "file": "./rules/no-dupe-keys.md"}
 {"gitdown": "include", "file": "./rules/no-existential-type.md"}
 {"gitdown": "include", "file": "./rules/no-flow-fix-me-comments.md"}
+{"gitdown": "include", "file": "./rules/no-mixed.md"}
 {"gitdown": "include", "file": "./rules/no-mutable-array.md"}
 {"gitdown": "include", "file": "./rules/no-primitive-constructor-types.md"}
 {"gitdown": "include", "file": "./rules/no-types-missing-file-annotation.md"}

.README/rules/no-mixed.md

@@ -0,0 +1,8 @@
+### `no-mixed`
+
+Warns against "mixed" type annotations.
+These types are not strict enough and could often be made more specific.
+
+The following patterns are considered problems:
+
+<!-- assertions noMixed -->

src/configs/recommended.json

@@ -14,6 +14,7 @@
       2,
       "never"
     ],
+    "flowtype/no-mixed": 0,
     "flowtype/no-types-missing-file-annotation": 2,
     "flowtype/no-weak-types": 0,
     "flowtype/require-parameter-type": 0,

src/index.js

@@ -15,6 +15,7 @@ import noPrimitiveConstructorTypes from './rules/noPrimitiveConstructorTypes';
 import noTypesMissingFileAnnotation from './rules/noTypesMissingFileAnnotation';
 import noUnusedExpressions from './rules/noUnusedExpressions';
 import noWeakTypes from './rules/noWeakTypes';
+import noMixed from './rules/noMixed';
 import objectTypeDelimiter from './rules/objectTypeDelimiter';
 import requireCompoundTypeAlias from './rules/requireCompoundTypeAlias';
 import requireExactType from './rules/requireExactType';
@@ -46,6 +47,7 @@ const rules = {
   'no-dupe-keys': noDupeKeys,
   'no-existential-type': noExistentialType,
   'no-flow-fix-me-comments': noFlowFixMeComments,
+  'no-mixed': noMixed,
   'no-mutable-array': noMutableArray,
   'no-primitive-constructor-types': noPrimitiveConstructorTypes,
   'no-types-missing-file-annotation': noTypesMissingFileAnnotation,
@@ -93,6 +95,7 @@ export default {
     'newline-after-flow-annotation': 0,
     'no-dupe-keys': 0,
     'no-flow-fix-me-comments': 0,
+    'no-mixed': 0,
     'no-mutable-array': 0,
     'no-weak-types': 0,
     'object-type-delimiter': 0,

src/rules/noMixed.js

@@ -0,0 +1,17 @@
+const schema = [];
+
+const create = (context) => {
+  return {
+    MixedTypeAnnotation (node) {
+      context.report({
+        message: 'Unexpected use of mixed type',
+        node
+      });
+    }
+  };
+};
+
+export default {
+  create,
+  schema
+};

tests/rules/assertions/noMixed.js

@@ -0,0 +1,66 @@
+export default {
+  invalid: [
+    {
+      code: 'function foo(thing): mixed {}',
+      errors: [{
+        message: 'Unexpected use of mixed type'
+      }]
+    },
+    {
+      code: 'function foo(thing): Promise<mixed> {}',
+      errors: [{
+        message: 'Unexpected use of mixed type'
+      }]
+    },
+    {
+      code: 'function foo(thing): Promise<Promise<mixed>> {}',
+      errors: [{
+        message: 'Unexpected use of mixed type'
+      }]
+    }
+  ],
+  valid: [
+    {
+      code: 'function foo(thing): string {}'
+    },
+    {
+      code: 'function foo(thing): Promise<string> {}'
+    },
+    {
+      code: 'function foo(thing): Promise<Promise<string>> {}'
+    },
+    {
+      code: '(foo?: string) => {}'
+    },
+    {
+      code: '(foo: ?string) => {}'
+    },
+    {
+      code: '(foo: { a: string }) => {}'
+    },
+    {
+      code: '(foo: { a: ?string }) => {}'
+    },
+    {
+      code: '(foo: string[]) => {}'
+    },
+    {
+      code: 'type Foo = string'
+    },
+    {
+      code: 'type Foo = { a: string }'
+    },
+    {
+      code: 'type Foo = { (a: string): string }'
+    },
+    {
+      code: 'function foo(thing: string) {}'
+    },
+    {
+      code: 'var foo: string'
+    },
+    {
+      code: 'class Foo { props: string }'
+    }
+  ]
+};

tests/rules/index.js

@@ -26,6 +26,7 @@ const reportingRules = [
   'no-types-missing-file-annotation',
   'no-unused-expressions',
   'no-weak-types',
+  'no-mixed',
   'object-type-delimiter',
   'require-compound-type-alias',
   'require-exact-type',