feat: add rule to make sure that object type that is spread has exact…

… type (#391)

.README/README.md

@@ -181,6 +181,7 @@ When `true`, only checks files with a [`@flow` annotation](http://flowtype.org/d
 {"gitdown": "include", "file": "./rules/space-after-type-colon.md"}
 {"gitdown": "include", "file": "./rules/space-before-generic-bracket.md"}
 {"gitdown": "include", "file": "./rules/space-before-type-colon.md"}
+{"gitdown": "include", "file": "./rules/spread-exact-type.md"}
 {"gitdown": "include", "file": "./rules/type-id-match.md"}
 {"gitdown": "include", "file": "./rules/type-import-style.md"}
 {"gitdown": "include", "file": "./rules/union-intersection-spacing.md"}

.README/rules/spread-exact-type.md

@@ -0,0 +1,5 @@
+### `spread-exact-type`
+
+Enforce object types, that are spread to be exact type explicitly.
+
+<!-- assertions spreadExactType -->

src/index.js

@@ -34,6 +34,7 @@ import typeImportStyle from './rules/typeImportStyle';
 import unionIntersectionSpacing from './rules/unionIntersectionSpacing';
 import useFlowType from './rules/useFlowType';
 import validSyntax from './rules/validSyntax';
+import spreadExactType from './rules/spreadExactType';
 import {checkFlowFileAnnotation} from './utilities';
 
 const rules = {
@@ -66,6 +67,7 @@ const rules = {
   'space-after-type-colon': spaceAfterTypeColon,
   'space-before-generic-bracket': spaceBeforeGenericBracket,
   'space-before-type-colon': spaceBeforeTypeColon,
+  'spread-exact-type': spreadExactType,
   'type-id-match': typeIdMatch,
   'type-import-style': typeImportStyle,
   'union-intersection-spacing': unionIntersectionSpacing,
@@ -109,6 +111,7 @@ export default {
     'space-after-type-colon': 0,
     'space-before-generic-bracket': 0,
     'space-before-type-colon': 0,
+    'spread-exact-type': 0,
     'type-id-match': 0,
     'type-import-style': 0,
     'union-intersection-spacing': 0,

src/rules/spreadExactType.js

@@ -0,0 +1,33 @@
+const schema = [
+  {
+    enum: ['always', 'never'],
+    type: 'string'
+  }
+];
+
+const create = (context) => {
+  return {
+    ObjectTypeAnnotation (node) {
+      const {properties} = node;
+
+      properties.forEach((property) => {
+        const {type} = property;
+        if (type === 'ObjectTypeSpreadProperty') {
+          const {argument: {type: argumentType, id: argumentId}} = property;
+          if (
+            argumentType !== 'GenericTypeAnnotation' || argumentId.name !== '$Exact') {
+            context.report({
+              message: 'Use $Exact to make type spreading safe.',
+              node
+            });
+          }
+        }
+      });
+    }
+  };
+};
+
+export default {
+  create,
+  schema
+};

tests/rules/assertions/spreadExactType.js

@@ -0,0 +1,20 @@
+export default {
+  invalid: [
+    {
+      code: 'type bar = {...{test: string}}',
+      errors: [{message: 'Use $Exact to make type spreading safe.'}]
+    },
+    {
+      code: 'type foo = {test: number}; type bar = {...foo}',
+      errors: [{message: 'Use $Exact to make type spreading safe.'}]
+    }
+  ],
+  valid: [
+    {
+      code: 'type bar = {...$Exact<{test: string}>}'
+    },
+    {
+      code: 'type foo = {test: number}; type bar = {...$Exact<foo>}'
+    }
+  ]
+};

tests/rules/index.js

@@ -40,6 +40,7 @@ const reportingRules = [
   'space-after-type-colon',
   'space-before-generic-bracket',
   'space-before-type-colon',
+  'spread-exact-type',
   'type-id-match',
   'type-import-style',
   'union-intersection-spacing',