Add jsx-uses-react rule

This rule somewhat unusual, in that it modifies the behaviour
of a core eslint rule. The alternative would be to copy-paste
the original rule and modify it. That would require users to
disable the core rule and replace with this one

docs/rules/jsx-uses-react.md

@@ -0,0 +1,29 @@
+# Make JSX count towards use of a declared variable (jsx-uses-react)
+
+JSX expands to a call to `React.createElement`, a file which includes `React`
+but only uses JSX should still consider the `React` variable used.
+
+This rule has no effect if the `no-unused-vars` rule is not enabled.
+
+## Rule Details
+
+The following patterns are considered warnings:
+
+```js
+var React = require('react'); // and other equivalent imports
+
+// nothing to do with React
+```
+
+The following patterns are not considered warnings:
+
+```js
+var React = require('react');
+
+var elem = <div>Some Stuff</div>;
+```
+
+## When Not To Use It
+
+If you are not using JSX, or React is delcared as global variable, this rule
+will not be useful.

index.js

@@ -2,6 +2,7 @@
 
 module.exports = {
   rules: {
+    'jsx-uses-react': require('./lib/rules/jsx-uses-react'),
     'no-multi-comp': require('./lib/rules/no-multi-comp'),
     'prop-types': require('./lib/rules/prop-types'),
     'display-name': require('./lib/rules/display-name'),
@@ -11,6 +12,7 @@ module.exports = {
     'no-did-update-set-state': require('./lib/rules/no-did-update-set-state')
   },
   rulesConfig: {
+    'jsx-uses-react': 0,
     'no-multi-comp': 0,
     'prop-types': 0,
     'display-name': 0,

lib/rules/jsx-uses-react.js

@@ -0,0 +1,44 @@
+/**
+ * @fileoverview Count <Jsx /> as use of the React variable
+ * @author Glen Mailer
+ */
+'use strict';
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = function(context) {
+
+  function flagReactAsUsedInJSX() {
+    var scope = context.getScope(),
+      variables = scope.variables,
+      i,
+      len;
+
+    while (scope.type !== "global") {
+      scope = scope.upper;
+      variables = [].concat.apply(scope.variables, variables);
+    }
+
+    // mark first React found with the same special flag used by no-unused-vars
+    for (i = 0, len = variables.length; i < len; i++) {
+      if (variables[i].name === 'React') {
+        variables[i].eslintJSXUsed = true;
+        return;
+      }
+    }
+  }
+
+  // --------------------------------------------------------------------------
+  // Public
+  // --------------------------------------------------------------------------
+
+  return {
+
+    'JSXOpeningElement': function() {
+      flagReactAsUsedInJSX();
+    }
+  };
+
+};

tests/lib/rules/jsx-uses-react.js

@@ -0,0 +1,31 @@
+/**
+ * @fileoverview Tests for jsx-uses-react
+ * @author Glen Mailer
+ */
+
+"use strict";
+
+// -----------------------------------------------------------------------------
+// Requirements
+// -----------------------------------------------------------------------------
+
+var eslint = require('eslint').linter;
+var ESLintTester = require('eslint-tester');
+
+// -----------------------------------------------------------------------------
+// Tests
+// -----------------------------------------------------------------------------
+
+var eslintTester = new ESLintTester(eslint);
+eslint.defineRule('jsx-uses-react', require('../../../lib/rules/jsx-uses-react'));
+eslintTester.addRuleTest("node_modules/eslint/lib/rules/no-unused-vars", {
+    valid: [
+        {code: "/*eslint jsx-uses-react:1*/ var App, React; <App />;", ecmaFeatures: {jsx: true}},
+        {code: "/*eslint jsx-uses-react:1*/ var React; <div />;", ecmaFeatures: {jsx: true}},
+        {code: "/*eslint jsx-uses-react:1*/ var React; (function () { <div /> })();", ecmaFeatures: {jsx: true}}
+    ],
+    invalid: [
+        {code: "/*eslint jsx-uses-react:1*/ var React;",
+         errors: [{message: "React is defined but never used"}], ecmaFeatures: {jsx: true}}
+    ]
+});