[ESLint] Forbid top-level use*() calls

packages/eslint-plugin-react-hooks/__tests__/ESLintRulesOfHooks-test.js

@@ -19,8 +19,17 @@ ESLintTester.setDefaultConfig({
   },
 });
 
-const eslintTester = new ESLintTester();
-eslintTester.run('react-hooks', ReactHooksESLintRule, {
+// ***************************************************
+// For easier local testing, you can add to any case:
+// {
+//   skip: true,
+//   --or--
+//   only: true,
+//   ...
+// }
+// ***************************************************
+
+const tests = {
   valid: [
     `
       // Valid because components can use hooks.
@@ -223,21 +232,20 @@ eslintTester.run('react-hooks', ReactHooksESLintRule, {
       (class {i() { useState(); }});
     `,
     `
-      // Currently valid although we *could* consider these invalid.
-      // It doesn't make a lot of difference because it would crash early.
+      // Valid because they're not matching use[A-Z].
+      fooState();
       use();
       _use();
-      useState();
       _useState();
-      use42();
-      useHook();
       use_hook();
-      React.useState();
     `,
     `
-      // Regression test for the popular "history" library
-      const {createHistory, useBasename} = require('history-2.1.2');
-      const browserHistory = useBasename(createHistory)({
+      // This is grey area.
+      // Currently it's valid (although React.useCallback would fail here).
+      // We could also get stricter and disallow it, just like we did
+      // with non-namespace use*() top-level calls.
+      const History = require('history-2.1.2');
+      const browserHistory = History.useBasename(History.createHistory)({
         basename: '/',
       });
     `,
@@ -669,8 +677,63 @@ eslintTester.run('react-hooks', ReactHooksESLintRule, {
         conditionalError('useState'),
       ],
     },
+    {
+      code: `
+        // Invalid because it's dangerous and might not warn otherwise.
+        // This *must* be invalid.
+        function useHook({ bar }) {
+          let foo1 = bar && useState();
+          let foo2 = bar || useState();
+          let foo3 = bar ?? useState();
+        }
+      `,
+      errors: [
+        conditionalError('useState'),
+        conditionalError('useState'),
+        // TODO: ideally this *should* warn, but ESLint
+        // doesn't plan full support for ?? until it advances.
+        // conditionalError('useState'),
+      ],
+    },
+    {
+      code: `
+        // Invalid because it's dangerous.
+        // Normally, this would crash, but not if you use inline requires.
+        // This *must* be invalid.
+        // It's expected to have some false positives, but arguably
+        // they are confusing anyway due to the use*() convention
+        // already being associated with Hooks.
+        useState();
+        if (foo) {
+          const foo = React.useCallback(() => {});
+        }
+        useCustomHook();
+      `,
+      errors: [
+        topLevelError('useState'),
+        topLevelError('React.useCallback'),
+        topLevelError('useCustomHook'),
+      ],
+    },
+    {
+      code: `
+        // Technically this is a false positive.
+        // We *could* make it valid (and it used to be).
+        //
+        // However, top-level Hook-like calls can be very dangerous
+        // in environments with inline requires because they can mask
+        // the runtime error by accident.
+        // So we prefer to disallow it despite the false positive.
+
+        const {createHistory, useBasename} = require('history-2.1.2');
+        const browserHistory = useBasename(createHistory)({
+          basename: '/',
+        });
+      `,
+      errors: [topLevelError('useBasename')],
+    },
   ],
-});
+};
 
 function conditionalError(hook, hasPreviousFinalizer = false) {
   return {
@@ -708,3 +771,42 @@ function genericError(hook) {
       'Hook function.',
   };
 }
+
+function topLevelError(hook) {
+  return {
+    message:
+      `React Hook "${hook}" cannot be called at the top level. React Hooks ` +
+      'must be called in a React function component or a custom React ' +
+      'Hook function.',
+  };
+}
+
+// For easier local testing
+if (!process.env.CI) {
+  let only = [];
+  let skipped = [];
+  [...tests.valid, ...tests.invalid].forEach(t => {
+    if (t.skip) {
+      delete t.skip;
+      skipped.push(t);
+    }
+    if (t.only) {
+      delete t.only;
+      only.push(t);
+    }
+  });
+  const predicate = t => {
+    if (only.length > 0) {
+      return only.indexOf(t) !== -1;
+    }
+    if (skipped.length > 0) {
+      return skipped.indexOf(t) === -1;
+    }
+    return true;
+  };
+  tests.valid = tests.valid.filter(predicate);
+  tests.invalid = tests.invalid.filter(predicate);
+}
+
+const eslintTester = new ESLintTester();
+eslintTester.run('react-hooks', ReactHooksESLintRule, tests);

packages/eslint-plugin-react-hooks/src/RulesOfHooks.js

@@ -432,9 +432,13 @@ export default {
                 'React Hook function.';
               context.report({node: hook, message});
             } else if (codePathNode.type === 'Program') {
-              // For now, ignore if it's in top level scope.
               // We could warn here but there are false positives related
               // configuring libraries like `history`.
+              const message =
+                `React Hook "${context.getSource(hook)}" cannot be called ` +
+                'at the top level. React Hooks must be called in a ' +
+                'React function component or a custom React Hook function.';
+              context.report({node: hook, message});
             } else {
               // Assume in all other cases the user called a hook in some
               // random function callback. This should usually be true for