Add lint rule

From #3835

Author: devongovett

.eslintrc.js

@@ -169,6 +169,7 @@ module.exports = {
     'rulesdir/sort-imports': [ERROR],
     'rulesdir/imports': [ERROR],
     'rulesdir/useLayoutEffectRule': [ERROR],
+    'rulesdir/pure-render': [ERROR],
 
     // jsx-a11y rules
     'jsx-a11y/accessible-emoji': ERROR,

bin/pure-render.js

@@ -0,0 +1,189 @@
+/*
+ * Copyright 2020 Adobe. All rights reserved.
+ * This file is licensed to you under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License. You may obtain a copy
+ * of the License at http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under
+ * the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR REPRESENTATIONS
+ * OF ANY KIND, either express or implied. See the License for the specific language
+ * governing permissions and limitations under the License.
+ */
+
+// Copied from https://github.com/facebook/react/pull/24506
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'enforces component render purity',
+      recommended: true,
+      url: 'https://beta.reactjs.org/learn/keeping-components-pure'
+    }
+  },
+  create(context) {
+    return {
+      MemberExpression(member) {
+        // Look for member expressions that look like refs (i.e. `ref.current`).
+        if (
+          member.object.type !== 'Identifier' ||
+          member.computed ||
+          member.property.type !== 'Identifier' ||
+          member.property.name !== 'current'
+        ) {
+          return;
+        }
+
+        // Find the parent function of this node, as well as any if statement matching against the ref value
+        // (i.e. lazy init pattern shown in React docs).
+        let node = member;
+        let fn;
+        let conditional;
+        while (node) {
+          if (
+            node.type === 'FunctionDeclaration' ||
+            node.type === 'FunctionExpression' ||
+            node.type === 'ArrowFunctionExpression'
+          ) {
+            fn = node;
+            break;
+          }
+
+          if (
+            node.type === 'IfStatement' &&
+            node.test.type === 'BinaryExpression' &&
+            (node.test.operator === '==' || node.test.operator === '===') &&
+            isMemberExpressionEqual(node.test.left, member)
+          ) {
+            conditional = node.test;
+          }
+
+          node = node.parent;
+        }
+
+        if (!fn) {
+          return;
+        }
+
+        // Find the variable definition for the object.
+        const variable = getVariable(context.getScope(), member.object.name);
+        if (!variable) {
+          return;
+        }
+
+        // Find the initialization of the variable and see if it's a call to useRef.
+        const refDefinition = variable.defs.find(def => {
+          const init = def.node.init;
+          if (!init) {
+            return false;
+          }
+
+          return (
+            init.type === 'CallExpression' &&
+            ((init.callee.type === 'Identifier' &&
+              init.callee.name === 'useRef') ||
+              (init.callee.type === 'MemberExpression' &&
+                init.callee.object.type === 'Identifier' &&
+                init.callee.object.name === 'React' &&
+                init.callee.property.type === 'Identifier' &&
+                init.callee.property.name === 'useRef')) &&
+            parentFunction(def.node) === fn
+          );
+        });
+
+        if (refDefinition) {
+          // If within an if statement, check if comparing with the initial value passed to useRef.
+          // This indicates the lazy init pattern, which is allowed.
+          if (conditional) {
+            const init = refDefinition.node.init.arguments[0] || {
+              type: 'Identifier',
+              name: 'undefined'
+            };
+            if (isLiteralEqual(conditional.operator, init, conditional.right)) {
+              return;
+            }
+          }
+
+          // Otherwise, report an error for either writing or reading to this ref based on parent expression.
+          context.report({
+            node: member,
+            message:
+              member.parent.type === 'AssignmentExpression' &&
+              member.parent.left === member
+                ? 'Writing to refs during rendering is not allowed. Move this into a useEffect or useLayoutEffect. See https://beta.reactjs.org/apis/useref'
+                : 'Reading from refs during rendering is not allowed. See https://beta.reactjs.org/apis/useref'
+          });
+        }
+      }
+    };
+  }
+};
+
+function getVariable(scope, name) {
+  while (scope) {
+    const variable = scope.set.get(name);
+    if (variable) {
+      return variable;
+    }
+
+    scope = scope.upper;
+  }
+}
+
+function parentFunction(node) {
+  while (node) {
+    if (
+      node.type === 'FunctionDeclaration' ||
+      node.type === 'FunctionExpression' ||
+      node.type === 'ArrowFunctionExpression'
+    ) {
+      return node;
+    }
+
+    node = node.parent;
+  }
+}
+
+function isMemberExpressionEqual(a, b) {
+  if (a === b) {
+    return true;
+  }
+
+  return (
+    a.type === 'MemberExpression' &&
+    b.type === 'MemberExpression' &&
+    a.object.type === 'Identifier' &&
+    b.object.type === 'Identifier' &&
+    a.object.name === b.object.name &&
+    a.property.type === 'Identifier' &&
+    b.property.type === 'Identifier' &&
+    a.property.name === b.property.name
+  );
+}
+
+function isLiteralEqual(operator, a, b) {
+  let aValue, bValue;
+  if (a.type === 'Identifier' && a.name === 'undefined') {
+    aValue = undefined;
+  } else if (a.type === 'Literal') {
+    aValue = a.value;
+  } else {
+    return;
+  }
+
+  if (b.type === 'Identifier' && b.name === 'undefined') {
+    bValue = undefined;
+  } else if (b.type === 'Literal') {
+    bValue = b.value;
+  } else {
+    return;
+  }
+
+  if (operator === '===') {
+    return aValue === bValue;
+  } else if (operator === '==') {
+    // eslint-disable-next-line
+    return aValue == bValue;
+  }
+
+  return false;
+}