Add an ESLint rule to validate imports. (#1444)

This addresses part of #1431 by adding a new ESLint rule to ensure that our production code never imports code from our test suite.

.eslintrc

@@ -10,5 +10,8 @@
   ],
   "parserOptions": {
     "sourceType": "module"
+  },
+  "rules": {
+    "justfix-validate-import": "error"
   }
 }

frontend/eslint/rules/justfix-validate-import.js

@@ -0,0 +1,35 @@
+// @ts-check
+
+const TESTS_DIR_RE = /[\/\\]tests[\/\\]/;
+
+/**
+ * Assert that the given condition is true; propagates to TypeScript.
+ *
+ * @param {boolean} condition
+ * @returns {asserts condition}
+ */
+function assert(condition) {
+  if (!condition) {
+    throw new Error("Assertion failed!");
+  }
+}
+
+/** @type import("eslint").Rule.RuleModule */
+module.exports = {
+  create: function (context) {
+    return {
+      ImportDeclaration(node) {
+        assert(node.type === "ImportDeclaration");
+        assert(typeof node.source.value === "string");
+        const importStr = node.source.value;
+        const filename = context.getFilename();
+        if (TESTS_DIR_RE.test(importStr) && !TESTS_DIR_RE.test(filename)) {
+          context.report({
+            node,
+            message: `Production code is importing test suite code at "${importStr}"!`,
+          });
+        }
+      },
+    };
+  },
+};

frontend/eslint/tests/justfix-validate-import.test.ts

@@ -0,0 +1,28 @@
+import { RuleTester } from "eslint";
+
+import rule from "../rules/justfix-validate-import";
+
+const ruleTester = new RuleTester({
+  parserOptions: { ecmaVersion: 2015, sourceType: "module" },
+});
+
+ruleTester.run("justfix-validate-import", rule, {
+  valid: [
+    {
+      code: "import boop from '../boop'",
+      filename: "lib/tests/boop.test.ts",
+    },
+  ],
+  invalid: [
+    {
+      code: "import boop from '../tests/boop'",
+      filename: "lib/main.ts",
+      errors: [
+        {
+          message:
+            'Production code is importing test suite code at "../tests/boop"!',
+        },
+      ],
+    },
+  ],
+});

package.json

@@ -14,7 +14,7 @@
     "lint:fix": "yarn typecheck && yarn eslint && yarn prettier:fix",
     "typecheck": "tsc --noEmit",
     "typecheck:watch": "tsc --noEmit --watch --preserveWatchOutput",
-    "eslint": "eslint frontend/lib --ext .ts,.tsx",
+    "eslint": "eslint frontend/lib --ext .ts,.tsx --rulesdir frontend/eslint/rules",
     "test": "jest",
     "test:watch": "jest --watch",
     "sass_norent": "node-sass-chokidar frontend/sass/norent/styles.scss frontend/static/frontend/styles-norent.css --source-map frontend/static/frontend/styles-norent.css.map --source-map-contents --output-style compressed",
@@ -46,6 +46,7 @@
   "devDependencies": {
     "@testing-library/dom": "7.5.2",
     "@testing-library/react": "10.0.4",
+    "@types/eslint": "6.8.0",
     "@types/jest": "25.1.4",
     "@types/webpack": "4.41.0",
     "@types/webpack-bundle-analyzer": "2.13.3",

yarn.lock

@@ -1596,6 +1596,19 @@
   resolved "https://registry.yarnpkg.com/@types/eslint-visitor-keys/-/eslint-visitor-keys-1.0.0.tgz#1ee30d79544ca84d68d4b3cdb0af4f205663dd2d"
   integrity sha512-OCutwjDZ4aFS6PB1UZ988C4YgwlBHJd6wCeQqaLdmadZ/7e+w79+hbMUFC1QXDNCmdyoRfAFdm0RypzwR+Qpag==
 
+"@types/[email protected]":
+  version "6.8.0"
+  resolved "https://registry.yarnpkg.com/@types/eslint/-/eslint-6.8.0.tgz#5f2289b9f01316da7cf31c9e63109a10602a23cb"
+  integrity sha512-hqzmggoxkOubpgTdcOltkfc5N8IftRJqU70d1jbOISjjZVPvjcr+CLi2CI70hx1SUIRkLgpglTy9w28nGe2Hsw==
+  dependencies:
+    "@types/estree" "*"
+    "@types/json-schema" "*"
+
+"@types/estree@*":
+  version "0.0.44"
+  resolved "https://registry.yarnpkg.com/@types/estree/-/estree-0.0.44.tgz#980cc5a29a3ef3bea6ff1f7d021047d7ea575e21"
+  integrity sha512-iaIVzr+w2ZJ5HkidlZ3EJM8VTZb2MJLCjw3V+505yVts0gRC4UMvjw0d1HPtGqI/HQC/KdsYtayfzl+AXY2R8g==
+
 "@types/events@*":
   version "3.0.0"
   resolved "https://registry.yarnpkg.com/@types/events/-/events-3.0.0.tgz#2862f3f58a9a7f7c3e78d79f130dd4d71c25c2a7"
@@ -1658,7 +1671,7 @@
     jest-diff "^25.1.0"
     pretty-format "^25.1.0"
 
-"@types/json-schema@^7.0.3":
+"@types/json-schema@*", "@types/json-schema@^7.0.3":
   version "7.0.4"
   resolved "https://registry.yarnpkg.com/@types/json-schema/-/json-schema-7.0.4.tgz#38fd73ddfd9b55abb1e1b2ed578cb55bd7b7d339"
   integrity sha512-8+KAKzEvSUdeo+kmqnKrqgeE+LcA0tjYWFY7RPProVYwnqDjukzO+3b6dLD56rYX5TdWejnEOLJYOIeh4CXKuA==