New: prefer-placeholders rule

Author: not-an-aardvark

Diff for: README.md

@@ -53,6 +53,7 @@ Name | ✔️ | 🛠 | Description
 [require-meta-fixable](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/require-meta-fixable.md) | ✔️ | | Requires a `meta.fixable` property for fixable rules
 [no-missing-placeholders](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/no-missing-placeholders.md) | ✔️ | | Disallows missing placeholders in rule report messages
 [test-case-shorthand-strings](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/test-case-shorthand-strings.md) | | | Enforces consistent usage of shorthand strings for test cases with no options
+[prefer-placeholders](https://github.com/not-an-aardvark/eslint-plugin-eslint-plugin/blob/master/docs/rules/prefer-placeholders.md) | | | Disallows template literals as report messages
 
 ## Supported Presets
 

Diff for: docs/rules/prefer-placeholders.md

@@ -0,0 +1,64 @@
+# disallow template literals as report messages (prefer-placeholders)
+
+Report messages in rules can have placeholders surrounded by curly brackets.
+
+```js
+context.report({
+  node,
+  message: '{{disallowedNode}} nodes are not allowed.',
+  data: { disallowedNode: node.type }
+});
+```
+
+Using placeholders is often preferred over using dynamic report messages, for a few reasons:
+
+* They can help enforce a separation of the message and the data.
+* It will be easier to migrate when ESLint starts supporting placing lint messages in metadata (see [#6740](https://github.com/eslint/eslint/issues/6740))
+
+## Rule Details
+
+This rule aims to report string concatenation and template literals in report messages.
+
+Examples of **incorrect** code for this rule:
+
+```js
+/* eslint eslint-plugin/prefer-placeholders: error */
+
+module.exports = {
+  create(context) {
+    context.report({
+      node,
+      message: `The node ${node.name} is not allowed to be used.`
+    });
+
+    context.report({
+      node,
+      message: 'The node ' + node.name + ' is not allowed to be used.'
+    });
+  }
+};
+```
+
+Examples of **correct** code for this rule:
+
+```js
+/* eslint eslint-plugin/prefer-placeholders: error */
+
+module.exports = {
+  create(context) {
+    context.report({
+      node,
+      message: 'The node {{name}} is not allowed to be used.',
+      data: { name: node.name }
+    });
+  }
+};
+```
+
+## When Not To Use It
+
+If you need to use string concatenation in your report messages for some reason, don't turn on this rule.
+
+## Further Reading
+
+* [context.report() API](http://eslint.org/docs/developer-guide/working-with-rules#contextreport)

Diff for: lib/rules/prefer-placeholders.js

@@ -0,0 +1,59 @@
+/**
+ * @fileoverview disallow template literals as report messages
+ * @author Teddy Katz
+ */
+
+'use strict';
+
+const utils = require('../utils');
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'disallow template literals as report messages',
+      category: 'Rules',
+      recommended: false,
+    },
+    fixable: null,
+    schema: [],
+  },
+
+  create (context) {
+    let contextIdentifiers;
+
+    // ----------------------------------------------------------------------
+    // Public
+    // ----------------------------------------------------------------------
+
+    return {
+      Program (node) {
+        contextIdentifiers = utils.getContextIdentifiers(context, node);
+      },
+      CallExpression (node) {
+        if (
+          node.callee.type === 'MemberExpression' &&
+          contextIdentifiers.has(node.callee.object) &&
+          node.callee.property.type === 'Identifier' && node.callee.property.name === 'report'
+        ) {
+          const reportInfo = utils.getReportInfo(node.arguments);
+
+          if (
+            reportInfo && reportInfo.message && (
+              (reportInfo.message.type === 'TemplateLiteral' && reportInfo.message.expressions.length) ||
+              (reportInfo.message.type === 'BinaryExpression' && reportInfo.message.operator === '+')
+            )
+          ) {
+            context.report({
+              node: reportInfo.message,
+              message: 'Use report message placeholders instead of string concatenation.',
+            });
+          }
+        }
+      },
+    };
+  },
+};

Diff for: tests/lib/rules/prefer-placeholders.js

@@ -0,0 +1,103 @@
+/**
+ * @fileoverview disallow template literals as report messages
+ * @author Teddy Katz
+ */
+
+'use strict';
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/prefer-placeholders');
+const RuleTester = require('eslint').RuleTester;
+
+// ------------------------------------------------------------------------------
+// Tests
+// ------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester({ parserOptions: { ecmaVersion: 6 } });
+const ERROR = { message: 'Use report message placeholders instead of string concatenation.' };
+
+
+ruleTester.run('prefer-placeholders', rule, {
+  valid: [
+    `
+      module.exports = {
+        create(context) {
+          context.report({
+            node,
+            message: '{{foo}} is bad.',
+            data: { foo },
+          });
+        }
+      };
+    `,
+    `
+      module.exports = {
+        create(context) {
+          context.report({
+            node,
+            message: 'foo is bad.'
+          });
+        }
+      };
+    `,
+    `
+      module.exports = {
+        create(context) {
+          context.report({
+            node,
+            message: foo
+          });
+        }
+      };
+    `,
+    `
+      module.exports = {
+        create(context) {
+          context.report(node, 'foo is bad.');
+        }
+      };
+    `,
+  ],
+
+  invalid: [
+    {
+      code: `
+        module.exports = {
+          create(context) {
+            context.report({
+              node,
+              message: \`\${foo} is bad.\`
+            });
+          }
+        };
+      `,
+      errors: [ERROR],
+    },
+    {
+      code: `
+        module.exports = {
+          create(context) {
+            context.report({
+              node,
+              message: foo + ' is bad.'
+            });
+          }
+        };
+      `,
+      errors: [ERROR],
+    },
+    {
+      code: `
+        module.exports = {
+          create(context) {
+            context.report(node, \`\${foo} is bad.\`);
+          }
+        };
+      `,
+      errors: [ERROR],
+    },
+  ],
+});