tools: add lint fixer for require-buffer

Adds `require-buffer` lint fixer.
If the buffer module is not required while `Buffer` is used, require the `buffer` module.
If the file has a `'use strict';` line, add the require after it on a separate line.
If the file does not have any (currently impossible with the `strict` rule) add it after the first comment (before the real code starts).

Fixes: #16636
PR-URL: #17144
Reviewed-By: Michaël Zasso <targos@protonmail.com>

test/parallel/test-eslint-require-buffer.js

@@ -12,15 +12,36 @@ const ruleTester = new RuleTester({
 const message = "Use const Buffer = require('buffer').Buffer; " +
                 'at the beginning of this file';
 
+const useStrict = '\'use strict\';\n\n';
+const bufferModule = 'const { Buffer } = require(\'buffer\');\n';
+const mockComment = '// Some Comment\n//\n// Another Comment\n\n';
+const useBuffer = 'Buffer;';
 ruleTester.run('require-buffer', rule, {
   valid: [
     'foo',
-    'const Buffer = require("Buffer"); Buffer;'
+    'const Buffer = require("Buffer"); Buffer;',
+    'const { Buffer } = require(\'buffer\'); Buffer;',
   ],
   invalid: [
     {
-      code: 'Buffer;',
-      errors: [{ message }]
-    }
+      code: useBuffer,
+      errors: [{ message }],
+      output: bufferModule + useBuffer,
+    },
+    {
+      code: useStrict + useBuffer,
+      errors: [{ message }],
+      output: useStrict + bufferModule + useBuffer,
+    },
+    {
+      code:  mockComment + useBuffer,
+      errors: [{ message }],
+      output:  mockComment + bufferModule + useBuffer,
+    },
+    {
+      code:  mockComment + useStrict + useBuffer,
+      errors: [{ message }],
+      output:  mockComment + useStrict + bufferModule + useBuffer,
+    },
   ]
 });

tools/eslint-rules/require-buffer.js

@@ -1,10 +1,26 @@
 'use strict';
+const BUFFER_REQUIRE = 'const { Buffer } = require(\'buffer\');\n';
 
 module.exports = function(context) {
+
   function flagIt(reference) {
     const msg = 'Use const Buffer = require(\'buffer\').Buffer; ' +
                 'at the beginning of this file';
-    context.report(reference.identifier, msg);
+
+    context.report({
+      node: reference.identifier,
+      message: msg,
+      fix: (fixer) => {
+        const sourceCode = context.getSourceCode();
+
+        const useStrict = /'use strict';\n\n?/g;
+        const hasUseStrict = !!useStrict.exec(sourceCode.text);
+        const firstLOC = sourceCode.ast.range[0];
+        const rangeNeedle = hasUseStrict ? useStrict.lastIndex : firstLOC;
+
+        return fixer.insertTextBeforeRange([rangeNeedle], BUFFER_REQUIRE);
+      }
+    });
   }
 
   return {