http: runtime deprecate legacy HTTP parser

The legacy HTTP parser, used by default in versions of Node.js prior to
12.0.0, is deprecated. The legacy HTTP parser cannot be guaranteed to be
supported after April 2021. This commit introduces a deprecation warning
for the legacy HTTP parser.

doc/api/deprecations.md

@@ -2469,12 +2469,15 @@ Module.createRequireFromPath() is deprecated. Please use [`module.createRequire(
 ### DEP0131: Legacy HTTP parser
 <!-- YAML
 changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/TBD
+    description: Runtime deprecation.
   - version: v12.3.0
     pr-url: https://github.com/nodejs/node/pull/27498
     description: Documentation-only.
 -->
 
-Type: Documentation-only
+Type: Runtime
 
 The legacy HTTP parser, used by default in versions of Node.js prior to 12.0.0,
 is deprecated. This deprecation applies to users of the

lib/_http_common.js

@@ -29,6 +29,10 @@ const { setImmediate } = require('timers');
 
 const { getOptionValue } = require('internal/options');
 
+if (getOptionValue('--http-parser') === 'legacy') {
+  process.emitWarning('The legacy HTTP parser is deprecated.',
+                      'DeprecationWarning', 'DEP0131');
+}
 const { methods, HTTPParser } =
   getOptionValue('--http-parser') === 'legacy' ?
     internalBinding('http_parser') : internalBinding('http_parser_llhttp');

test/parallel/test-http-parser-legacy-deprecation.js

@@ -0,0 +1,11 @@
+'use strict';
+const common = require('../common');
+
+// Flags: --http-parser=legacy
+require('http');
+
+common.expectWarning({
+  DeprecationWarning:
+    ['The legacy HTTP parser is deprecated.',
+     'DEP0131']
+});