[v12.x] http: runtime deprecate legacy HTTP parser

doc/api/cli.md

@@ -383,6 +383,10 @@ Specify the file name of the heap profile generated by `--heap-prof`.
 ### `--http-parser=library`
 <!-- YAML
 added: v11.4.0
+changes:
+  - version: REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/37603
+    description: The legacy HTTP parser will emit a deprecation warning.
 -->
 
 Chooses an HTTP parser library. Available values are:
@@ -392,6 +396,8 @@ Chooses an HTTP parser library. Available values are:
 
 The default is `llhttp`, unless otherwise specified when building Node.js.
 
+The `legacy` HTTP parser is deprecated and will emit a deprecation warning.
+
 This flag exists to aid in experimentation with the internal implementation of
 the Node.js http parser.
 This flag is likely to become a no-op and removed at some point in the future.

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/37603
+    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

src/node_http_parser_impl.h

@@ -26,6 +26,9 @@
 
 #include "node.h"
 #include "node_buffer.h"
+#ifndef NODE_EXPERIMENTAL_HTTP
+#include "node_process.h"
+#endif  /* NODE_EXPERIMENTAL_HTTP */
 #include "util.h"
 
 #include "async_wrap-inl.h"
@@ -1021,6 +1024,10 @@ void InitializeHttpParser(Local<Object> target,
 #ifndef NODE_EXPERIMENTAL_HTTP
   static uv_once_t init_once = UV_ONCE_INIT;
   uv_once(&init_once, InitMaxHttpHeaderSizeOnce);
+  ProcessEmitDeprecationWarning(
+    env,
+    "The legacy HTTP parser is deprecated.",
+    "DEP0131").IsNothing();
 #endif  /* NODE_EXPERIMENTAL_HTTP */
 }
 

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']
+});