feature(doc_cfg): set cfg(rustdoc) when rustdoc is running

src/librustdoc/lib.rs

@@ -466,7 +466,8 @@ fn main_args(args: &[String]) -> isize {
 
     let output = matches.opt_str("o").map(|s| PathBuf::from(&s));
     let css_file_extension = matches.opt_str("e").map(|s| PathBuf::from(&s));
-    let cfgs = matches.opt_strs("cfg");
+    let mut cfgs = matches.opt_strs("cfg");
+    cfgs.push("rustdoc".to_string());
 
     if let Some(ref p) = css_file_extension {
         if !p.is_file() {
@@ -643,7 +644,8 @@ where R: 'static + Send,
     for s in &matches.opt_strs("L") {
         paths.add_path(s, ErrorOutputType::default());
     }
-    let cfgs = matches.opt_strs("cfg");
+    let mut cfgs = matches.opt_strs("cfg");
+    cfgs.push("rustdoc".to_string());
     let triple = matches.opt_str("target").map(|target| {
         if target.ends_with(".json") {
             TargetTriple::TargetPath(PathBuf::from(target))

src/libsyntax/feature_gate.rs

@@ -1148,6 +1148,7 @@ const GATED_CFGS: &[(&str, &str, fn(&Features) -> bool)] = &[
     ("target_vendor", "cfg_target_vendor", cfg_fn!(cfg_target_vendor)),
     ("target_thread_local", "cfg_target_thread_local", cfg_fn!(cfg_target_thread_local)),
     ("target_has_atomic", "cfg_target_has_atomic", cfg_fn!(cfg_target_has_atomic)),
+    ("rustdoc", "doc_cfg", cfg_fn!(doc_cfg)),
 ];
 
 #[derive(Debug)]

src/test/ui/feature-gate-doc_cfg-cfg-rustdoc.rs

@@ -0,0 +1,14 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[cfg(rustdoc)] //~ ERROR: `cfg(rustdoc)` is experimental and subject to change
+pub struct SomeStruct;
+
+fn main() {}

src/test/ui/feature-gate-doc_cfg-cfg-rustdoc.stderr

@@ -0,0 +1,11 @@
+error[E0658]: `cfg(rustdoc)` is experimental and subject to change (see issue #43781)
+  --> $DIR/feature-gate-doc_cfg-cfg-rustdoc.rs:11:7
+   |
+LL | #[cfg(rustdoc)] //~ ERROR: `cfg(rustdoc)` is experimental and subject to change
+   |       ^^^^^^^
+   |
+   = help: add #![feature(doc_cfg)] to the crate attributes to enable
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0658`.