From a31ae159bc29ae26396160b1f3cb0fb358b4eae7 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lo=C3=AFc=20BRANSTETT?= <lolo.branstett@numericable.fr>
Date: Sun, 20 Feb 2022 17:13:38 +0100
Subject: [PATCH] Add documentation for `--check-cfg` in rustdoc book

---
 src/doc/rustdoc/src/unstable-features.md | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/src/doc/rustdoc/src/unstable-features.md b/src/doc/rustdoc/src/unstable-features.md
index c7fd5ed6fcb2e..141d5d2d2b2d0 100644
--- a/src/doc/rustdoc/src/unstable-features.md
+++ b/src/doc/rustdoc/src/unstable-features.md
@@ -512,3 +512,17 @@ crate being documented (`foobar`) and a path to output the calls
 
 To scrape examples from test code, e.g. functions marked `#[test]`, then
 add the `--scrape-tests` flag.
+
+### `--check-cfg`: check configuration flags
+
+This flag accepts the same values as `rustc --check-cfg`, and uses it to check configuration flags.
+
+Using this flag looks like this:
+
+```bash
+$ rustdoc src/lib.rs -Z unstable-options \
+    --check-cfg='names()' --check-cfg='values(feature, "foo", "bar")'
+```
+
+The example above check every well known names (`target_os`, `doc`, `test`, ... via `names()`)
+and check the values of `feature`: `foo` and `bar`.