From e06c51553ddc202a79f2c0b76822ad56c4a30f80 Mon Sep 17 00:00:00 2001
From: Corey Farwell <coreyf@rwell.org>
Date: Sat, 11 Mar 2017 17:10:05 -0500
Subject: [PATCH] Rust unstable book: basic desc and example for `const_fn`.

---
 cargo                                 |  2 +-
 src/doc/unstable-book/src/const-fn.md | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/cargo b/cargo
index 4a3c0a63b07..5f3b9c4c6a7 160000
--- a/cargo
+++ b/cargo
@@ -1 +1 @@
-Subproject commit 4a3c0a63b07e9a4feb41cb11de37c92a09db5a60
+Subproject commit 5f3b9c4c6a7be1f177d6024cb83d150b6479148a
diff --git a/src/doc/unstable-book/src/const-fn.md b/src/doc/unstable-book/src/const-fn.md
index 9b7942c408a..d5a22436838 100644
--- a/src/doc/unstable-book/src/const-fn.md
+++ b/src/doc/unstable-book/src/const-fn.md
@@ -6,5 +6,24 @@ The tracking issue for this feature is: [#24111]
 
 ------------------------
 
+The `const_fn` feature allows marking free functions and inherent methods as
+`const`, enabling them to be called in constants contexts, with constant
+arguments.
 
+## Examples
 
+```rust
+#![feature(const_fn)]
+
+const fn double(x: i32) -> i32 {
+    x * 2
+}
+
+const FIVE: i32 = 5;
+const TEN: i32 = double(FIVE);
+
+fn main() {
+    assert_eq!(5, FIVE);
+    assert_eq!(10, TEN);
+}
+```