Auto merge of #41095 - clarcharr:as_extras, r=alexcrichton

Add as_c_str

Again, tying up some consistencies with `CString`.

src/doc/unstable-book/src/SUMMARY.md

@@ -12,6 +12,7 @@
 - [alloc_system](alloc-system.md)
 - [allocator](allocator.md)
 - [allow_internal_unstable](allow-internal-unstable.md)
+- [as_c_str](as-c-str.md)
 - [as_unsafe_cell](as-unsafe-cell.md)
 - [ascii_ctype](ascii-ctype.md)
 - [asm](asm.md)

src/doc/unstable-book/src/as-c-str.md

@@ -0,0 +1,8 @@
+# `as_c_str`
+
+The tracking issue for this feature is: [#40380]
+
+[#40380]: https://github.com/rust-lang/rust/issues/40380
+
+------------------------
+

src/libstd/ffi/c_str.rs

@@ -324,6 +324,12 @@ pub fn as_bytes_with_nul(&self) -> &[u8] {
         &self.inner
     }
 
+    /// Extracts a `CStr` slice containing the entire string.
+    #[unstable(feature = "as_c_str", issue = "40380")]
+    pub fn as_c_str(&self) -> &CStr {
+        &*self
+    }
+
     /// Converts this `CString` into a boxed `CStr`.
     #[unstable(feature = "into_boxed_c_str", issue = "40380")]
     pub fn into_boxed_c_str(self) -> Box<CStr> {