Add Error::as_errno

This method is useful when it can be statically determined that a
nix::Error be an errno, which I find to be very common.

CHANGELOG.md

@@ -36,6 +36,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   ([#952](https://github.com/nix-rust/nix/pull/952))
 - Added the `time_t` and `suseconds_t` public aliases within `sys::time`.
   ([#968](https://github.com/nix-rust/nix/pull/968))
+- Added `Error::as_errno`.
+  ([#975](https://github.com/nix-rust/nix/pull/975))
 
 ### Changed
 - Increased required Rust version to 1.24.1

src/lib.rs

@@ -110,6 +110,23 @@ pub enum Error {
 }
 
 impl Error {
+    /// Convert this `Error` to an [`Errno`](enum.Errno.html).
+    ///
+    /// # Example
+    ///
+    /// ```
+    /// # use nix::Error;
+    /// # use nix::errno::Errno;
+    /// let e = Error::from(Errno::EPERM);
+    /// assert_eq!(Some(Errno::EPERM), e.as_errno());
+    /// ```
+    pub fn as_errno(&self) -> Option<Errno> {
+        if let Error::Sys(e) = self {
+            Some(*e)
+        } else {
+            None
+        }
+    }
 
     /// Create a nix Error from a given errno
     pub fn from_errno(errno: Errno) -> Error {