Add infallible conversion from uid_t and gid_t

Implements the following traits:
* From<uid_t> for Uid
* From<gid_t> for Gid

Author: vkkoskie

CHANGELOG.md

@@ -9,6 +9,8 @@ This project adheres to [Semantic Versioning](https://semver.org/).
 - Added `aio_writev` and `aio_readv`.
   (#[1713](https://github.com/nix-rust/nix/pull/1713))
 
+- impl `From<uid_t>` for `Uid` and `From<gid_t>` for `Gid`
+  (#[1727](https://github.com/nix-rust/nix/pull/1727))
 - impl From<SockaddrIn> for std::net::SocketAddrV4 and
   impl From<SockaddrIn6> for std::net::SocketAddrV6.
   (#[1711](https://github.com/nix-rust/nix/pull/1711))

src/unistd.rs

@@ -87,6 +87,12 @@ impl From<Uid> for uid_t {
     }
 }
 
+impl From<uid_t> for Uid {
+    fn from(uid: uid_t) -> Self {
+        Uid(uid)
+    }
+}
+
 impl fmt::Display for Uid {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Display::fmt(&self.0, f)
@@ -131,6 +137,12 @@ impl From<Gid> for gid_t {
     }
 }
 
+impl From<gid_t> for Gid {
+    fn from(gid: gid_t) -> Self {
+        Gid(gid)
+    }
+}
+
 impl fmt::Display for Gid {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         fmt::Display::fmt(&self.0, f)