Apply repr(transparent) to several FFI types

repr(transparent) is required in order to safely cast between an FFI
type and its NewType.  This commit applies that attribute to PollFd,
EpollEvent, IpMembershipRequest, Ipv6MembershipRequest, TimeVal, and
IoVec.

Fixes #1241

Author: asomers

CHANGELOG.md

@@ -46,6 +46,11 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   16KB. (#[1198](https://github.com/nix-rust/nix/pull/1198))
 - Fixed unaligned casting of `cmsg_data` to `af_alg_iv` (#[1206](https://github.com/nix-rust/nix/pull/1206))
 - Fixed `readlink`/`readlinkat` when reading symlinks longer than `PATH_MAX` (#[1231](https://github.com/nix-rust/nix/pull/1231))
+- `PollFd`, `EpollEvent`, `IpMembershipRequest`, `Ipv6MembershipRequest`,
+  `TimeVal`, and `IoVec` are now `repr(transparent)`.  This is required for
+  correctness's sake across all architectures and compilers, though now bugs
+  have been reported so far.
+  (#[1243](https://github.com/nix-rust/nix/pull/1243))
 
 ### Removed
 

src/poll.rs

@@ -17,7 +17,7 @@ use errno::Errno;
 ///
 /// After a call to `poll` or `ppoll`, the events that occured can be
 /// retrieved by calling [`revents()`](#method.revents) on the `PollFd`.
-#[repr(C)]
+#[repr(transparent)]
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub struct PollFd {
     pollfd: libc::pollfd,

src/sys/epoll.rs

@@ -43,7 +43,7 @@ libc_bitflags!{
 }
 
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
-#[repr(C)]
+#[repr(transparent)]
 pub struct EpollEvent {
     event: libc::epoll_event,
 }

src/sys/socket/mod.rs

@@ -278,7 +278,7 @@ cfg_if! {
 /// Request for multicast socket operations
 ///
 /// This is a wrapper type around `ip_mreq`.
-#[repr(C)]
+#[repr(transparent)]
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub struct IpMembershipRequest(libc::ip_mreq);
 
@@ -297,7 +297,7 @@ impl IpMembershipRequest {
 /// Request for ipv6 multicast socket operations
 ///
 /// This is a wrapper type around `ipv6_mreq`.
-#[repr(C)]
+#[repr(transparent)]
 #[derive(Clone, Copy, Debug, Eq, PartialEq)]
 pub struct Ipv6MembershipRequest(libc::ipv6_mreq);
 

src/sys/time.rs

@@ -245,7 +245,7 @@ impl fmt::Display for TimeSpec {
 
 
 
-#[repr(C)]
+#[repr(transparent)]
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub struct TimeVal(timeval);
 

src/sys/uio.rs

@@ -158,7 +158,7 @@ pub fn process_vm_readv(pid: ::unistd::Pid, local_iov: &[IoVec<&mut [u8]>], remo
     Errno::result(res).map(|r| r as usize)
 }
 
-#[repr(C)]
+#[repr(transparent)]
 #[derive(Clone, Copy, Debug, Eq, Hash, PartialEq)]
 pub struct IoVec<T>(libc::iovec, PhantomData<T>);