Merge #1923

1923: feat: I/O safety for 'sys/wait' r=asomers a=SteveLauC

#### What this PR does:
1. Adds I/O safety for `sys/wait`

----------

Actually, I am not sure about which type to use here:
```rust
pub enum Id<'fd> {
    /// Wait for the child referred to by the given PID file descriptor
    #[cfg(any(target_os = "android", target_os = "linux"))]
    PIDFd(RawFd),
    PIDFd(BorrowedFd<'fd>),
}
```

If we use `Fd: AsFd`

```rust
pub enum Id<'fd, Fd: AsFd> {
    /// Wait for the child referred to by the given PID file descriptor
    #[cfg(any(target_os = "android", target_os = "linux"))]
    PIDFd(RawFd),
    PIDFd(&'fd Fd),
}
```

 then the user has to specify that generic type when using this interface, which is kinda user-unfriendly...


------

The typical usage of this interface will be something like:

```rust
// Thought currently we don't have pidfd_open(2) in `Nix`
let fd_referring_to_a_process: OwnedFd  = pidfd_open().unwrap();
let status = waitid(Id::PIDFd(fd_referring_to_a_process), WaitPidFlag::XXXX).unwrap();
```

UPDATE: `pidfd_open(2)` will be added in #1859 or #1868 .


Co-authored-by: Steve Lau <stevelauc@outlook.com>

Author: bors[bot]

src/sys/wait.rs

@@ -10,7 +10,7 @@ use std::convert::TryFrom;
     target_os = "android",
     all(target_os = "linux", not(target_env = "uclibc")),
 ))]
-use std::os::unix::io::RawFd;
+use std::os::unix::io::{AsRawFd, BorrowedFd};
 
 libc_bitflags!(
     /// Controls the behavior of [`waitpid`].
@@ -343,8 +343,8 @@ pub fn wait() -> Result<WaitStatus> {
     target_os = "haiku",
     all(target_os = "linux", not(target_env = "uclibc")),
 ))]
-#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
-pub enum Id {
+#[derive(Debug)]
+pub enum Id<'fd> {
     /// Wait for any child
     All,
     /// Wait for the child whose process ID matches the given PID
@@ -355,7 +355,11 @@ pub enum Id {
     PGid(Pid),
     /// Wait for the child referred to by the given PID file descriptor
     #[cfg(any(target_os = "android", target_os = "linux"))]
-    PIDFd(RawFd),
+    PIDFd(BorrowedFd<'fd>),
+    /// A helper variant to resolve the unused parameter (`'fd`) problem on platforms
+    /// other than Linux and Android.
+    #[doc(hidden)]
+    _Unreachable(std::marker::PhantomData<&'fd std::convert::Infallible>),
 }
 
 /// Wait for a process to change status
@@ -373,7 +377,8 @@ pub fn waitid(id: Id, flags: WaitPidFlag) -> Result<WaitStatus> {
         Id::Pid(pid) => (libc::P_PID, pid.as_raw() as libc::id_t),
         Id::PGid(pid) => (libc::P_PGID, pid.as_raw() as libc::id_t),
         #[cfg(any(target_os = "android", target_os = "linux"))]
-        Id::PIDFd(fd) => (libc::P_PIDFD, fd as libc::id_t),
+        Id::PIDFd(fd) => (libc::P_PIDFD, fd.as_raw_fd() as libc::id_t),
+        Id::_Unreachable(_) => unreachable!("This variant could never be constructed"),
     };
 
     let siginfo = unsafe {