Implement and use Read and Write traits for AutoCloseFd

This lets us use any std::io functions that build on top of these, such as
`write_all()` in place of our own `write_loop()`.

fish-rust/src/fds.rs

@@ -1,5 +1,6 @@
 use crate::ffi;
 use nix::unistd;
+use std::io::{Read, Write};
 use std::os::unix::io::{AsRawFd, FromRawFd, RawFd};
 
 /// A helper type for managing and automatically closing a file descriptor
@@ -11,6 +12,33 @@ pub struct AutoCloseFd {
     fd_: RawFd,
 }
 
+impl Read for AutoCloseFd {
+    fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
+        unsafe {
+            match libc::read(self.as_raw_fd(), buf.as_mut_ptr() as *mut _, buf.len()) {
+                -1 => Err(std::io::Error::from_raw_os_error(errno::errno().0)),
+                bytes => Ok(bytes as usize),
+            }
+        }
+    }
+}
+
+impl Write for AutoCloseFd {
+    fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
+        unsafe {
+            match libc::write(self.as_raw_fd(), buf.as_ptr() as *const _, buf.len()) {
+                -1 => Err(std::io::Error::from_raw_os_error(errno::errno().0)),
+                bytes => Ok(bytes as usize),
+            }
+        }
+    }
+
+    fn flush(&mut self) -> std::io::Result<()> {
+        // We don't buffer anything so this is a no-op.
+        Ok(())
+    }
+}
+
 #[cxx::bridge]
 mod autoclose_fd_t {
     extern "Rust" {

fish-rust/src/tests/fd_monitor.rs

@@ -1,9 +1,9 @@
+use std::io::Write;
 use std::os::fd::AsRawFd;
 use std::sync::atomic::{AtomicBool, AtomicU64, AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex};
 use std::time::Duration;
 
-use crate::common::write_loop;
 use crate::fd_monitor::{FdMonitor, FdMonitorItem, FdMonitorItemId, ItemWakeReason};
 use crate::fds::{make_autoclose_pipes, AutoCloseFd};
 use crate::ffi_tests::add_test;
@@ -96,7 +96,7 @@ impl ItemMaker {
     fn write42(&self) {
         let buf = [0u8; 42];
         let mut writer = self.writer.lock().expect("Mutex poisoned!");
-        write_loop(&mut *writer, &buf).expect("Error writing 42 bytes to pipe!");
+        writer.write_all(&buf).expect("Error writing 42 bytes to pipe!");
     }
 }