Replace libc::c_int with core::ffi::c_int

And remove the libc crate when it isn't needed

library/std/src/lib.rs

@@ -431,6 +431,9 @@ extern crate test;
 #[allow(unused_imports)] // macros from `alloc` are not used on all platforms
 #[macro_use]
 extern crate alloc as alloc_crate;
+
+// Many compiler tests depend on libc being pulled in by std
+// so include it here even if it's unused.
 #[doc(masked)]
 #[allow(unused_extern_crates)]
 extern crate libc;

library/std/src/os/unix/net/addr.rs

@@ -11,7 +11,7 @@ use crate::{fmt, io, mem, ptr};
 #[cfg(not(unix))]
 #[allow(non_camel_case_types)]
 mod libc {
-    pub use libc::c_int;
+    pub use core::ffi::c_int;
     pub type socklen_t = u32;
     pub struct sockaddr;
     #[derive(Clone)]

library/std/src/os/unix/net/ancillary.rs

@@ -20,7 +20,7 @@ use crate::sys::net::Socket;
 ))]
 #[allow(non_camel_case_types)]
 mod libc {
-    pub use libc::c_int;
+    pub use core::ffi::c_int;
     pub struct ucred;
     pub struct cmsghdr;
     pub struct sockcred2;

library/std/src/os/unix/net/datagram.rs

@@ -34,7 +34,7 @@ use libc::MSG_NOSIGNAL;
     target_os = "haiku",
     target_os = "nto",
 )))]
-const MSG_NOSIGNAL: libc::c_int = 0x0;
+const MSG_NOSIGNAL: core::ffi::c_int = 0x0;
 
 /// A Unix datagram socket.
 ///
@@ -317,7 +317,7 @@ impl UnixDatagram {
     fn recv_from_flags(
         &self,
         buf: &mut [u8],
-        flags: libc::c_int,
+        flags: core::ffi::c_int,
     ) -> io::Result<(usize, SocketAddr)> {
         let mut count = 0;
         let addr = SocketAddr::new(|addr, len| unsafe {

library/std/src/os/unix/net/listener.rs

@@ -79,14 +79,14 @@ impl UnixListener {
                 target_os = "espidf",
                 target_os = "horizon"
             ))]
-            const backlog: libc::c_int = 128;
+            const backlog: core::ffi::c_int = 128;
             #[cfg(any(
                 target_os = "linux",
                 target_os = "freebsd",
                 target_os = "openbsd",
                 target_os = "macos"
             ))]
-            const backlog: libc::c_int = -1;
+            const backlog: core::ffi::c_int = -1;
             #[cfg(not(any(
                 target_os = "windows",
                 target_os = "redox",
@@ -138,9 +138,9 @@ impl UnixListener {
         unsafe {
             let inner = Socket::new_raw(libc::AF_UNIX, libc::SOCK_STREAM)?;
             #[cfg(target_os = "linux")]
-            const backlog: libc::c_int = -1;
+            const backlog: core::ffi::c_int = -1;
             #[cfg(not(target_os = "linux"))]
-            const backlog: libc::c_int = 128;
+            const backlog: core::ffi::c_int = 128;
             cvt(libc::bind(
                 inner.as_raw_fd(),
                 core::ptr::addr_of!(socket_addr.addr) as *const _,