library: Migrate from `cfg_if` to `cfg_select`

Migrate the standard library from using the external `cfg_if` crate to
using the now-built-in `cfg_select` macro.

This does not yet eliminate the dependency from
`library/std/Cargo.toml`, because while the standard library itself no
longer uses `cfg_if`, it also incorporates the `backtrace` crate, which
does.

Migration assisted by the following vim command (after selecting the
full `cfg_if!` invocation):

```
'<,'>s/\(cfg_if::\)\?cfg_if/cfg_select/ | '<,'>s/^\( *\)} else {/\1}\r\1_ => {/c | '<,'>s/^\( *\)} else if #\[cfg(\(.*\))\] /\1}\r\1\2 => /e | '<,'>s/if #\[cfg(\(.*\))\] {/\1 => {/e
```

This is imperfect, but substantially accelerated the process. This
prompts for confirmation on the `} else {` since that can also appear
inside one of the arms. This also requires manual intervention to handle
any multi-line conditions.

1 files changed, 6 insertions, 6 deletions

diff --git a/library/std/src/os/unix/process.rs b/library/std/src/os/unix/process.rs
index 76e63a69e45..09429af06e3 100644
--- a/library/std/src/os/unix/process.rs
+++ b/library/std/src/os/unix/process.rs
@@ -4,8 +4,6 @@
 
 #![stable(feature = "rust1", since = "1.0.0")]
 
-use cfg_if::cfg_if;
-
 use crate::ffi::OsStr;
 use crate::os::unix::io::{AsFd, AsRawFd, BorrowedFd, FromRawFd, IntoRawFd, OwnedFd, RawFd};
 use crate::path::Path;
@@ -13,17 +11,19 @@ use crate::sealed::Sealed;
 use crate::sys_common::{AsInner, AsInnerMut, FromInner, IntoInner};
 use crate::{io, process, sys};
 
-cfg_if! {
-    if #[cfg(any(target_os = "vxworks", target_os = "espidf", target_os = "horizon", target_os = "vita"))] {
+cfg_select! {
+    any(target_os = "vxworks", target_os = "espidf", target_os = "horizon", target_os = "vita") => {
         type UserId = u16;
         type GroupId = u16;
-    } else if #[cfg(target_os = "nto")] {
+    }
+    target_os = "nto" => {
         // Both IDs are signed, see `sys/target_nto.h` of the QNX Neutrino SDP.
         // Only positive values should be used, see e.g.
         // https://www.qnx.com/developers/docs/7.1/#com.qnx.doc.neutrino.lib_ref/topic/s/setuid.html
         type UserId = i32;
         type GroupId = i32;
-    } else {
+    }
+    _ => {
         type UserId = u32;
         type GroupId = u32;
     }