Auto merge of #145489 - joshtriplett:cfg-if-not, r=Amanieu

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, 4 insertions, 6 deletions

diff --git a/library/std/src/sys/alloc/unix.rs b/library/std/src/sys/alloc/unix.rs
index a7ac4117ec9..3d369b08abc 100644
--- a/library/std/src/sys/alloc/unix.rs
+++ b/library/std/src/sys/alloc/unix.rs
@@ -58,18 +58,16 @@ unsafe impl GlobalAlloc for System {
     }
 }
 
-cfg_if::cfg_if! {
+cfg_select! {
     // We use posix_memalign wherever possible, but some targets have very incomplete POSIX coverage
     // so we need a fallback for those.
-    if #[cfg(any(
-        target_os = "horizon",
-        target_os = "vita",
-    ))] {
+    any(target_os = "horizon", target_os = "vita") => {
         #[inline]
         unsafe fn aligned_malloc(layout: &Layout) -> *mut u8 {
             unsafe { libc::memalign(layout.align(), layout.size()) as *mut u8 }
         }
-    } else {
+    }
+    _ => {
         #[inline]
         #[cfg_attr(target_os = "vxworks", allow(unused_unsafe))]
         unsafe fn aligned_malloc(layout: &Layout) -> *mut u8 {