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, 8 insertions, 5 deletions

diff --git a/library/std/src/sys/process/unix/mod.rs b/library/std/src/sys/process/unix/mod.rs
index ee8fd8b2ca3..b4cf060fba9 100644
--- a/library/std/src/sys/process/unix/mod.rs
+++ b/library/std/src/sys/process/unix/mod.rs
@@ -1,18 +1,21 @@
 #[cfg_attr(any(target_os = "espidf", target_os = "horizon", target_os = "nuttx"), allow(unused))]
 mod common;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "fuchsia")] {
+cfg_select! {
+    target_os = "fuchsia" => {
         mod fuchsia;
         use fuchsia as imp;
-    } else if #[cfg(target_os = "vxworks")] {
+    }
+    target_os = "vxworks" => {
         mod vxworks;
         use vxworks as imp;
-    } else if #[cfg(any(target_os = "espidf", target_os = "horizon", target_os = "vita", target_os = "nuttx"))] {
+    }
+    any(target_os = "espidf", target_os = "horizon", target_os = "vita", target_os = "nuttx") => {
         mod unsupported;
         use unsupported as imp;
         pub use unsupported::output;
-    } else {
+    }
+    _ => {
         mod unix;
         use unix as imp;
     }