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, 10 insertions, 7 deletions

diff --git a/library/std/src/sys/process/unix/common.rs b/library/std/src/sys/process/unix/common.rs
index 6219be60caf..ea45b08e90a 100644
--- a/library/std/src/sys/process/unix/common.rs
+++ b/library/std/src/sys/process/unix/common.rs
@@ -20,12 +20,14 @@ use crate::{fmt, io};
 
 mod cstring_array;
 
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "fuchsia")] {
+cfg_select! {
+    target_os = "fuchsia" => {
         // fuchsia doesn't have /dev/null
-    } else if #[cfg(target_os = "vxworks")] {
+    }
+    target_os = "vxworks" => {
         const DEV_NULL: &CStr = c"/null";
-    } else {
+    }
+    _ => {
         const DEV_NULL: &CStr = c"/dev/null";
     }
 }
@@ -35,8 +37,8 @@ cfg_if::cfg_if! {
 // to support older Android version (independent of libc version).
 // The following implementations are based on
 // https://github.com/aosp-mirror/platform_bionic/blob/ad8dcd6023294b646e5a8288c0ed431b0845da49/libc/include/android/legacy_signal_inlines.h
-cfg_if::cfg_if! {
-    if #[cfg(target_os = "android")] {
+cfg_select! {
+    target_os = "android" => {
         #[allow(dead_code)]
         pub unsafe fn sigemptyset(set: *mut libc::sigset_t) -> libc::c_int {
             set.write_bytes(0u8, 1);
@@ -69,7 +71,8 @@ cfg_if::cfg_if! {
             raw[bit / LONG_BIT] |= 1 << (bit % LONG_BIT);
             return 0;
         }
-    } else {
+    }
+    _ => {
         #[allow(unused_imports)]
         pub use libc::{sigemptyset, sigaddset};
     }