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, 3 deletions

diff --git a/library/std/tests/env_modify.rs b/library/std/tests/env_modify.rs
index ba84978b35f..fe0ae68806e 100644
--- a/library/std/tests/env_modify.rs
+++ b/library/std/tests/env_modify.rs
@@ -1,5 +1,6 @@
 // These tests are in a separate integration test as they modify the environment,
 // and would otherwise cause some other tests to fail.
+#![feature(cfg_select)]
 
 use std::env::*;
 use std::ffi::{OsStr, OsString};
@@ -110,8 +111,8 @@ fn env_home_dir() {
         }
     }
 
-    cfg_if::cfg_if! {
-        if #[cfg(unix)] {
+    cfg_select! {
+        unix => {
             let oldhome = var_to_os_string(var("HOME"));
 
             unsafe {
@@ -130,7 +131,8 @@ fn env_home_dir() {
             }
 
             if let Some(oldhome) = oldhome { unsafe { set_var("HOME", oldhome); } }
-        } else if #[cfg(windows)] {
+        }
+        windows => {
             let oldhome = var_to_os_string(var("HOME"));
             let olduserprofile = var_to_os_string(var("USERPROFILE"));
 
@@ -159,6 +161,7 @@ fn env_home_dir() {
                 if let Some(olduserprofile) = olduserprofile { set_var("USERPROFILE", olduserprofile); }
             }
         }
+        _ => {}
     }
 }