Rollup merge of #126854 - devnexen:std_unix_os_fallback_upd, r=Mark-S…

…imulacrum

std::unix::os::home_dir: fallback's optimisation.

we're using a guaranteed initialised field on success.

library/std/src/sys/pal/unix/os.rs

@@ -738,17 +738,17 @@ pub fn home_dir() -> Option<PathBuf> {
  n => n as usize,
  };
  let mut buf = Vec::with_capacity(amt);
- let mut passwd: libc::passwd = mem::zeroed();
+ let mut p = mem::MaybeUninit::<libc::passwd>::uninit();
  let mut result = ptr::null_mut();
  match libc::getpwuid_r(
  libc::getuid(),
- &mut passwd,
+ p.as_mut_ptr(),
  buf.as_mut_ptr(),
  buf.capacity(),
  &mut result,
  ) {
  0 if !result.is_null() => {
- let ptr = passwd.pw_dir as *const _;
+ let ptr = (*result).pw_dir as *const _;
  let bytes = CStr::from_ptr(ptr).to_bytes().to_vec();
  Some(OsStringExt::from_vec(bytes))
  }