Avoid mem::uninitialized in the demo cpu_time

For both Unix `getrusage` and Windows `GetProcessTimes`, we can pass
`MaybeUninit` pointers to those FFI calls and `assume_init()` afterward.

rayon-demo/src/cpu_time/unix.rs

@@ -1,14 +1,17 @@
-use libc::{getrusage, rusage, RUSAGE_SELF};
-use std::mem;
+use libc::{getrusage, RUSAGE_SELF};
+use std::mem::MaybeUninit;
 
 pub fn get_cpu_time() -> Option<u64> {
-    unsafe {
-        let mut usage: rusage = mem::uninitialized();
-        getrusage(RUSAGE_SELF, &mut usage);
-        let user = 1_000_000_000 * (usage.ru_utime.tv_sec as u64)
-            + 1_000 * (usage.ru_utime.tv_usec as u64);
-        let system = 1_000_000_000 * (usage.ru_stime.tv_sec as u64)
-            + 1_000 * (usage.ru_stime.tv_usec as u64);
-        Some(user + system)
-    }
+    let usage = unsafe {
+        let mut usage = MaybeUninit::uninit();
+        if getrusage(RUSAGE_SELF, usage.as_mut_ptr()) != 0 {
+            return None;
+        }
+        usage.assume_init()
+    };
+    let user =
+        1_000_000_000 * (usage.ru_utime.tv_sec as u64) + 1_000 * (usage.ru_utime.tv_usec as u64);
+    let system =
+        1_000_000_000 * (usage.ru_stime.tv_sec as u64) + 1_000 * (usage.ru_stime.tv_usec as u64);
+    Some(user + system)
 }

rayon-demo/src/cpu_time/win.rs

@@ -1,17 +1,26 @@
-use std::mem;
-use winapi::shared::minwindef::FILETIME;
+use std::mem::MaybeUninit;
 use winapi::um::processthreadsapi::{GetCurrentProcess, GetProcessTimes};
 
 pub fn get_cpu_time() -> Option<u64> {
-    unsafe {
+    let (kernel, user) = unsafe {
         let process = GetCurrentProcess();
-        let mut _creation: FILETIME = mem::uninitialized();
-        let mut _exit: FILETIME = mem::uninitialized();
-        let mut kernel: FILETIME = mem::uninitialized();
-        let mut user: FILETIME = mem::uninitialized();
-        GetProcessTimes(process, &mut _creation, &mut _exit, &mut kernel, &mut user);
-        let kernel = (kernel.dwHighDateTime as u64) << 32 | kernel.dwLowDateTime as u64;
-        let user = (user.dwHighDateTime as u64) << 32 | user.dwLowDateTime as u64;
-        Some(100 * (kernel + user))
-    }
+        let mut _creation = MaybeUninit::uninit();
+        let mut _exit = MaybeUninit::uninit();
+        let mut kernel = MaybeUninit::uninit();
+        let mut user = MaybeUninit::uninit();
+        if GetProcessTimes(
+            process,
+            _creation.as_mut_ptr(),
+            _exit.as_mut_ptr(),
+            kernel.as_mut_ptr(),
+            user.as_mut_ptr(),
+        ) == 0
+        {
+            return None;
+        }
+        (kernel.assume_init(), user.assume_init())
+    };
+    let kernel = (kernel.dwHighDateTime as u64) << 32 | kernel.dwLowDateTime as u64;
+    let user = (user.dwHighDateTime as u64) << 32 | user.dwLowDateTime as u64;
+    Some(100 * (kernel + user))
 }