Rollup merge of rust-lang#63546 - lzutao:clouldabi-maybeuninit, r=RalfJung

Remove uses of `mem::uninitialized()` from cloudabi

This PR removes uses of `mem::uninitialized` from `cloudabi` module,
excluding the layout test in `src/libstd/sys/cloudabi/abi/cloudabi.rs`.

r? @RalfJung
cc @EdSchouten
cc rust-lang#62397

Author: Centril

src/libstd/sys/cloudabi/abi/cloudabi.rs

@@ -115,6 +115,7 @@
 
 #![no_std]
 #![allow(non_camel_case_types)]
+#![allow(deprecated)] // FIXME: using `mem::uninitialized()`
 
 include!("bitflags.rs");
 

src/libstd/sys/cloudabi/mod.rs

@@ -1,5 +1,3 @@
-#![allow(deprecated)] // mem::uninitialized
-
 use crate::io::ErrorKind;
 use crate::mem;
 

src/libstd/sys/cloudabi/mutex.rs

@@ -104,10 +104,11 @@ impl ReentrantMutex {
                 },
                 ..mem::zeroed()
             };
-            let mut event: abi::event = mem::uninitialized();
-            let mut nevents: usize = mem::uninitialized();
-            let ret = abi::poll(&subscription, &mut event, 1, &mut nevents);
+            let mut event = MaybeUninit::<abi::event>::uninit();
+            let mut nevents = MaybeUninit::<usize>::uninit();
+            let ret = abi::poll(&subscription, event.as_mut_ptr(), 1, nevents.as_mut_ptr());
             assert_eq!(ret, abi::errno::SUCCESS, "Failed to acquire mutex");
+            let event = event.assume_init();
             assert_eq!(event.error, abi::errno::SUCCESS, "Failed to acquire mutex");
         }
     }

src/libstd/sys/cloudabi/rwlock.rs

@@ -1,5 +1,6 @@
 use crate::cell::UnsafeCell;
 use crate::mem;
+use crate::mem::MaybeUninit;
 use crate::sync::atomic::{AtomicU32, Ordering};
 use crate::sys::cloudabi::abi;
 
@@ -73,10 +74,11 @@ impl RWLock {
                 },
                 ..mem::zeroed()
             };
-            let mut event: abi::event = mem::uninitialized();
-            let mut nevents: usize = mem::uninitialized();
-            let ret = abi::poll(&subscription, &mut event, 1, &mut nevents);
+            let mut event = MaybeUninit::<abi::event>::uninit();
+            let mut nevents = MaybeUninit::<usize>::uninit();
+            let ret = abi::poll(&subscription, event.as_mut_ptr(), 1, nevents.as_mut_ptr());
             assert_eq!(ret, abi::errno::SUCCESS, "Failed to acquire read lock");
+            let event = event.assume_init();
             assert_eq!(
                 event.error,
                 abi::errno::SUCCESS,
@@ -182,10 +184,11 @@ impl RWLock {
                 },
                 ..mem::zeroed()
             };
-            let mut event: abi::event = mem::uninitialized();
-            let mut nevents: usize = mem::uninitialized();
-            let ret = abi::poll(&subscription, &mut event, 1, &mut nevents);
+            let mut event = MaybeUninit::<abi::event>::uninit();
+            let mut nevents = MaybeUninit::<usize>::uninit();
+            let ret = abi::poll(&subscription, event.as_mut_ptr(), 1, nevents.as_mut_ptr());
             assert_eq!(ret, abi::errno::SUCCESS, "Failed to acquire write lock");
+            let event = event.assume_init();
             assert_eq!(
                 event.error,
                 abi::errno::SUCCESS,

src/libstd/sys/cloudabi/thread.rs

@@ -72,10 +72,11 @@ impl Thread {
                 },
                 ..mem::zeroed()
             };
-            let mut event: abi::event = mem::uninitialized();
-            let mut nevents: usize = mem::uninitialized();
-            let ret = abi::poll(&subscription, &mut event, 1, &mut nevents);
+            let mut event = mem::MaybeUninit::<abi::event>::uninit();
+            let mut nevents = mem::MaybeUninit::<usize>::uninit();
+            let ret = abi::poll(&subscription, event.as_mut_ptr(), 1, nevents.as_mut_ptr());
             assert_eq!(ret, abi::errno::SUCCESS);
+            let event = event.assume_init();
             assert_eq!(event.error, abi::errno::SUCCESS);
         }
     }