use MaybeUninit in core::ptr::{read,read_unaligned}

Code by @japaric, I just split it into individual commits

Author: RalfJung

src/libcore/ptr.rs

@@ -79,7 +79,7 @@ use ops::{CoerceUnsized, DispatchFromDyn};
 use fmt;
 use hash;
 use marker::{PhantomData, Unsize};
-use mem;
+use mem::{self, MaybeUninit};
 use nonzero::NonZero;
 
 use cmp::Ordering::{self, Less, Equal, Greater};
@@ -575,9 +575,9 @@ pub unsafe fn replace<T>(dst: *mut T, mut src: T) -> T {
 #[inline]
 #[stable(feature = "rust1", since = "1.0.0")]
 pub unsafe fn read<T>(src: *const T) -> T {
-    let mut tmp: T = mem::uninitialized();
-    copy_nonoverlapping(src, &mut tmp, 1);
-    tmp
+    let mut tmp = MaybeUninit::<T>::uninitialized();
+    copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
+    tmp.into_inner()
 }
 
 /// Reads the value from `src` without moving it. This leaves the
@@ -642,11 +642,11 @@ pub unsafe fn read<T>(src: *const T) -> T {
 #[inline]
 #[stable(feature = "ptr_unaligned", since = "1.17.0")]
 pub unsafe fn read_unaligned<T>(src: *const T) -> T {
-    let mut tmp: T = mem::uninitialized();
+    let mut tmp = MaybeUninit::<T>::uninitialized();
     copy_nonoverlapping(src as *const u8,
-                        &mut tmp as *mut T as *mut u8,
+                        tmp.as_mut_ptr() as *mut u8,
                         mem::size_of::<T>());
-    tmp
+    tmp.into_inner()
 }
 
 /// Overwrites a memory location with the given value without reading or