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Rollup merge of #62634 - llogiq:uninit-array-docs, r=RalfJung

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Less unsafe in the array example of MaybeUninit docs

I believe this is an acceptable way to initialize elements of `[MaybeUninit<T>; _]` arrays. Miri agrees. Conceptually, we are working at the array level, above the `MaybeUninit`, and as we are replacing it wholesale, this should pose no problem to soundness. And the code is easier to read.

r? @RalfJung
This commit is contained in:
Mark Rousskov 2019-07-15 19:55:04 -04:00 • committed by GitHub
commit caf10b5ffe
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1 changed files with 7 additions and 5 deletions
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12
src/libcore/mem/maybe_uninit.rs
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@ -112,7 +112,6 @@ use crate::mem::ManuallyDrop;
///
/// ```
/// use std::mem::{self, MaybeUninit};
/// use std::ptr;
///
/// let data = {
/// // Create an uninitialized array of `MaybeUninit`. The `assume_init` is
@ -122,10 +121,13 @@ use crate::mem::ManuallyDrop;
/// MaybeUninit::uninit().assume_init()
/// };
///
/// // Dropping a `MaybeUninit` does nothing, so if there is a panic during this loop,
/// // we have a memory leak, but there is no memory safety issue.
/// // Dropping a `MaybeUninit` does nothing. Thus using raw pointer
/// // assignment instead of `ptr::write` does not cause the old
/// // uninitialized value to be dropped. Also if there is a panic during
/// // this loop, we have a memory leak, but there is no memory safety
/// // issue.
/// for elem in &mut data[..] {
/// unsafe { ptr::write(elem.as_mut_ptr(), vec![42]); }
/// *elem = MaybeUninit::new(vec![42]);
/// }
///
/// // Everything is initialized. Transmute the array to the
@ -151,7 +153,7 @@ use crate::mem::ManuallyDrop;
/// let mut data_len: usize = 0;
///
/// for elem in &mut data[0..500] {
/// unsafe { ptr::write(elem.as_mut_ptr(), String::from("hello")); }
/// *elem = MaybeUninit::new(String::from("hello"));
/// data_len += 1;
/// }
///