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Add MaybeUninit method array_assume_init #80600

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merged 6 commits into from
Jan 12, 2021
Merged

Add MaybeUninit method array_assume_init #80600

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826bc36
Implement MaybeUninit::array_assume_init
joboet Jan 1, 2021
0ff1e6c
Add test for MaybeUninit::array_assume_init
joboet Jan 1, 2021
72a3dee
Format code
joboet Jan 1, 2021
dec8c03
Add tracking issue for array_assume_init
joboet Jan 11, 2021
5d65b7e
Simplify array_assume_init
joboet Jan 11, 2021
985071b
Fix implementation
joboet Jan 12, 2021
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40 changes: 40 additions & 0 deletions library/core/src/mem/maybe_uninit.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -804,6 +804,46 @@ impl<T> MaybeUninit<T> {
}
}

/// Extracts the values from an array of `MaybeUninit` containers.
///
/// # Safety
///
/// It is up to the caller to guarantee that all elements of the array are
/// in an initialized state.
///
/// # Examples
///
/// ```
/// #![feature(maybe_uninit_uninit_array)]
/// #![feature(maybe_uninit_array_assume_init)]
/// use std::mem::MaybeUninit;
///
/// let mut array: [MaybeUninit<i32>; 3] = MaybeUninit::uninit_array();
/// array[0] = MaybeUninit::new(0);
/// array[1] = MaybeUninit::new(1);
/// array[2] = MaybeUninit::new(2);
///
/// // SAFETY: Now safe as we initialised all elements
/// let array = unsafe {
/// MaybeUninit::array_assume_init(array)
/// };
///
/// assert_eq!(array, [0, 1, 2]);
/// ```
#[unstable(feature = "maybe_uninit_array_assume_init", issue = "80908")]
#[inline(always)]
pub unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] {
// SAFETY:
// * The caller guarantees that all elements of the array are initialized
// * `MaybeUninit<T>` and T are guaranteed to have the same layout
// * MaybeUnint does not drop, so there are no double-frees
// And thus the conversion is safe
unsafe {
intrinsics::assert_inhabited::<T>();
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Is this assertion really correct?

I tried this code:

#![feature(maybe_uninit_array_assume_init)]

use std::mem::MaybeUninit;

enum Uninhabited {}

fn main() {
    unsafe {
        MaybeUninit::<Uninhabited>::array_assume_init([]);
    }
}
And it panicked with attempted to instantiate uninhabited type `Uninhabited`. 
   Compiling playground v0.0.1 (/playground)
    Finished dev [unoptimized + debuginfo] target(s) in 1.12s
     Running `target/debug/playground`
thread 'main' panicked at 'attempted to instantiate uninhabited type `Uninhabited`', /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/core/src/mem/maybe_uninit.rs:842:13
stack backtrace:
   0: rust_begin_unwind
             at /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/std/src/panicking.rs:493:5
   1: core::panicking::panic_fmt
             at /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/core/src/panicking.rs:92:14
   2: core::panicking::panic
             at /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/core/src/panicking.rs:50:5
   3: core::mem::maybe_uninit::MaybeUninit<T>::array_assume_init
             at /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/core/src/mem/maybe_uninit.rs:842:13
   4: playground::main
             at ./src/main.rs:9:9
   5: core::ops::function::FnOnce::call_once
             at /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/core/src/ops/function.rs:227:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

But it seems that creating an array of uninhabited type with length of zero is not UB (playground):

#![forbid(unsafe_code)]

#[derive(Debug)]
enum Uninhabited {}

fn main() {
    let not_ub: [Uninhabited; 0] = [];
    dbg!(not_ub);
}

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Good point! This should probably be

if N > 0 { assert_inhabited(); }

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I'll submit a PR to fix it.

(&array as *const _ as *const [T; N]).read()
}
}

/// Assuming all the elements are initialized, get a slice to them.
///
/// # Safety
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2 changes: 2 additions & 0 deletions library/core/tests/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -36,6 +36,8 @@
#![feature(raw)]
#![feature(sort_internals)]
#![feature(slice_partition_at_index)]
#![feature(maybe_uninit_uninit_array)]
#![feature(maybe_uninit_array_assume_init)]
#![feature(maybe_uninit_extra)]
#![feature(maybe_uninit_write_slice)]
#![feature(min_specialization)]
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14 changes: 14 additions & 0 deletions library/core/tests/mem.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -140,6 +140,20 @@ fn assume_init_good() {
assert!(TRUE);
}

#[test]
fn uninit_array_assume_init() {
let mut array: [MaybeUninit<i16>; 5] = MaybeUninit::uninit_array();
array[0].write(3);
array[1].write(1);
array[2].write(4);
array[3].write(1);
array[4].write(5);

let array = unsafe { MaybeUninit::array_assume_init(array) };

assert_eq!(array, [3, 1, 4, 1, 5]);
}

#[test]
fn uninit_write_slice() {
let mut dst = [MaybeUninit::new(255); 64];
Expand Down