<[MaybeUninit<T>; N]>::transpose() results in miscompilation #144645
Description
I tried this code:
#![feature(maybe_uninit_uninit_array_transpose)]
#[derive(Debug)]
struct S {
a: [[u8; 32]; 2],
}
use std::mem::MaybeUninit;
fn main() {
let mut s = Box::<S>::new_uninit();
let s_ptr = s.as_mut_ptr();
let a_ptr = unsafe { &raw mut (*s_ptr).a };
let a = unsafe {
&mut *a_ptr.cast::<[MaybeUninit<[u8; 32]>; 2]>()
};
a.transpose().write([[1u8; 32]; 2]);
dbg!(unsafe { s.assume_init() });
}I expected to see this happen: compiles and runs correctly, including under Miri.
Instead, this happened: results in UB, including as detected by Miri.
Variations of the code that do work correctly:
#![feature(maybe_uninit_uninit_array_transpose)]
#[derive(Debug)]
struct S {
a: [[u8; 32]; 2],
}
use std::mem::MaybeUninit;
fn main() {
let mut s = Box::<S>::new_uninit();
let s_ptr = s.as_mut_ptr();
let a_ptr = unsafe { &raw mut (*s_ptr).a };
let a = unsafe {
&mut *a_ptr.cast::<MaybeUninit<[[u8; 32]; 2]>>()
};
a.write([[1u8; 32]; 2]);
dbg!(unsafe { s.assume_init() });
}The difference is MaybeUninit<[[u8; 32]; 2]> instead of [MaybeUninit<[u8; 32]>; 2], which makes .transpose() unnecessary. My understanding is that both should be perfectly valid.
To verify that theory, I modified original code to this, just to avoid .transpose(), while leaving everything else the same:
#![feature(maybe_uninit_uninit_array_transpose)]
#[derive(Debug)]
struct S {
a: [[u8; 32]; 2],
}
use std::mem::MaybeUninit;
fn main() {
let mut s = Box::<S>::new_uninit();
let s_ptr = s.as_mut_ptr();
let a_ptr = unsafe { &raw mut (*s_ptr).a };
let a = unsafe {
&mut *a_ptr.cast::<[MaybeUninit<[u8; 32]>; 2]>()
};
for a in a {
a.write([1u8; 32]);
}
dbg!(unsafe { s.assume_init() });
}This compiles and runs correctly, including under Miri.
From this I make a conclusion that .transpose() is to blame here.
Meta
rustc --version --verbose:
rustc 1.90.0-nightly (498ae9fed 2025-07-28)
binary: rustc
commit-hash: 498ae9fed2e7d90821d70a048f3770f91af08957
commit-date: 2025-07-28
host: x86_64-unknown-linux-gnu
release: 1.90.0-nightly
LLVM version: 20.1.8