Closed
Description
This code causes a const_err somewhere in the standard library:
#![feature(const_swap)]
#![feature(const_mut_refs)]
use std::{
mem::{self, MaybeUninit},
ptr,
};
const X: () = {
let mut ptr1 = &1;
let mut ptr2 = &2;
// Swap them, bytewise.
unsafe {
ptr::swap_nonoverlapping(
&mut ptr1 as *mut _ as *mut MaybeUninit<u8>,
&mut ptr2 as *mut _ as *mut MaybeUninit<u8>,
mem::size_of::<&i32>(),
);
}
};When the rust-src package is not installed, the error is pretty unreadable:
error: any use of this value will cause an error
|
::: src/lib.rs:8:1
|
8 | / const X: () = {
9 | | let mut ptr1 = &1;
10 | | let mut ptr2 = &2;
11 | |
... |
19 | | }
20 | | };
| |__-
|
= note: `#[deny(const_err)]` on by default
= warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
= note: for more information, [see issue #71800 <https://github.com/rust-lang/rust/issues/71800>](https://github.com/rust-lang/rust/issues/71800)
This is missing the key information of what goes wrong! With rust-src, the following is added, making it much clearer:
--> /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1151:9
|
1151 | copy_nonoverlapping(src, tmp.as_mut_ptr(), 1);
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
| |
| unable to turn pointer into raw bytes
| inside `std::ptr::read::<MaybeUninit<u8>>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:1151:9
| inside `mem::swap_simple::<MaybeUninit<u8>>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/mem/mod.rs:768:17
| inside `ptr::swap_nonoverlapping_simple::<MaybeUninit<u8>>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:968:9
| inside `swap_nonoverlapping::<MaybeUninit<u8>>` at /home/r/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/ptr/mod.rs:949:14
| inside `X` at test.rs:14:9
If the sources are missing, we should probably still show the error, even if we cannot attach it to any code.
Cc @rust-lang/wg-const-eval