Closed
Description
While making a PoC for #67456 it was discovered that the code beblow fails to compile in debug mode (#67456 (comment)), saying that "pointer-to-integer cast needs an rfc before being allowed in constants", while in release mode, the code compiles and runs.
Here's a playground link.
#![feature(const_ptr_is_null)]
#![feature(const_raw_ptr_to_usize_cast)]
#![feature(const_raw_ptr_deref)]
#![feature(const_slice_from_raw_parts)]
#![feature(const_panic)]
mod slice {
use core::mem;
use core::ptr;
pub(crate) const unsafe fn is_aligned_and_not_null<T>(ptr: *const T) -> bool {
!ptr.is_null() && ptr as usize % mem::align_of::<T>() == 0
}
pub const unsafe fn from_raw_parts<'a, T>(data: *const T, len: usize) -> &'a [T] {
debug_assert!(
is_aligned_and_not_null(data),
"attempt to create unaligned or null slice"
);
debug_assert!(
mem::size_of::<T>().saturating_mul(len) <= isize::MAX as usize,
"attempt to create slice covering at least half the address space"
);
// SAFETY: the caller must uphold the safety contract for `from_raw_parts`.
unsafe { &*ptr::slice_from_raw_parts(data, len) }
}
pub const fn from_ref<T>(reference: &T) -> &[T] {
unsafe {
from_raw_parts(reference, 1)
}
}
}
const X: &[usize] = slice::from_ref(&1);
fn main() {
dbg!(X);
}Here's the error message in debug mode:
error: any use of this value will cause an error
--> src/main.rs:13:27
|
13 | !ptr.is_null() && ptr as usize % mem::align_of::<T>() == 0
| ^^^^^^^^^^^^
| |
| "pointer-to-integer cast" needs an rfc before being allowed inside constants
| inside `is_aligned_and_not_null::<usize>` at src/main.rs:13:27
| inside `slice::from_raw_parts::<usize>` at src/main.rs:18:13
| inside `slice::from_ref::<usize>` at src/main.rs:31:13
| inside `X` at src/main.rs:36:21
...
36 | const X: &[usize] = slice::from_ref(&1);
| ----------------------------------------
|
= note: `#[deny(const_err)]` on by default
error: aborting due to previous error; 1 warning emitted
error: could not compile `playground`
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