This is not necessarily a bug, but I think it's worth noting as an unintended(?) consequence of the MaybeDangling changes, probably in #150446. See also #118166. Note that the ManuallyDrop type currently contains a MaybeDangling inside.
The following code compiles since 1.96.0:
use std::mem::{ManuallyDrop, transmute};
const DATA: ManuallyDrop<&i32> = unsafe { transmute(4_usize) };(Note: We're using 4 since i32 has alignment 4.)
In 1.95.0, it produced the following error:
error[E0080]: constructing invalid value at .value.0: encountered a dangling reference (0x4[noalloc] has no provenance)
--> <source>:2:1
|
2 | const DATA: ManuallyDrop<&i32> = unsafe { transmute(4_usize) };
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ it is undefined behavior to use this value
|
= note: the rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
= note: the raw bytes of the constant (size: 8, align: 8) {
04 00 00 00 00 00 00 00 │ ........
}
error: aborting due to 1 previous error
For more information about this error, try `rustc --explain E0080`.
Attempting to use the const in a pattern results in a strange diagnostic:
use std::mem::{ManuallyDrop, transmute};
const DATA: ManuallyDrop<&i32> = unsafe { transmute(4_usize) };
fn main() {
if let DATA = DATA {}
}error: constant DATA cannot be used as pattern
--> src/main.rs:4:12
|
4 | if let DATA = DATA {}
| ^^^^
|
= note: constants that reference mutable or external memory cannot be used as patterns
Is this the correct behavior?
cc @WaffleLapkin @RalfJung
Meta
Tested on 1.99.0-nightly (2026-07-20 87e5904f5eb6398af6b2) on the playground
This is not necessarily a bug, but I think it's worth noting as an unintended(?) consequence of the
MaybeDanglingchanges, probably in #150446. See also #118166. Note that theManuallyDroptype currently contains aMaybeDanglinginside.The following code compiles since 1.96.0:
(Note: We're using
4sincei32has alignment4.)In 1.95.0, it produced the following error:
Attempting to use the
constin a pattern results in a strange diagnostic:Is this the correct behavior?
cc @WaffleLapkin @RalfJung
Meta
Tested on
1.99.0-nightly (2026-07-20 87e5904f5eb6398af6b2)on the playground