`ManuallyDrop` allows storing invalid references in consts and statics.

[theemathas]

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

[RalfJung]

I would say this is mostly expected behavior, except that I did not expect that one can use patterns of ManuallyDrop.

Cc @rust-lang/wg-const-eval

[WaffleLapkin]

Note that the addition of MaybeDangling briefly made ManuallyDrop not StructuralPartialEq (and thus not being able to use it in patterns), see #154890 and #154891.

I think this is expected behavior, except for the fact that ManuallyDrop probably shouldn't be usable in patterns...

[theemathas]

Another fun case: this doesn't compile, due to...... a dangling pointer in the MaybeDangling.

use std::mem::{ManuallyDrop, transmute};
const DATA: ManuallyDrop<&i32> = unsafe {
    let x = 1_i32;
    transmute(&raw const x)
};

error: encountered dangling pointer in final value of constant
 --> src/lib.rs:2:1
  |
2 | const DATA: ManuallyDrop<&i32> = unsafe {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

[RalfJung]

Yes, you get that error even if you use a raw pointer. Dangling pointers are fully disallowed in the final value of consts.