Tracking Issue for using std::mem::transmute() in const fn (const_fn_transmute)

[TheDarkula]

Using std::mem::transmute() in constant functions is behind the const_transmute/const_fn_transmute feature gate.
(In constants, it is stable.)

Blocked on rust-lang/const-eval#14.

[mjbshaw]

I, for one, would love this, because I've had to use the union transmute hack more times than I feel comfortable. std::mem::transmute is still (super) unsafe, but at least it makes sure the types have the same size.

[TheDarkula]

@mjbshaw It's already been merged :)

[mjbshaw]

@TheDarkula I know. I'm just trying to show support for eventually stabilizing this feature (rather than being removed because people are opposed to it).

[jamesmunns]

Even with the flag, is this possible? I am getting an error stating that the use of unsafe functions are not allowed in const fn:

pub const unsafe fn rx_buffer_init() -> [BufferDescriptor; BUFFER_CT] {
    transmute::<
        [u8; size_of::<BufferDescriptor>() * BUFFER_CT],
        [BufferDescriptor; BUFFER_CT]
    >([0u8; size_of::<BufferDescriptor>() * BUFFER_CT])
}

gives me

error: call to unsafe function is unsafe and unsafe operations are not allowed in const fn
  --> foo/src/test.rs:18:5
   |
18 | /     transmute::<
19 | |         [u8; size_of::<BufferDescriptor>() * BUFFER_CT],
20 | |         [BufferDescriptor; BUFFER_CT]
21 | |     >([0u8; size_of::<BufferDescriptor>() * BUFFER_CT])
   | |____________________________________________________________^ call to unsafe function
   |
   = note: consult the function's documentation for information on how to avoid undefined behavior

[mgostIH]

Is there some blocker on this or could this be starting to get stabilized? I'm mostly looking out for a way to define a function from a u8 array without any indirection but I have no idea whether this is supposed to be supported or not in the design of const fn.

[MSxDOS]

unsafe in const fn has been stabilized in 1.33. Is there anything else left preventing this from being stabilized as well?

EDIT: Ugh, in fact, there is. For example, usize -> function pointer throws

error[E0080]: it is undefined behavior to use this value
...
type validation failed: encountered 10220800, but expected a pointer
|
= note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior

... which makes const_transmute pretty much useless for me.