Tracking issue for `#![feature(const_float_bits_conv)]`

[ecstatic-morse]

Tracking issue for the #![feature(const_float_bits_conv)] feature gate, which makes the following methods on f32 and f64 const fn:

• formerly #![feature(float_bits_conv)]
  • to_bits
  • from_bits
• formerly #![feature(float_to_from_bytes)]
  • to_bytes_be
  • to_bytes_le
  • to_bytes_ne
  • from_bytes_be
  • from_bytes_le
  • from_bytes_ne

These require transmute within a const fn to stabilize, and–unlike the recently stabilized #![feature(const_int_conversion)]–do not have a const-stable alternate implementation. Nevertheless, these may be useful for nightly users.

Stabilization is blocked on #57241.

[lebensterben]

Is there anyway to hard-code this without nightly?
I've a real world use case, I need to define
pub const NA_REAL: f64 = f64::from_le_bytes([0xa2, 0x07, 0x00, 0x00, 0x00, 0x00, 0xf0, 0x7f]);
which is the NA (not NaN) value used in R.

[ecstatic-morse]

transmute is stable in constants and statics (but not in const fn) as of 1.46.

[lebensterben]

pub const NA_REAL: f64 = unsafe {
    std::mem::transmute(u64::from_le_bytes([
        0xa2, 0x07, 0x00, 0x00, 0x00, 0x00, 0xf0, 0x7f,
    ]))
};

This works for me now.

[bstrie]

Stabilization is blocked on #57241.

Does that issue need to be a blocker for this? The functions in question aren't doing arithmetic on floats, they're just casting bit representations. AIUI while float arithmetic might be nonportable, bit representations are well-specified, yes?