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

[Centril]

Sub-tracking issue for rust-lang/rfcs#911.

This tracks arithmetic for floating point types in const fn. These operations are already stable in const/static, and are also being promoted on stable (&(1.2*2.1) has lifetime 'static). However, in all of these situations, the code is definitely executed at compile-time -- only const fn code could be executed both at compile-time and run-time, making possible differences in FP behavior observable. Also see #77745.

Open Questions

• Must a const fn behave exactly the same at runtime as at compile-time? #77745: Must a const fn behave exactly the same at runtime as at compile-time?
• See Our floating point semantics were a mess unsafe-code-guidelines#237 for a collection of issues around floating-point semantics.

[burrbull]

Can from_bits / to_bits be made const fn in respect that #53535 ?

[RalfJung]

One problem with stabilizing floats in consts is that their run-time semantics are broken, and likely unsound: #55131, #73288.