Tracking Issue for const_num_from_num

[usbalbin]

Feature gate: #![feature(const_num_from_num)]

This is a tracking issue for constifying the impl From<U> for T and impl TryFrom<U> for T for conversions between the basic built in numeric types(both T and U are one of f32, f64, iX, uX). It also constifies the corresponding From conversions for NonZeroUX and NonZeroIX and from NonZeroUX/NonZeroIX to uX/iX of same type and signedness. Additionally AtomicUX from uX and AtomicIX from iX, this is however only for types of same size.

Note that all of this does not add any new implementations, it only constifies the existing ones.

In short, this will enable things like below without having to use as casts

const FOO_U32: u32 = u32::from(bar_returing_u16()); // u16 -> u32
const BAZ_F32: f32 = f32::from(bar_returing_u8());  // u8 -> f32

const FROM_NONZERO_U8: u8 = u8::from(bar_returing_nonzero_u8()); // NonZeroU8 -> u8
const NONZERO_FROM_SMALLER: NonZeroU32 = NonZeroU32::from(bar_returing_nonzero_u8()); // NonZeroU8 -> NonZeroU32

const ATOMIC_FROM_U8: AtomicU8 = AtomicU8::from(bar_returing_u8()); // u8 -> AtomicU8

Public API

# TODO

Steps / History

• Implementation of conversions of primitive numbers: Constify implementations of (Try)From for int types #86840
• Implementation of conversions of atomic and non-zero integers: Make From impls of NonZero integer const. #90077
• Final comment period (FCP)
• Stabilization PR

Unresolved Questions

• None yet.

[lilasta]

From implementations of atomic and non-zero integers is now const (Integers from non-zero integers and atomic integers from normal integers).

[chrisbouchard]

Is this still feature a thing? Rust doesn't recognize the feature name when I enable it on the latest nightly.

error[E0635]: unknown feature `const_num_from_num`
 --> src/lib.rs:1:12
  |
1 | #![feature(const_num_from_num)]
  |            ^^^^^^^^^^^^^^^^^^

But also numeric From and TryFroms are still non-const.

Playground Link

[oli-obk]

All const trait impls are currently disabled while the backing implementation is being reworked

[RalfJung]

This has been removed by #110393.