`Pow` trait

[Oakchris1955]

Proposal

Problem statement

Add a Pow trait in the standard and core libraries that group all pow traits of signed/unsigned integers

Motivating examples or use cases

There are already traits for the Add, Sub, Mul, Div and Rem operations, thus some argue that Rust should also have a Pow operator. There are also people out there that would love to see a Pow trait in Rust, according to this poll

A quick GitHub search also shows quite a lot of places where people have to rely on the third-party num-traits macro.

Solution sketch

The code itself already exists, it just needs to be moved out of the respective pow methods to a trait impl. We still need to see how we will deal with floats, since they have 2 exponentiation methods, powf and powi.

Alternatives

N/A

Links and related work

Initial Rust Internals discussion about adding a pow operation, with the consensus being that Rust shouldn't add an exponentiation operator, however several people expressed their opinion that an ACP for a Pow operator should be made: https://internals.rust-lang.org/t/pow-trait-raise-to-power-operator/23340

What happens now?

This issue contains an API change proposal (or ACP) and is part of the libs-api team feature lifecycle. Once this issue is filed, the libs-api team will review open proposals as capability becomes available. Current response times do not have a clear estimate, but may be up to several months.

Possible responses

The libs team may respond in various different ways. First, the team will consider the problem (this doesn't require any concrete solution or alternatives to have been proposed):

• We think this problem seems worth solving, and the standard library might be the right place to solve it.
• We think that this probably doesn't belong in the standard library.

Second, if there's a concrete solution:

• We think this specific solution looks roughly right, approved, you or someone else should implement this. (Further review will still happen on the subsequent implementation PR.)
• We're not sure this is the right solution, and the alternatives or other materials don't give us enough information to be sure about that. Here are some questions we have that aren't answered, or rough ideas about alternatives we'd want to see discussed.

[hanna-kruppe]

As far as I’m aware there are no stable (or intended to ever be stable) traits in the standard library that exist only to house some methods that are common to integer or numeric types. Add and friends also enable operator overloading. But there’s no traits for, e.g., wrapping_add, saturating_mul, isqrt, , leading_zeros, rem_euclid, count_ones, div_floor, midpoint, next_multiple_of, etc. etc. — what’s different about pow that motivates a trait for it and not for the other methods?

[ChrisDenton]

It seems like what would be more useful is some way to be generic over number types. Not just special case pow.

Unless there's a particular reason that pow is special?

[scottmcm]

It seems like what would be more useful is some way to be generic over number types.

Obligatory mention of rust-lang/rfcs#3686 with u<N> and i<N>.

[joshtriplett]

We talked about this in today's @rust-lang/libs-api meeting. We'd like to defer this and any other factoring out of methods from integers/floats/etc into traits until we have 1) inherent traits and 2) supertrait item shadowing. Once we have those, we can then systematically go through inherent methods and factor common ones out into traits.