Wrapping neg is not implemented for PrimInt

[cuviper]

From @gnzlbg on December 23, 2015 18:40

When constraining on PrimInt there is no way of performing wrapping neg in a generic way.

The "generic" way to do this seems to be to call the .wrapping_neg() method which is offered by all primitive integers (but not by PrimInt).

Copied from original issue: rust-num/num#146

[cuviper]

Hmm, it's odd that std::num::Wrapping doesn't implement Neg.

AFAICS .wrapping_neg() was only added in Rust 1.1, but we're still trying to maintain 1.0 compatibility. Bumping that minimum version is a breaking change IMO.

I suppose you could do manual 2's complement, x.not().wrapping_add(1).

[cuviper]

From @gnzlbg on December 23, 2015 19:2

rust-lang/rust#30524 was just oppened, looks like an overlook.

[cuviper]

From @gnzlbg on December 23, 2015 19:3

I suppose you could do manual 2's complement, x.not().wrapping_add(1).

Yes I could, thanks for the tip.

AFAICS .wrapping_neg() was only added in Rust 1.1, but we're still trying to maintain 1.0 compatibility. Bumping that minimum version is a breaking change IMO.

Could we use a cfg macro for that? E.g. if rust version >= 1.1 offer wrapping_neg in PrimInt, otherwise, not offer it?

[cuviper]

There's no cfg for the rust version, unfortunately.

[cuviper]

From @briansmith on January 8, 2016 22:48

In the interim, you could add a new, off-by-default feature for wrapping_neg. Then people who need it could opt in to it, knowing that it requires Rust 1.1 or later.

[cuviper]

That's possible, if there's strong demand. I don't want a combinatorial explosion of features though...

[cuviper]

AFAICS .wrapping_neg() was only added in Rust 1.1, but we're still trying to maintain 1.0 compatibility.

We were later forced to 1.8 (rust-num/num#257), so I think we can do this now without breaking.