Make the `NonZero*` methods const fn

[oli-obk]

and undo the const -> fn changes in https://github.com/rust-lang/rust/pull/53315/files#diff-a4a060a3c734b42317d2b9136d68918eL186

This requires changing applicable methods on NonZero* from fn to const fn, adding the #[rustc_const_unstable="const_nonzero*_methods"] attribute to all of them and adding some tests ensuring that the methods are not const fn on stable rust.

[eddyb]

Doesn't this require panicking in constants?
Ah, no, it requires conditional control-flow.

[oli-obk]

Ah I thought there were more trivial methods.

This issue is only acctionable for get at the moment

[MagnumOpus21]

Hi, I'd like to help out on this. I would appreciate if some one gave me pointers on what needs to be accomplished.

[oli-obk]

Steps:

1. add const to https://github.com/rust-lang/rust/blob/master/src/libcore/num/mod.rs#L80
2. add the rustc_const_unstable attribute
3. add a test in src/test/ui/const that shows a const usage of the method (needs the feature gate specified in rustc_const_unstable
4. See whether the linked commit above can be partially reverted

[TimDiekmann]

@MagnumOpus21 you may take a look at #53697. These are the steps you have to do in order to add const.

[MagnumOpus21]

@cyres I will do that sir. Was busy with my internship.

[TimDiekmann]

@MagnumOpus21 If you need help, don't hesitate to ping me here, in Discord (Cyres#8322) or on StackOverflow in the rust chat room.

[MagnumOpus21]

Thank you 😄 @cyres