Add Hash derive

[fredszaq]

Synopsis

This PR adds a new derive for the Hash trait. This does mostly the same as the derive bundled with rust std but with a few niceties :

• no overcontrain of generic types
• possibility to ignore fields with #[hash(skip)] (also honors #[eq(skip)] and #[partial_eq(skip)] to ensure trait expectations)
• possibility to use custom hash function on fields with #[hash(with(my_custom_hash_function)]

Solution

I used the PartialEq derive as an example to implement this, didn't have to do exotic things to get things to work

Checklist

• Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)

[tyranron]

@fredszaq that's a solid work!

One major concern should be addressed before merging this. Otherwise, the PR makes a very good impression!

Thank you very much for contributing this and being pedantic to the implementation!

UPD: The concern appeared to be not major and non-concern at all. Then, will try to merge it the next few days!

[tyranron]

@fredszaq I think, deriving Hash is a little bit trickier than it seems to be.

For the correctness, we should definitely consider and work correctly with the prefix collisions. They also should have good coverage in tests, since seem to be quite error-prone to edge cases.

[tyranron]

Hmmm... okay, it seems that std impls for tuples doesn't bother with that. It's all up to "leaf" types to bother with such things.

Good, so we don't need to bother about that. However, some tests for such cases still would be good to see.

[fredszaq]

@tyranron I added a few more tests around possible collisions, see 3fd8115 did you have other use cases in mind ?

[sebschrader]

The rustc also doesn't bother for its derived impls (pass -Zunpretty=expanded to rustc or see playground):

#[derive(Hash)]
struct Test {
    a: u32,
    b: u32,
}

becomes

#![feature(prelude_import)]
extern crate std;
#[prelude_import]
use std::prelude::rust_2024::*;
struct Test {
    a: u32,
    b: u32,
}
#[automatically_derived]
impl ::core::hash::Hash for Test {
    #[inline]
    fn hash<__H: ::core::hash::Hasher>(&self, state: &mut __H) {
        ::core::hash::Hash::hash(&self.a, state);
        ::core::hash::Hash::hash(&self.b, state)
    }
}

[Ten0]

I'd like to PR support for with in PartialEq as well, but I see this would conflict with this PR that already adds the attribute for the Hash case. Would love to see this merged so I can build on top! 😃
(Also I would also have uses for this PR 🙂.)

[fredszaq]

@Ten0 #535 already open here 😄

[fredszaq]

Hey @tyranron, pinging back on this, anything I can do to help you get this merged ?

[tyranron]

@fredszaq

anything I can do to help you get this merged ?

Nah... I would like to, but don't see how you could possibly decrease the load on my current job for me to get back on this 😅

Anyway, thanks for asking! I will reall try to merge this on these weekends.

[fredszaq]

No pressure^^ do not hesitate if you have any questions or want me to make changes !