Implement Hash for Number.

[timothee-haudebourg]

This is the minimal version of my previous PR #790. I implemented the Eq and Hash traits for the Number type, regardless of its definition. Contrarily to my previous PR, this is strictly limited to the Number type.

I was originally going to use the ordered_float crate for that, but it turns out it is much simpler to do it manually. It avoids a dependency, and this library is not compatible with Rust 1.31.0 because it uses const parameters somewhere in the code of an optional feature, which has a syntax that is not supported with this version of Rust.

I had to manually implement PartialEq for Number to please clippy.

[dtolnay]

Thanks, this mostly looks good.

[dtolnay]

https://doc.rust-lang.org/1.56.0/std/hash/trait.Hash.html#hash-and-eq
Hash has a documented requirement that k1 == k2 implies hash(k1) == hash(k2), which this implementation violates.

use serde_json::Number;
use std::hash::{Hash, Hasher};
use std::collections::hash_map::DefaultHasher;

fn main() {
    let k1 = serde_json::from_str::<Number>("0.0").unwrap();
    let k2 = serde_json::from_str::<Number>("-0.0").unwrap();
    dbg!(k1 == k2);
    dbg!(hash(k1) == hash(k2));
}

fn hash(obj: impl Hash) -> u64 {
    let mut hasher = DefaultHasher::new();
    obj.hash(&mut hasher);
    hasher.finish()
}

[src/main.rs:8] k1 == k2 = true
[src/main.rs:9] hash(k1) == hash(k2) = false

[timothee-haudebourg]

Right, I forgot about that.

[timothee-haudebourg]

I've added a test in the hash function to use the +0 hash for both +0 and -0. I've also added a test that cover this case.

With the arbitrary_precision feature, +0 and -0 are not equals so no problem here.

[timothee-haudebourg]

Oh right, my test does not work with arbitrary_precision of course, let me fix that.

[dtolnay]

Thanks!