Breaking change between 1.5.0 and 1.5.1: `U256` cannot be used in a match pattern

[gregorydemay]

Starting 1.5.1 U256 can no longer be used in a match pattern.

The following example pass with 1.5.0 and Rust 1.87.0

#[test]
fn should_be_less_than_two() {
    let value = ethnum::u256::new(1);
    let less_than_two = match value {
        ethnum::u256::ZERO => true,
        ethnum::u256::ONE => true,
        _ => false,
    };
    assert!(less_than_two);
}

However, the same test will no longer compile once updating ethnum to 1.5.1. The compilation error is as follows:

error: constant of non-structural type `U256` in a pattern
  --> src/tests.rs:5:9
   |
5  |         ethnum::u256::ZERO => true,
   |         ^^^^^^^^^^^^^^^^^^ constant of non-structural type
   |
  ::: /Users/greg/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/ethnum-1.5.1/src/uint.rs:18:1
   |
18 | pub struct U256(pub [u128; 2]);
   | --------------- `U256` must be annotated with `#[derive(PartialEq)]` to be usable in patterns
...
22 |     pub const ZERO: Self = U256([0; 2]);
   |     -------------------- constant defined here
   |
   = note: see https://doc.rust-lang.org/stable/std/marker/trait.StructuralPartialEq.html for details

Looking at the changelog, I suspect this might be related to #38.

[nlordell]

Ouuf, thanks for flagging this. I didn't know that match relied on #[derive(PartialEq)] and thought implementing it would be fine.

Will add a test to catch this kind of thing in the future and try to do a hot fix release this weekend.

[nlordell]

Looks like what we actually want, is to implement StructuralPartialEq marker trait with the manual PartialEq implementation (for the improved speed). Unfortunately, that marker trait is unstable for now, so I will rollback to deriving PartialEq and switch to the manual implementation once StructuralPartialEq stabilizes.

[nlordell]

Should be fixed in #52 and released in v1.5.2.

[gregorydemay]

Thanks a lot @nlordell for the quick fix!

I didn't know that match relied on #[derive(PartialEq)] and thought implementing it would be fine.

I agree, this is very surprising and unexpected.