Feature request: hash_with at the enum-variant level

[Pr0methean]

Describe the bug
It would be useful to be able to apply the hash_with attribute to an enum variant rather than to individual fields, so that fields can interact if necessary.

To Reproduce
The enum I'd use this on is:

#[derive(Derivative)]
#[derivative(Clone, Debug, Hash, Eq)]
#[repr(u8)]
pub enum ComplexFactor {
    AddSub {
        #[derivative(Hash(hash_with = "crate::algebraic::hash_add_sub"))]
        terms: (Factor, Factor),
        subtract: bool,
    },
    /* and other variants */
}
fn hash_add_sub<H: Hasher>(terms: &(Factor, Factor), state: &mut H) {
    let (left, right) = terms;
    let invariant_hasher_state = RandomState::with_seed(0x1337c0de);
    let left_hash = invariant_hasher_state.hash_one(left);
    let right_hash = invariant_hasher_state.hash_one(right);
    state.write_u64(left_hash.wrapping_add(right_hash));
}

This way I could ensure that x+y and y+x have the same hash, but x-y and y-x don't, and avoid the double hashing of subtraction terms.
Expected behavior
I can replace the above code with:

#[derive(Derivative)]
#[derivative(Clone, Debug, Hash, Eq)]
#[repr(u8)]
pub enum ComplexFactor {
    #[derivative(Hash(hash_with = "crate::algebraic::hash_add_sub"))]
    AddSub {
        terms: (Factor, Factor),
        subtract: bool,
    },
    /* and other variants */
}
fn hash_add_sub<H: Hasher>(terms: &(Factor, Factor), subtract: bool, state: &mut H) {
    let (left, right) = terms;
    if subtract {
        left.hash(state);
        right.hash(state);
    } else {
        let invariant_hasher_state = RandomState::with_seed(0x1337c0de);
        let left_hash = invariant_hasher_state.hash_one(left);
        let right_hash = invariant_hasher_state.hash_one(right);
        state.write_u64(left_hash.wrapping_add(right_hash));
    }
}

Errors

error: Unknown attribute `hash_with` for trait `Hash`
   --> src/algebraic.rs:628:23
    |
628 |     #[derivative(Hash(hash_with = "crate::algebraic::hash...
    |                       ^^^^^^^^^

Version:

$ rustup --version
rustup 1.28.2 (e4f3ad6f8 2025-04-28)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: The currently active `rustc` version is `rustc 1.94.0-nightly (7b5cde737 2025-12-27)`
$ cargo --version
cargo 1.94.0-nightly (94c368ad2 2025-12-26)

• Version of derivative: 2.2.0