Hash doesn't work for generic enums

[brunocodutra]

Describe the bug

The code generated by deriving Hash for generic enumerations leads to issues with type inference.

To Reproduce

#[derive(derivative::Derivative)]
#[derivative(Hash)]
enum Enumeration<T> {
    Variant(T),
}

Expected behavior

I expect valid code akin to the example below to be generated

impl<T: Hash> Hash for Enumeration<T> {
    fn hash<H: Hasher>(&self, state: &mut H) {
        use Enumeration::*;
        match self {
            Variant(t) => t.hash(state),
        }

        discriminant(self).hash(state);
    }
}

Errors

error[E0282]: type annotations needed
 --> src/test.rs:3:6
  |
3 |   enum Enumeration<T> {
  |  ______^
4 | |     Variant(T),
  | |___________^ cannot infer type for type parameter `T` declared on the enum `Enumeration`

Version (please complete the following information):

rustup --version
rustup 1.21.1 (2020-02-23)

cargo --version
cargo 1.43.0-nightly (bda50510d 2020-03-02)

rustc --version
rustc 1.43.0-nightly (c20d7eecb 2020-03-11)

• Version of derivative: 2.0.2

Additional context

N/A

[mcarton]

Thanks for the report and the example!

This should now be fixed in v2.1.0.