The documentation for EcKdf says:

In the current version of this crate, only the null KDF is
supported, which takes no shared data. Therefore 'a can always be inferred
'static.

But there are two methods: null and sha256. sha256's signature is:

fn sha256() -> Self

That is, it doesn't have an elegant way to fail fast. Is sha256 supported? Is the error returned later? Is the above comment out of date?

Thanks!