For the other part of the issue (the truncation in SHA512), in Rust you could've had:

    struct Input<'a>(&'a [u8]);
    

    impl<'a> Input<'a> {
      fn new(input: &'a [u8]) -> Result<Self, Error> {
        if input.len() >= 256 {
           return Err(Errro...);
        }
        
        Ok(Self(input))
      }
    }

Now the truncation is a terrible idea in the first place, but at least with this it would have been entirely obvious to the caller that it was there.