Introduce Message type to simplify the API

[tomusdrw]

Split off from #93

It would be nice to have SecretKey::sing directly use an array [u8; 32] instead of slice &[u8]. This would improve the api since the message should always be 32 bytes.

I guess using a slice is a carry-over form secp256k1 library. I agree that using [u8; 32] would be more convenient and could reduce the number of Results returned. However in some cases this might trigger unnecessary copying of data, so perhaps instead we could introduce a Message type:

pub struct Message<'a> {
  msg: InternalMessage<'a>,
}

impl<'a> Message {
  pub fn from_slice(slice: &'a [u8]) -> Result<Self, _> {
    // verify length
  }
}
impl<'a> From<[u8; 32]> for Message<'a> {
  ...
}
impl<'a> From<&'a [u8; 32]> for Message<'a> {
...
}

enum InternalMessage<'a> {
   ValidatedSlice(&'a [u8]),
   Borrowed(&'a [u8; 32]),
   Owned([u8; 32]),
}

And take impl Into<Message<'a>> in the API. Wondering how ergonomic the new API would be, since the errors might sometimes be confusing.

@elpiel wdyt?

[elpiel]

It might be a good approach although it would make the API a bit less user-friendly.

Since arrays are what's used I suspect that it fits the API better but then the user of the crate should manually convert a slice to a fixed-length array.

There is an impl of TryFrom for slice to array so it's not that bad.

https://doc.rust-lang.org/std/primitive.array.html#impl-TryFrom%3C%26'a+%5BT%5D%3E-for-%26'a+%5BT;+N%5D

[tomusdrw]

Ah, the conversion looks ideal, I didn't know about it! Yeah, I think the Message type is not needed in that case and just having &[u8; 32] would be good enough, thanks!

[elpiel]

Thank you as well for taking the time to review this idea 😋