Potential panic in `Signature::try_from`

[nbaztec]

What
Using Signature::try_from can panic in a specific case, due to the interactions between rust's auto-implementations and user oversight.

Expected Behavior
try_from should either return an Ok or Error

Actual Behavior
Given the following code, if the user makes the mistake of passing the bytes as value [u8; 64] instead of a reference &[u8], try_from will accept that input and potentially panic at runtime for invalid input.

let sig = [255u8; 64];
if let Ok(s) = Signature::try_from(sig) {  // should've been `sig.as_ref()`
  ...
}

thread 'main' panicked at 'invalid signature: signature::Error { source: None }', /home/user/.cargo/registry/src/github.zerozr99.workers.dev-1ecc6299db9ec823/ed25519-1.5.2/src/lib.rs:331:38
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Explanation
This is because the From<[u8; 64]> albeit deprecated, is still implemented. Even though TryFrom<&[u8]> is the only impl available, Rust auto-impls TryFrom<[u8; 64]> via From<u8; 64>.

This is quite significant as a single error can give the user the impression that they'll get a Result, whereas the codepath can potentially panic instead.

Removing From<[u8; 64]> implementation should fix this behavior.

[tarcieri]

I assume this is in regard to the `ed25519 crate? There are several crates in this repo.

Please see the comments on the conversion:

https://github.com/RustCrypto/signatures/blob/1eec6b4/ed25519/src/lib.rs#L365-L376

/// DEPRECATED: use `TryFrom<&[u8]>` instead.
///
/// # Warning
///
/// This conversion will panic if a signature is invalid.
// TODO(tarcieri): remove this in the next breaking release

As you can see, all of this is already clearly documented.

Fortunately, as it were, we literally merged the first PR to move towards an ed25519 v2.0 release yesterday: #562

So yes, that conversion can now be removed.

[nbaztec]

Thanks, yes this was with regards to ed25519. The issue came up due to try_from implicitly using from, which is easy to miss with the current design. Thanks for removing it!