go/doc/next/6-stdlib/99-minor/crypto/63405.md

[MessageSigner] is a new signing interface that can be implemented by signers that wish to hash the message to be signed themselves. A new function is also introduced, [SignMessage] which attempts to update a [Signer] interface to [MessageSigner], using the [MessageSigner.SignMessage] method if successful, and [Signer.Sign] if not. This can be used when code wishes to support both [Signer] and [MessageSigner].