Code shared from the Rust Playground

playground.rs

/// The trait interesting for everyone wanting to sign PSBTs, be it using their
/// own code or ours
pub trait PsbtSigner {
    type Error: Debug;
    
    fn verify(_psbt: &Psbt) -> Result<(), Self::Error> {
        // Some default validation code
        Ok(())
    }
    
    fn sign(&self, psbt: Psbt) -> Result<Psbt, Self::Error>;
}



/// We alos supply a default signer with an interface similar to what you
/// initially proposed
pub struct DefaultPsbtSigner {
    pub sign_fn: Box<dyn Fn(Message, DerivationPath) -> Result<Option<Signature>, DefaultPsbtSignerError>>,
}

#[derive(Debug)]
pub enum DefaultPsbtSignerError {}

impl PsbtSigner for DefaultPsbtSigner {
    type Error = DefaultPsbtSignerError;

    #[allow(unused_mut)]
    fn sign(&self, mut psbt: Psbt) -> Result<Psbt, Self::Error> {
        Self::verify(&psbt)?;
        // do things with psbt, call sign_fn, add signature to psbt
        Ok(psbt)
    }
}



/// But we can even extend our default signer to hold its own secrets
pub struct SecretPsbtSigner {
    signer: DefaultPsbtSigner,
}

impl PsbtSigner for SecretPsbtSigner {
    type Error = DefaultPsbtSignerError;

    #[allow(unused_mut)]
    fn sign(&self, mut psbt: Psbt) -> Result<Psbt, Self::Error> {
        self.signer.sign(psbt)
    }
}

impl SecretPsbtSigner {
    pub fn from_xkey(master: XPrivKey) -> Self {
        SecretPsbtSigner {
            signer: DefaultPsbtSigner {
                sign_fn: Box::new(move |_msg, _path| {
                    // sign using XPrivKey
                    let _ = master;
                    // could return Ok(None) if it doesn't have the key
                    Ok(Some(Signature {}))
                })
            }
        }
    }
}

use std::fmt::Debug;

pub struct Psbt {}
pub struct Signature {}
pub struct Message {}
pub struct DerivationPath {}
pub struct XPrivKey {}

fn main() {}