BIP-174 defines not just a structure and serialization for PSBTs, but also responsibilities — a well-defined roles of how PSBT can be modified by different actors, which is required for correct workflow organization. Right now PSBT implementation in the current rust-bitcoin code does not address this part of the spec; neither there any other rust implementation of it. Even PSBT signers (Firma and MagicalBitcoin) misses validation parts that are required by BIP-174.

While this library is not a wallet library, it seems that implementation of well-defined API for the responsibilities will be beneficial; plus we can implement non-signing part of PSBT validation described in this section of BIP-174. I am planning to work on that and do it as a number of decorators+facades, one per responsibility, at the same time limiting possible PSBT operations (facade) and implementing common responsibility business logic (like validation of the internal structure) as methods (decorator).

pub trait Responsibility {
  fn from_psbt(psbt: PartiallySignedTransaction) -> Self;
  fn into_psbt(self) -> PartiallySignedTransaction;
}

pub struct PsbtSigner {
  psbt: PartiallySignedTransaction,
}

impl Responsibility for PsbtSigner { /* ... */ }

impl PsbtSigner {
  pub fn valdiate(&self) -> Result<(), ValidationError> {
     // ....
  }

  pub fn sign<F>(&mut self, signer: F) -> Result<PartiallySignedTransaction> 
      where F: FnMut(Message, Fingerprint, DerivationPath) -> Signature {
     // ....
  }
}

// ... other responsibilities

Related PR in miniscript: rust-bitcoin/rust-miniscript#119