`sign_psbt` should be told what policy to sign for.

[darosior]

This was reported to me by @bigspider, and i think it makes sense. Copy paste of the report follows.

async fn sign_tx(&self, tx: &mut Psbt) -> Result<(), Error>;

I think you shouldn't abstract the policy in the HWI trait; sign_psbt without specifying what policy you're signing with puts the burden of figuring it out to the library.
But the caller of sign_psbt always knows which policy you're signing with.
if you have multiple inputs with different policies, most likely it's going to be from different signers − so multiple calls to sign_psbt with different policies
sure, one might imagine signing with multiple policies at the same time (if they are all controlled by the same hardware wallet), but I wouldn't bother yet, can generalize in the future

[edouardparis]

It does indirectly.
Before signing with ledger, the policy must be loaded in the client struct:
https://github.com/wizardsardine/async-hwi/blob/master/src/ledger.rs#L39

I defined the interface in a way that it is the simplest and the most common with specter which does not need the policy in the sign_psbt request (Already loaded in specter device memory).

But maybe we should explore other way to do it like for example:

harware_wallet.with_policy(&policy).sign_psbt(&mut psbt)