The Miniscript repository and the Miniscript integration PR here have been a moving target for the past months, and some final cleanups were done there that were not included here. I initially intended to add some small followup commits to #24148 but i think there are enough of them to be worth a followup PR on its own.
Some parts of the code did not change since it was initially written in 2019, and the code could use some modernization. (Use std::optional instead of out args, remove old compiler workarounds). We refactored the helpers to be more meaningful, and also did some renaming. A new fuzz target was also added and both were merged in a single file. 2 more will be added in #24149 that will be contained in this file too.
The only behaviour change in this PR is to rule out Miniscript with duplicate keys from sane Miniscripts. In a P2WSH context, signatures can be rebounded (Miniscript does not use CODESEPARATOR) and it’s reasonable to assume that reusing keys across the Script drops the malleability guarantees. It was previously assumed such Miniscript would never exist in the first place since a compiler should never create them. We finally agreed that if one were to exist (say, written by hand or from a buggy compiler) it would be very confusing if an imported Miniscript descriptor (after #24148) with duplicate keys was deemed sane (ie, “safe to use”) by Bitcoin Core. We now check for duplicate keys in the constructor.
This is (still) joint work with Pieter Wuille. (Actually he entirely authored the cleanups and code modernization.)