descriptor: support hash terminals in WalletPolicy translator

[bg002h]

AI authorship disclosure

This patch was authored by Claude Opus 4.7 under direction from a human (the project author of a BIP 388-aware backup format that uses WalletPolicy as its policy type — see "Context" below). Every algorithmic decision, the test matrix, and the commit text was produced by the model; the human reviewed and directed the work but did not hand-write the diff. The patch was tested locally as described in the "Test coverage" section before submission. Reviewing it with the same scrutiny you'd apply to any AI-generated contribution is appropriate.

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The WalletPolicyTranslator used translate_hash_fail! in both directions, which made WalletPolicy::into_descriptor() and from_descriptor() panic on any descriptor containing a sha256/hash256/ripemd160/hash160 terminal. Hash terminals are perfectly valid in segwit-v0 miniscript, and BIP 388 wallet policies place no restriction on them — HTLC-style spending paths (sig + preimage) are a primary use case.

Replaces the panicking translator with manual hash methods that bridge the impedance mismatch between the two key types' associated hash types: KeyExpression uses String (storing hex for template round-tripping), DescriptorPublicKey uses the concrete bitcoin::hashes::*::Hash types.

• KeyExpression -> DescriptorPublicKey: parse the hex String into the binary Hash via the existing FromStr impl. New WalletPolicyError::TranslatorInvalidHashHex(kind, raw) variant for invalid hex (defensive — the template parser already validates length, so unreachable in practice through the public API).
• DescriptorPublicKey -> KeyExpression: render the binary Hash to its canonical lowercase-hex Display form. Infallible.

Test coverage

All six new tests are round-trip-shaped:

• {sha256,hash256,ripemd160,hash160}_terminal_round_trips_through_translator — one named test per hash type, end-to-end through Descriptor::from_str -> WalletPolicy::from_str -> into_descriptor.
• all_ordered_pairs_of_distinct_hash_types_round_trip — 4·3 = 12 ordered pairs of distinct hash types in wsh(and_v(v:and_v(v:pk(@0/**),A(...)),B(...))). Guards against cross-type interference.
• translator_invalid_hash_hex_corrupted_round_trip — drives each hash type's translator forward (Hash → hex, infallible) and reverse (hex → Hash, fallible) for canonical input, then verifies corrupted hex surfaces the new error variant. Reaches the variant via the private translator API since the public parser validates hex up-front.

Default-feature: 152 → 158 tests. All-features: 171 → 176. clippy --lib -D warnings clean, cargo fmt --check clean.

Context

Discovered while building a BIP 388-aware backup format on top of WalletPolicy; the panic blocked a corpus entry that uses sha256 inside an HTLC pattern. The published miniscript v12 doesn't expose WalletPolicy yet, so this is the first downstream user surfacing the bug.

[bg002h]

Feel free to ignore this...I can see all sorts of footguns/scam opportunities that could come from seemingly secure home-rolled constructions implementing minimal security.

I appreciate your work on this and codex32, both of which are key to my attempt at a bip & implementation for doing something like bip 39 for wallet descriptors.

[bg002h]

Sorry for the mess…I’m targeting PR to the wrong fork

[bg002h]

Closing in favor of rust-bitcoin/rust-miniscript#935 — re-routed to true upstream (since wallet_policy/ already lives at rust-bitcoin/rust-miniscript:master). Same branch head (bg002h:fix/wallet-policy-hash-terminals), same content. Apologies for the spam.

—Brian (via Claude — forgive me, I'm a physician!)