Does not compile with 1.56.1:

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
   --> bitcoin/src/blockdata/script/witness_program.rs:108:64
    |
108 |         WitnessProgram { version: WitnessVersion::V1, program: ArrayVec::from_slice(&P2A_PROGRAM)}
    |                                                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0015`.
error: could not compile `bitcoin` due to previous error

Oh, it's specifically cargo test --all-features --locked --doc which fails, which is why CI didn't see it.

I would prefer we fix this because it will let me pass my local CI, though I don't really feel it's essential that our doctests meet MSRV.

In bitcoin this now works for me locally: cargo +1.51.1 test --all-features --locked --doc

Oof, sorry for the long delay. But now I get

error: unexpected `cfg` condition name: `rust_v_1_61`

This is "just clippy" but it wasn't happening before and the fix is easy: just add cfg(rust_v_1_61) to the whitelist of cfg parameters in bitcoin/Cargo.toml.

Added. No other changes.