Since the introduction of `Script` `unsafe` started slowly creeping in
as more types with similar semantics were added. The `unsafe` in these
cases is just for trivial conversions between various pointer-like
types. As such, it's possible to move these into a single macro that
takes care of the conversions at one place and avoid repeating the same
`unsafe` code in the codebase. This decreases the cost of audits which
now only need to happen in `internals`, focuses any changes to happen in
that single macro and decreases the chance that we will mess up
similarly to the recent `try_into().expect()` issue (but this time with
UB rather than panic).
The new macro accepts syntax very similar to the already-existing struct
declarations with these differences:
* The struct MUST NOT have `#[repr(transparent)]` - it's added by the
macro
* If the struct uses `PhantomData` it must be the first field and the
real data must be the second field (to allow unsized types).
* The struct must be immediately followed by an impl block containing at
least on conversion function.
* If the struct has generics the impl block has to use the same names of
generics.
* The conversion functions don't have bodies (similarly to required
trait methods) and have a fixed set of allowed signatures.
* Underscore (`_`) must be used in place of the inner type in the
conversion function parameters.
The existing code can simply call the macro with simple changes and get
the same behavior without any direct use of `unsafe`. This change
already calls the macro for all relevant existing types. There are still
some usages left unrelated to the macro, except one additional
conversion in reverse direction on `Script`. It could be moved as well
but since it's on a single place so far it's not really required.
`TaprootMerkleBranchBuf` being a vec introduced intermediate allocation
when creating or decoding `Witness`. However the representation on the
wire is the same as in-memory (aside from `#[repr(transparent)]`) so
this allocation wasn't really needed.
This commit introduces `TaprootMerkleBranch` type which is unsized and
can be used in place of `TaprootMerkleBranchBuf` within `ControlBlock`.
Aside from removing the intermediate allocation, this improves the API a
bit: the conversion from array to other type is no longer needed because
it's performed by `ControlBlock` in its methods. Thus, consumers who
have an array can simply set it as `merkle_branch` field and then encode
the `ControlBlock` into witness. A convenience method is also provided
to push the `ControlBlock` along with other parts at the end of the
`Witness`.
Fmt Bot
Martin Habovstiak