With the new `bitcoin_io` library, implementing `io::Write`
manually is somewhat tricky - for `std` users we really want to
provide an `std::io::Write` implementation, however for `no-std`
users we want to implement against our internal trait.
Sadly we cannot provide a blanket implementation of
`std::io::Write` for all types whcih implement our `io::Write`
trait as its an out-of-crate impl.
Instead, we provide a macro which will either implement
`std::io::Write` or our `io::Write` depending on the feature flags
set on `bitcoin_io`.
In order to move towards our own I/O traits in the `rust-bitcoin`
ecosystem, we have to slowly replace our use of the `std` and
`core2` traits.
Here we take the first real step, replacing
`{std,core2}::io::Write` withour own `bitcoin_io::io::Write`. We
provide a blanket impl for our trait for all `std::io::Write`, if
the `std` feature is enabled, allowing users who use their own
streams or `std` streams to call `rust-bitcoin` methods directly.
In the coming commits we'll move our `io` module to having its own
implementations of the usual I/O traits and structs. Here we first
move to re-exporting only exactly what we need, allowing us to
whittle the list down from a fixed set.
In order to support standard (de)serialization of structs, the
`rust-bitcoin` ecosystem uses the standard `std::io::{Read,Write}`
traits. This works great for environments with `std`, however sadly
the `std::io` module has not yet been added to the `core` crate.
Thus, in `no-std`, the `rust-bitcoin` ecosystem has historically
used the `core2` crate to provide copies of the `std::io` module
without any major dependencies. Sadly, its one dependency,
`memchr`, recently broke our MSRV.
Worse, because we didn't want to take on any excess dependencies
for `std` builds, `rust-bitcoin` has had to have
mutually-exclusive `std` and `no-std` builds. This breaks general
assumptions about how features work in Rust, causing substantial
pain for applications far downstream of `rust-bitcoin` crates.
Here, we add a new `bitcoin_io` crate, making it an unconditional
dependency and using its `io` module in the in-repository crates
in place of `std::io` and `core2::io`. As it is not substantial
additional code, the `hashes` io implementations are no longer
feature-gated.
This doesn't actually accomplish anything on its own, only adding
the new crate which still depends on `core2`.
Matt Corallo