Conditional compilation depending on Rust version using `cfg` had the
disadvantage that we had to write the same code multiple times, compile
it multiple times, execute it multiple times, update it multiple
times... Apart from obvious maintenance issues the build script wasn't
generating the list of allowed `cfg`s so those had to be maintained
manually in `Cargo.toml`. This was fixable by printing an appropriate
line but it's best to do it together with the other changes.
Because we cannot export `cfg` flags from a crate to different crates we
take a completely different approach: we define a macro called
`rust_version` that takes a very naturally looking condition such as
`if >= 1.70 {}`. This macro is auto-generated so that it produces
different results based on the compiler version - it either expands to
first block or the second block (after `else`).
This way, the other crates can simply call the macro when needed.
Unfortunately some minimal maintenance is still needed: to update the
max version number when a newer version is used. (Note that code will
still work with higher versions, it only limits which conditions can be
used in downstream code.) This can be automated with the pin update
script or we could just put the pin file into the `internals` directory
and read the value from there. Not automating isn't terrible either
since anyone adding a cfg with higher version will see a nice error
about unknown version of Rust and can update it manually.
Because this changes syntax to a more naturally looking version number,
as a side effect the `cond_const` macro could be also updated to use the
new macro under the hood, providing much nicer experience - it is no
longer needed to provide human-readable version of the version string to
put in the note about `const`ness requiring a newer version. As such the
note is now always there using a single source of truth.
It's also a great moment to introduce this change right now since
there's currently no conditional compilation used in `bitcoin` crate
making the changes minimal.
There are various challenges when making code `const`: making it
conditional, awkward copying of slices... This change adds tools that
help dealing with these challenges.
Martin Habovstiak