constmuck VS cargo-xtask

Working towards the first real release of constmuck, which provides const fn equivalents of many bytemuckfunctions. constmuck uses bytemuck's traits, so any type that implements those traits can be used with the relevant functions in constmuck.

Using a cargo-xtask task to generate them as a manual step (inlyne currently does this)

As someone else said just is good for that job, or you could implement an xtask helper for these things and setup a suitable development environment with that: https://github.com/matklad/cargo-xtask/

Well tools like cornucopia, prisma-rust-client, protoc-gen-tonic, they don't generate in build.rs, but instead provide either a cli to be called ahead of time, or provide a library that can be called by your own binary (which should generally follow the xtask pattern)

If I understand, this is a tool for when working on the project itself? Akin to a helper script? You could go the cargo install route as already pointed out but there is also the xtask convention.

For simple cli apps for internal use, such as cargo-xtasks, I prefer pico_args due to its fast compile times.

For developer-oriented stuff, there's tools like xshell and cargo-xtask. For operator tasks that need to run in a deployed environment, it's not usually a big lift to add CLI subcommands to your binary. It's certainly more boilerplate and inertia than doing stuff in a live REPL, though, and sometimes difficult to recommend for truly one-off situations.