I don't know if it can be expressed as a compile-time constant, so it's
a comment for now.
About this series:
This is quite old patches for rust-chg. I heard from Octobus people that
there's a plan to do an experiment on merging hgcli + chg + some Rust?,
so I decided to respin the rust-chg series.
Maybe we'll rewrite the core to leverage the recent async/await functionality,
but I want to first make my old patches in so the rust-chg can be a drop-in
replacement for the chg of C. Compiler warnings will be removed later, and
the codebase will be upgraded to the 2018 edition later.