It's a common pattern to provide a foo crate with trait definitions, and foo-derive crate with a
proc-macro derive implementation. Typically, you want foo and foo-derive to be versioned in
lockstep, because derive crates like to use #[doc(hidden)] non-semver-guarded API. Usually, this
is solved by a derive feature, which makes foo depend on foo-derive with =x.y.z constraint.
This, however, is problematic for compile times! It means that compilation of foo-derive is
sequenced before compilation of foo. As foo-derive is a derive macro, it needs to parse the Rust
language. Rust is not a small language, so parsing it is fundamentally hard, and requires loads of
code to do correctly. So it takes some time to compile foo-derive. What's worse, while normally
Cargo pipelines compilation such that .rmeta files are all that's needed to unblock compilation of
dependent crates, for proc macros Cargo really needs to link the whole
...