It's hard to have zero deps - I put many hours into one to have no required deps in the end but it was not easy, and writing declarative macros to do anything complex takes work (and a proc macro often means a minimum of two crates). Both of the crates it requires are part of the same project, however.

One of my other crates (getaddrinfo) requires windows-sys and libc which would be challenging to get rid of.

I like the idea of low deps but zero is tough

https://crates.io/crates/ctor/1.0.4/dependencies