Just a nit: Zygote is not technically a language level library. In fact, none of the above libraries really are language level — they are all just Julia librari

Just a nit: Zygote is not technically a language level library. In fact, none of the above libraries really are language level — they are all just Julia libraries, some of which use macros. With the exception of metaprogramming with macros, none of these libraries customize inference/opt to do something funky. Zygote (for example) is based on a language level feature — generated functions which allow multistage programming in Julia — but beyond using this feature it does not configure or otherwise modify the normal compiler pipeline. With Julia, it is sometimes tough to tell — but Diffractor (for example) I would consider a language level library, as it modifies the traditional compiler pipeline to perform inference and optimization in a specific way.