Let's list here crates that enhance Rust as a language.
It not "batteries" like in stdx, but Rust-specific crates for workarounds for various missing features and experimental ideals from non-accepted/postponed RFCs, or just hacky tricks.
The list is supposed to contain (mostly) crates that are internal to Rust, not ones for making Rust deal with "external world" like other languages bindings, file formats, protocols and so on.
Primary focus should be on crates that are not easy to find by conventional means (e.g. no algorithm name, format or protocol to search for).
Note that quality of the listed crates may vary from proof-of-concept to stable-and-widely-used.