It’s a rust crate designed to be a native rust replacement for a rust c wrapper crate. It’s faster and easier to link to in rust projects.

How would you even tell people you made a better rust crate without using the word “rust?”

Why not spend the efforts to speed up the real zlib? So that the whole world actually gets to spin a bit faster.

Rust folks are claiming excellent interoperability with C binaries. Why the need for a rewrite then?

Maybe because the speed up is easier to attain in a language where you aren't constantly worrying about introducing bugs? Maybe development is easier in a language with more modern tooling?

Interoperability runs both ways, everyone currently taking a dependency on the C library can swap in the rust library in its place and see the same benefits

It’s a rust crate that depended on c and is now literally “rewritten in rust”

Why the hate? It's a genuine question. When you rewrite something, you need to justify the effort somehow. The GNU coreutils started out as "the BSD utilities, but with the GPL!".

Because the reimplementation authors skip all the complexities of designing the tool in the first place while getting right to the fun part (which is coding), and then they get to call themselves authors of a well known infrastructure tool.

Compare "I have typed a setuid() wrapper in rust" vs "I'm the author of sudo-rs".