Have you run AFL against git2-rs? I'd love to see that interface hammered on, to make sure it doesn't expose any unsafe behavior from the underlying library.
EDIT: I forgot to mention: It's possible to run AFL on uninstrumented code, it just won't be that smart about finding new code paths.
True, but the Rust bindings necessarily contain tons of unsafe FFI code, and those bindings enforces many required safety properties. Even without checking the underlying C code, running AFL to check for any unsafe holes in the bindings would help.
That said, yes, for best results you'd want to check the combination of C and Rust to find new paths and full coverage on both.