but anyway i'd say bring in an extra overlay to add a patch

        overlays = [
          (import rust-overlay)
          (final: prev: {
            mimalloc =
              if final.stdenv.hostPlatform.isMusl
              then
                prev.mimalloc.overrideAttrs (old: {
                  patches = (old.patches or []) + [
                    (final.fetchpatch {
                      url = "https://github.com/microsoft/mimalloc/commit/951538d469f72532e43a2437d556a73518f5dde4.patch";
                      hash = ""; # TOFU, update with hash
                    })
                  ];
                })
              else
                prev.mimalloc;
          })
        ];