The StringPtr struct under src/string.rs in the Rust project seems to be unnecessary. The Rust std provides FFI structs that wrap Rust str and String types, to allow communication with a C library. (See CString and CStr in the std).

Unless you can’t use the Rust std in Android (Which should be designated in lib.rs with a #![no_std] at the top), there is no reason to reimplement this.