I've managed to provide static options to jemalloc in my application via this code:

#[no_mangle]
pub static mut malloc_conf: *const libc::c_char =
    b"background_thread:true,metadata_thp:auto\0".as_ptr() as _;

This isn't very nice, is quite unsafe though no unsafe is present, and is dependent on prefixing being disabled. It would be nice if the crate provided a way to set this string in a safe(r) way.