It works!

librusty/.cargo/config

+[build]
+rustflags = ["-C", "lto", "-Z", "embed-bitcode"]

librusty/rust-toolchain

-nightly-2019-09-13
+nightly-2019-07-17

librusty/src/lib.rs

 use std::ffi::CString;
 use std::os::raw::c_char;
 
+#[no_mangle]
 #[allow(non_snake_case)]
 pub extern "C" fn DoSomeInterestingWork(count: i32, content: *const c_char) -> *const c_char {
     let content_string = unsafe { CStr::from_ptr(content) }
         .into_raw()
 }
 
+#[no_mangle]
 #[allow(non_snake_case)]
 pub extern "C" fn FreeString(content: *mut c_char) {
     unsafe {