rust: add common function to exchange CString objects from/to C

rust/src/common.rs

@@ -1,3 +1,6 @@
+use std::ffi::CString;
+use std::os::raw::c_char;
+
 #[macro_export]
 macro_rules! take_until_and_consume (
  ( $i:expr, $needle:expr ) => (
@@ -10,3 +13,29 @@ macro_rules! take_until_and_consume (
     }
   );
 );
+
+/// Convert a String to C-compatible string
+///
+/// This function will consume the provided data and use the underlying bytes to construct a new
+/// string, ensuring that there is a trailing 0 byte. This trailing 0 byte will be appended by this
+/// function; the provided data should *not* contain any 0 bytes in it.
+///
+/// Returns a valid pointer, or NULL
+pub fn rust_string_to_c(s: String) -> *mut c_char {
+    CString::new(s)
+        .map(|c_str| c_str.into_raw())
+        .unwrap_or(std::ptr::null_mut())
+}
+
+/// Free a CString allocated by Rust (for ex. using `rust_string_to_c`)
+///
+/// # Safety
+///
+/// s must be allocated by rust, using `CString::new`
+#[no_mangle]
+pub unsafe extern "C" fn rs_cstring_free(s: *mut c_char) {
+    if s.is_null() {
+        return;
+    }
+    drop(CString::from_raw(s));
+}