More Rust FFI adventures

We can now convert Rust Errors to C++ exceptions. At the Rust->C++ FFI
boundary, Result<T, Error> will cause Error to be converted to and
thrown as a C++ exception.

nix-rust/src/lib.rs

@@ -4,7 +4,30 @@ mod tarfile;
 
 pub use error::Error;
 
-#[no_mangle]
-pub extern "C" fn unpack_tarfile(source: foreign::Source, dest_dir: &str) {
-    tarfile::unpack_tarfile(source, dest_dir).unwrap();
+pub struct CBox<T> {
+    ptr: *mut libc::c_void,
+    phantom: std::marker::PhantomData<T>,
+}
+
+impl<T> CBox<T> {
+    fn new(t: T) -> Self {
+        unsafe {
+            let size = std::mem::size_of::<T>();
+            let ptr = libc::malloc(size);
+            eprintln!("PTR = {:?}, SIZE = {}", ptr, size);
+            *(ptr as *mut T) = t; // FIXME: probably UB
+            Self {
+                ptr,
+                phantom: std::marker::PhantomData,
+            }
+        }
+    }
+}
+
+#[no_mangle]
+pub extern "C" fn unpack_tarfile(
+    source: foreign::Source,
+    dest_dir: &str,
+) -> CBox<Result<(), error::CppException>> {
+    CBox::new(tarfile::unpack_tarfile(source, dest_dir).map_err(|err| err.into()))
 }