diff --git a/nix-rust/src/c.rs b/nix-rust/src/c.rs
index 3feeb71a3..e0462742f 100644
--- a/nix-rust/src/c.rs
+++ b/nix-rust/src/c.rs
@@ -1,20 +1,21 @@
 use super::{
     error,
-    foreign::{self, CBox},
+    foreign::{self},
     store::path,
     store::StorePath,
     util,
 };
 
 #[no_mangle]
-pub extern "C" fn unpack_tarfile(
+pub unsafe extern "C" fn unpack_tarfile(
     source: foreign::Source,
     dest_dir: &str,
-) -> CBox<Result<(), error::CppException>> {
-    CBox::new(
+    out: *mut Result<(), error::CppException>,
+) {
+    out.write(
         util::tarfile::unpack_tarfile(source, std::path::Path::new(dest_dir))
             .map_err(|err| err.into()),
-    )
+    );
 }
 
 #[no_mangle]
diff --git a/nix-rust/src/foreign.rs b/nix-rust/src/foreign.rs
index 8e04280f3..7bce7753c 100644
--- a/nix-rust/src/foreign.rs
+++ b/nix-rust/src/foreign.rs
@@ -12,22 +12,3 @@ impl std::io::Read for Source {
         Ok(n)
     }
 }
-
-pub struct CBox<T> {
-    pub ptr: *mut libc::c_void,
-    phantom: std::marker::PhantomData<T>,
-}
-
-impl<T> CBox<T> {
-    pub fn new(t: T) -> Self {
-        unsafe {
-            let size = std::mem::size_of::<T>();
-            let ptr = libc::malloc(size);
-            *(ptr as *mut T) = t; // FIXME: probably UB
-            Self {
-                ptr,
-                phantom: std::marker::PhantomData,
-            }
-        }
-    }
-}
diff --git a/src/libutil/rust-ffi.hh b/src/libutil/rust-ffi.hh
index a77f83ac5..1466eabec 100644
--- a/src/libutil/rust-ffi.hh
+++ b/src/libutil/rust-ffi.hh
@@ -180,24 +180,4 @@ struct Result
     }
 };
 
-template<typename T>
-struct CBox
-{
-    T * ptr;
-
-    T * operator ->()
-    {
-        return ptr;
-    }
-
-    CBox(T * ptr) : ptr(ptr) { }
-    CBox(const CBox &) = delete;
-    CBox(CBox &&) = delete;
-
-    ~CBox()
-    {
-        free(ptr);
-    }
-};
-
 }
diff --git a/src/libutil/tarfile.cc b/src/libutil/tarfile.cc
index 79fb9f721..1be0ba24c 100644
--- a/src/libutil/tarfile.cc
+++ b/src/libutil/tarfile.cc
@@ -3,7 +3,7 @@
 
 extern "C" {
     rust::Result<std::tuple<>> *
-    unpack_tarfile(rust::Source source, rust::StringSlice dest_dir);
+    unpack_tarfile(rust::Source source, rust::StringSlice dest_dir, rust::Result<std::tuple<>> & out);
 }
 
 namespace nix {
@@ -11,7 +11,9 @@ namespace nix {
 void unpackTarfile(Source & source, const Path & destDir)
 {
     rust::Source source2(source);
-    rust::CBox(unpack_tarfile(source2, destDir))->unwrap();
+    rust::Result<std::tuple<>> res;
+    unpack_tarfile(source2, destDir, res);
+    res.unwrap();
 }
 
 void unpackTarfile(const Path & tarFile, const Path & destDir,