diff --git a/nix-rust/src/error.rs b/nix-rust/src/error.rs
index 586a8f53d..85ac926e9 100644
--- a/nix-rust/src/error.rs
+++ b/nix-rust/src/error.rs
@@ -76,7 +76,7 @@ impl From<Error> for CppException {
     fn from(err: Error) -> Self {
         match err {
             Error::Foreign(ex) => ex,
-            _ => unsafe { make_error(&err.to_string()) },
+            _ => CppException::new(&err.to_string()),
         }
     }
 }
@@ -85,7 +85,23 @@ impl From<Error> for CppException {
 #[derive(Debug)]
 pub struct CppException(*const libc::c_void); // == std::exception_ptr*
 
+impl CppException {
+    fn new(s: &str) -> Self {
+        Self(unsafe { make_error(s) })
+    }
+}
+
+impl Drop for CppException {
+    fn drop(&mut self) {
+        unsafe {
+            destroy_error(self.0);
+        }
+    }
+}
+
 extern "C" {
     #[allow(improper_ctypes)] // YOLO
-    fn make_error(s: &str) -> CppException;
+    fn make_error(s: &str) -> *const libc::c_void;
+
+    fn destroy_error(exc: *const libc::c_void);
 }
diff --git a/src/libutil/rust-ffi.cc b/src/libutil/rust-ffi.cc
index accc5e22b..6f36b3192 100644
--- a/src/libutil/rust-ffi.cc
+++ b/src/libutil/rust-ffi.cc
@@ -3,10 +3,14 @@
 
 extern "C" std::exception_ptr * make_error(rust::StringSlice s)
 {
-    // FIXME: leak
     return new std::exception_ptr(std::make_exception_ptr(nix::Error(std::string(s.ptr, s.size))));
 }
 
+extern "C" void destroy_error(std::exception_ptr * ex)
+{
+    free(ex);
+}
+
 namespace rust {
 
 std::ostream & operator << (std::ostream & str, const String & s)
diff --git a/src/libutil/rust-ffi.hh b/src/libutil/rust-ffi.hh
index 1466eabec..4fecce606 100644
--- a/src/libutil/rust-ffi.hh
+++ b/src/libutil/rust-ffi.hh
@@ -152,28 +152,47 @@ struct Source
 template<typename T>
 struct Result
 {
-    unsigned int tag;
+    enum { Ok = 0, Err = 1, Uninit = 2 } tag;
 
     union {
         T data;
         std::exception_ptr * exc;
     };
 
+    Result() : tag(Uninit) { }; // FIXME: remove
+
+    Result(const Result &) = delete;
+
+    Result(Result && other)
+        : tag(other.tag)
+    {
+        other.tag = Uninit;
+        if (tag == Ok)
+            data = std::move(other.data);
+        else if (tag == Err)
+            exc = other.exc;
+    }
+
     ~Result()
     {
-        if (tag == 0)
+        if (tag == Ok)
             data.~T();
-        else if (tag == 1)
-            // FIXME: don't leak exc
+        else if (tag == Err)
+            free(exc);
+        else if (tag == Uninit)
             ;
+        else
+            abort();
     }
 
     /* Rethrow the wrapped exception or return the wrapped value. */
     T unwrap()
     {
-        if (tag == 0)
+        if (tag == Ok) {
+            tag = Uninit;
             return std::move(data);
-        else if (tag == 1)
+        }
+        else if (tag == Err)
             std::rethrow_exception(*exc);
         else
             abort();