Prevent direct instantiation of Pro and Storage

The Pro and Storage structs may only be created using the connect
functions.  This patch adds a private PhantomData field to the structs
to ensure that the compiler does not allow direct instantiation.

1 files changed, 23 insertions, 6 deletions

diff --git a/src/device.rs b/src/device.rs
index 16064c3..40d6ba4 100644
--- a/src/device.rs
+++ b/src/device.rs
@@ -1,4 +1,5 @@
 use std::fmt;
+use std::marker;
 
 use libc;
 use nitrokey_sys;
@@ -154,7 +155,11 @@ pub enum DeviceWrapper {
 /// [`connect`]: fn.connect.html
 /// [`Pro::connect`]: #method.connect
 #[derive(Debug)]
-pub struct Pro {}
+pub struct Pro {
+    // make sure that users cannot directly instantiate this type
+    #[doc(hidden)]
+    marker: marker::PhantomData<()>,
+}
 
 /// A Nitrokey Storage device without user or admin authentication.
 ///
@@ -196,7 +201,11 @@ pub struct Pro {}
 /// [`connect`]: fn.connect.html
 /// [`Storage::connect`]: #method.connect
 #[derive(Debug)]
-pub struct Storage {}
+pub struct Storage {
+    // make sure that users cannot directly instantiate this type
+    #[doc(hidden)]
+    marker: marker::PhantomData<()>,
+}
 
 /// The status of a volume on a Nitrokey Storage device.
 #[derive(Debug)]
@@ -713,8 +722,12 @@ fn get_connected_model() -> Option<Model> {
 
 fn create_device_wrapper(model: Model) -> DeviceWrapper {
     match model {
-        Model::Pro => DeviceWrapper::Pro(Pro {}),
-        Model::Storage => DeviceWrapper::Storage(Storage {}),
+        Model::Pro => DeviceWrapper::Pro(Pro {
+            marker: marker::PhantomData,
+        }),
+        Model::Storage => DeviceWrapper::Storage(Storage {
+            marker: marker::PhantomData,
+        }),
     }
 }
 
@@ -790,7 +803,9 @@ impl Pro {
     pub fn connect() -> Result<Pro, Error> {
         // TODO: maybe Option instead of Result?
         match connect_enum(Model::Pro) {
-            true => Ok(Pro {}),
+            true => Ok(Pro {
+                marker: marker::PhantomData,
+            }),
             false => Err(CommunicationError::NotConnected.into()),
         }
     }
@@ -836,7 +851,9 @@ impl Storage {
     pub fn connect() -> Result<Storage, Error> {
         // TODO: maybe Option instead of Result?
         match connect_enum(Model::Storage) {
-            true => Ok(Storage {}),
+            true => Ok(Storage {
+                marker: marker::PhantomData,
+            }),
             false => Err(CommunicationError::NotConnected.into()),
         }
     }