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.

3 files changed, 25 insertions, 7 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index a4df5c6..1e962e0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,8 @@
   - Return `CommunicationError::NotConnected` instead of
     `CommandError::Undefined` from the connect functions.
   - Remove the `CommandError::Undefined` variant.
+- Add a private `PhantomData` field to `Pro` and `Storage` to make direct
+  instantiation impossible.
 
 # v0.3.4 (2019-01-20)
 - Fix authentication methods that assumed that `char` is signed.
diff --git a/TODO.md b/TODO.md
index 487f56d..8f4e348 100644
--- a/TODO.md
+++ b/TODO.md
@@ -8,7 +8,6 @@
 - Fix timing issues with the `totp_no_pin` and `totp_pin` test cases.
 - Clear passwords from memory.
 - Find a nicer syntax for the `write_config` test.
-- Prevent construction of internal types.
 - Check integer conversions.
 - Consider implementing `Into<CommandError>` for `(Device, CommandError)`
 - Lock password safe in `PasswordSafe::drop()` (see [nitrokey-storage-firmware
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()),
         }
     }