openpgp: Mark methods as unreachable.

  - The private trait Stackable has two functions that are only
    implemented by the streaming Signer.  Mark these functions as such
    in the documentation as well as the stub implementations.

3 files changed, 14 insertions, 10 deletions

diff --git a/openpgp/src/serialize/writer/mod.rs b/openpgp/src/serialize/writer/mod.rs
index 3368c6ff..a02339b0 100644
--- a/openpgp/src/serialize/writer/mod.rs
+++ b/openpgp/src/serialize/writer/mod.rs
@@ -83,9 +83,13 @@ pub(crate) trait Stackable<'a, C> : io::Write + fmt::Debug {
     /// Pops the stackable from the stack, detaching it.
     ///
     /// Returns the detached stack.
+    ///
+    /// Note: Only the Signer implements this interface.
     fn pop(&mut self) -> Result<Option<BoxStack<'a, C>>>;
 
     /// Sets the inner stackable.
+    ///
+    /// Note: Only the Signer implements this interface.
     fn mount(&mut self, new: BoxStack<'a, C>);
 
     /// Returns a mutable reference to the inner `Writer`, if
@@ -380,10 +384,10 @@ impl<'a, C: 'a> Stackable<'a, C> for Encryptor<'a, C> {
         Ok(Some(inner))
     }
     fn pop(&mut self) -> Result<Option<BoxStack<'a, C>>> {
-        unimplemented!()
+        unreachable!("Only implemented by Signer")
     }
     fn mount(&mut self, _new: BoxStack<'a, C>) {
-        unimplemented!()
+        unreachable!("Only implemented by Signer")
     }
     fn inner_mut(&mut self) -> Option<&mut Stackable<'a, C>> {
         self.inner.inner_mut()
@@ -448,10 +452,10 @@ impl<'a, C: 'a> Stackable<'a, C> for AEADEncryptor<'a, C> {
         Ok(Some(inner))
     }
     fn pop(&mut self) -> Result<Option<BoxStack<'a, C>>> {
-        unimplemented!()
+        unreachable!("Only implemented by Signer")
     }
     fn mount(&mut self, _new: BoxStack<'a, C>) {
-        unimplemented!()
+        unreachable!("Only implemented by Signer")
     }
     fn inner_mut(&mut self) -> Option<&mut Stackable<'a, C>> {
         self.inner.inner_mut()
diff --git a/openpgp/src/serialize/writer/writer_bzip2.rs b/openpgp/src/serialize/writer/writer_bzip2.rs
index d0189821..513e95e6 100644
--- a/openpgp/src/serialize/writer/writer_bzip2.rs
+++ b/openpgp/src/serialize/writer/writer_bzip2.rs
@@ -46,10 +46,10 @@ impl<'a, C: 'a> Stackable<'a, C> for BZ<'a, C> {
         Ok(Some(inner))
     }
     fn pop(&mut self) -> Result<Option<BoxStack<'a, C>>> {
-        unimplemented!()
+        unreachable!("Only implemented by Signer")
     }
     fn mount(&mut self, _new: BoxStack<'a, C>) {
-        unimplemented!()
+        unreachable!("Only implemented by Signer")
     }
     fn inner_mut(&mut self) -> Option<&mut Stackable<'a, C>> {
         self.inner.inner_mut()
diff --git a/openpgp/src/serialize/writer/writer_deflate.rs b/openpgp/src/serialize/writer/writer_deflate.rs
index f5cdcb7d..64e547f1 100644
--- a/openpgp/src/serialize/writer/writer_deflate.rs
+++ b/openpgp/src/serialize/writer/writer_deflate.rs
@@ -46,10 +46,10 @@ impl<'a, C: 'a> Stackable<'a, C> for ZIP<'a, C> {
         Ok(Some(inner))
     }
     fn pop(&mut self) -> Result<Option<BoxStack<'a, C>>> {
-        unimplemented!()
+        unreachable!("Only implemented by Signer")
     }
     fn mount(&mut self, _new: BoxStack<'a, C>) {
-        unimplemented!()
+        unreachable!("Only implemented by Signer")
     }
     fn inner_mut(&mut self) -> Option<&mut Stackable<'a, C>> {
         self.inner.inner_mut()
@@ -108,10 +108,10 @@ impl<'a, C: 'a> Stackable<'a, C> for ZLIB<'a, C> {
         Ok(Some(inner))
     }
     fn pop(&mut self) -> Result<Option<BoxStack<'a, C>>> {
-        unimplemented!()
+        unreachable!("Only implemented by Signer")
     }
     fn mount(&mut self, _new: BoxStack<'a, C>) {
-        unimplemented!()
+        unreachable!("Only implemented by Signer")
     }
     fn inner_mut(&mut self) -> Option<&mut Stackable<'a, C>> {
         self.inner.inner_mut()