ser: sequences: Introduce SeqSerializer newtype

We're going to want to do something more complicated.

In particular, to handle nested arrays properly, we need to do some
work at the start and end of each array.

The `new` and (inherent) `end` methods of this newtype is where that
work will be done.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>
(cherry picked from commit 147e6c7)
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

src/ser.rs

@@ -12,6 +12,8 @@ pub struct ConfigSerializer {
     pub output: Config,
 }
 
+pub struct SeqSerializer<'a>(&'a mut ConfigSerializer);
+
 impl ConfigSerializer {
     fn serialize_primitive<T>(&mut self, value: T) -> Result<()>
     where
@@ -85,10 +87,10 @@ impl ConfigSerializer {
 impl<'a> ser::Serializer for &'a mut ConfigSerializer {
     type Ok = ();
     type Error = ConfigError;
-    type SerializeSeq = Self;
-    type SerializeTuple = Self;
-    type SerializeTupleStruct = Self;
-    type SerializeTupleVariant = Self;
+    type SerializeSeq = SeqSerializer<'a>;
+    type SerializeTuple = SeqSerializer<'a>;
+    type SerializeTupleStruct = SeqSerializer<'a>;
+    type SerializeTupleVariant = SeqSerializer<'a>;
     type SerializeMap = Self;
     type SerializeStruct = Self;
     type SerializeStructVariant = Self;
@@ -159,7 +161,8 @@ impl<'a> ser::Serializer for &'a mut ConfigSerializer {
         for byte in v {
             seq.serialize_element(byte)?;
         }
-        seq.end()
+        seq.end();
+        Ok(())
     }
 
     fn serialize_none(self) -> Result<Self::Ok> {
@@ -214,7 +217,7 @@ impl<'a> ser::Serializer for &'a mut ConfigSerializer {
     }
 
     fn serialize_seq(self, _len: Option<usize>) -> Result<Self::SerializeSeq> {
-        Ok(self)
+        SeqSerializer::new(self)
     }
 
     fn serialize_tuple(self, len: usize) -> Result<Self::SerializeTuple> {
@@ -260,25 +263,36 @@ impl<'a> ser::Serializer for &'a mut ConfigSerializer {
     }
 }
 
-impl<'a> ser::SerializeSeq for &'a mut ConfigSerializer {
+impl<'a> SeqSerializer<'a> {
+    fn new(inner: &'a mut ConfigSerializer) -> Result<Self> {
+        Ok(SeqSerializer(inner))
+    }
+
+    fn end(self) -> &'a mut ConfigSerializer {
+        self.0
+    }
+}
+
+impl<'a> ser::SerializeSeq for SeqSerializer<'a> {
     type Ok = ();
     type Error = ConfigError;
 
     fn serialize_element<T>(&mut self, value: &T) -> Result<()>
     where
         T: ?Sized + ser::Serialize,
     {
-        self.inc_last_key_index()?;
-        value.serialize(&mut **self)?;
+        self.0.inc_last_key_index()?;
+        value.serialize(&mut *(self.0))?;
         Ok(())
     }
 
     fn end(self) -> Result<Self::Ok> {
+        self.end();
         Ok(())
     }
 }
 
-impl<'a> ser::SerializeTuple for &'a mut ConfigSerializer {
+impl<'a> ser::SerializeTuple for SeqSerializer<'a> {
     type Ok = ();
     type Error = ConfigError;
 
@@ -294,7 +308,7 @@ impl<'a> ser::SerializeTuple for &'a mut ConfigSerializer {
     }
 }
 
-impl<'a> ser::SerializeTupleStruct for &'a mut ConfigSerializer {
+impl<'a> ser::SerializeTupleStruct for SeqSerializer<'a> {
     type Ok = ();
     type Error = ConfigError;
 
@@ -310,7 +324,7 @@ impl<'a> ser::SerializeTupleStruct for &'a mut ConfigSerializer {
     }
 }
 
-impl<'a> ser::SerializeTupleVariant for &'a mut ConfigSerializer {
+impl<'a> ser::SerializeTupleVariant for SeqSerializer<'a> {
     type Ok = ();
     type Error = ConfigError;
 
@@ -322,8 +336,8 @@ impl<'a> ser::SerializeTupleVariant for &'a mut ConfigSerializer {
     }
 
     fn end(self) -> Result<Self::Ok> {
-        ser::SerializeSeq::end(&mut *self)?;
-        self.pop_key();
+        let inner = self.end();
+        inner.pop_key();
         Ok(())
     }
 }