[Rust] Simplify done unwrapping to prevent dual mutable aliasing

rust/car_example/src/main.rs

@@ -139,10 +139,10 @@ fn decode_car_and_assert_expected_content(buffer: &[u8]) -> CodecResult<()> {
     println!("Activation Code: {}", activation_code);
     assert_eq!("abcdef", activation_code);
 
-    let (position, buffer_back) = dec_done.unwrap();
+    let position = dec_done.unwrap();
     println!("Finished decoding. Made it to position {0} out of {1}",
              position,
-             buffer_back.len());
+             buffer.len());
     Ok(())
 }
 
@@ -202,7 +202,7 @@ fn encode_car_from_scratch() -> CodecResult<Vec<u8>> {
         let enc_model = enc_manufacturer.manufacturer("Honda".as_bytes())?;
         let enc_activation_code = enc_model.model("Civic VTi".as_bytes())?;
         let done = enc_activation_code.activation_code("abcdef".as_bytes())?;
-        let (pos, _) = done.unwrap();
+        let pos = done.unwrap();
         pos
     };
     println!("encoded up to position {}", used_pos);

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/rust/RustCodecType.java

@@ -118,11 +118,10 @@ String generateDoneCoderType(
         {
             appendScratchWrappingStruct(writer, doneTypeName);
             RustGenerator.appendImplWithLifetimeHeader(writer, doneTypeName);
-            writer.append(INDENT).append(String.format("pub fn unwrap(mut self) -> (usize, &%s%s [u8]) {\n",
-                DATA_LIFETIME, this == Encoder ? " mut" : ""))
-                .append(INDENT).append(INDENT).append(format("(self.%s.pos, self.%s.data)\n",
-                scratchProperty(), scratchProperty()))
-                .append(INDENT).append("}\n");
+            indent(writer, 1, "/// Returns the number of bytes %s\n", this == Encoder ? "encoded" : "decoded");
+            indent(writer, 1, "pub fn unwrap(self) -> usize {\n");
+            indent(writer, 2, "self.%s.pos\n", scratchProperty());
+            indent(writer, 1, "}\n");
 
             appendWrapMethod(writer, doneTypeName);
             writer.append("}\n");