Fix deriving Encodable trait for unit structs

Closes #14021

src/libsyntax/ext/deriving/encodable.rs

@@ -175,6 +175,14 @@ fn encodable_substructure(cx: &mut ExtCtxt, trait_span: Span,
                 stmts.push(cx.stmt_expr(call));
             }
 
+            // unit structs have no fields and need to return Ok()
+            if stmts.is_empty() {
+                let ret_ok = cx.expr(trait_span,
+                                     ExprRet(Some(cx.expr_ok(trait_span,
+                                                             cx.expr_lit(trait_span, LitNil)))));
+                stmts.push(cx.stmt_expr(ret_ok));
+            }
+
             let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
             cx.expr_method_call(trait_span,
                                 encoder,

src/test/run-pass/issue-14021.rs

@@ -0,0 +1,29 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+
+extern crate serialize;
+
+use serialize::{Encodable, Decodable};
+use serialize::json;
+
+#[deriving(Encodable, Decodable, PartialEq, Show)]
+struct UnitLikeStruct;
+
+pub fn main() {
+    let obj = UnitLikeStruct;
+    let json_str: String = json::Encoder::str_encode(&obj);
+
+    let json_object = json::from_str(json_str.as_slice());
+    let mut decoder = json::Decoder::new(json_object.unwrap());
+    let mut decoded_obj: UnitLikeStruct = Decodable::decode(&mut decoder).unwrap();
+
+    assert_eq!(obj, decoded_obj);
+}