Auto merge of #49279 - varkor:generated-closure-return-type, r=alexcrichton

Fix implicit closure return type generation for libsyntax

The `lambda` function for constructing closures in libsyntax was explicitly setting the return type to `_`, which resulted in incorrect corresponding syntax (as `|| -> _ x` is not valid, without the enclosing brackets). This meant the generated code, when printed, was invalid.

I also took the opportunity to slightly improve the generated code for the `RustcEncodable::encode` method for unit structs.

Fixes #42213.

2 files changed, 6 insertions, 6 deletions

diff --git a/src/libsyntax_ext/deriving/encodable.rs b/src/libsyntax_ext/deriving/encodable.rs
index 743f22b6b31..88baa22e7fa 100644
--- a/src/libsyntax_ext/deriving/encodable.rs
+++ b/src/libsyntax_ext/deriving/encodable.rs
@@ -228,13 +228,13 @@ fn encodable_substructure(cx: &mut ExtCtxt,
             }
 
             // unit structs have no fields and need to return Ok()
-            if stmts.is_empty() {
+            let blk = if stmts.is_empty() {
                 let ok = cx.expr_ok(trait_span, cx.expr_tuple(trait_span, vec![]));
-                let ret_ok = cx.expr(trait_span, ExprKind::Ret(Some(ok)));
-                stmts.push(cx.stmt_expr(ret_ok));
-            }
+                cx.lambda1(trait_span, ok, blkarg)
+            } else {
+                cx.lambda_stmts_1(trait_span, stmts, blkarg)
+            };
 
-            let blk = cx.lambda_stmts_1(trait_span, stmts, blkarg);
             cx.expr_method_call(trait_span,
                                 encoder,
                                 cx.ident_of("emit_struct"),
diff --git a/src/libsyntax_ext/deriving/generic/mod.rs b/src/libsyntax_ext/deriving/generic/mod.rs
index 49c372b751b..3935f1722b6 100644
--- a/src/libsyntax_ext/deriving/generic/mod.rs
+++ b/src/libsyntax_ext/deriving/generic/mod.rs
@@ -962,7 +962,7 @@ impl<'a> MethodDef<'a> {
         let ret_type = self.get_ret_ty(cx, trait_, generics, type_ident);
 
         let method_ident = cx.ident_of(self.name);
-        let fn_decl = cx.fn_decl(args, ret_type);
+        let fn_decl = cx.fn_decl(args, ast::FunctionRetTy::Ty(ret_type));
         let body_block = cx.block_expr(body);
 
         let unsafety = if self.is_unsafe {