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.

src/libsyntax/ext/build.rs

@@ -214,7 +214,7 @@ pub trait AstBuilder {
 
     fn arg(&self, span: Span, name: Ident, ty: P<ast::Ty>) -> ast::Arg;
     // FIXME unused self
-    fn fn_decl(&self, inputs: Vec<ast::Arg> , output: P<ast::Ty>) -> P<ast::FnDecl>;
+    fn fn_decl(&self, inputs: Vec<ast::Arg> , output: ast::FunctionRetTy) -> P<ast::FnDecl>;
 
     fn item_fn_poly(&self,
                     span: Span,
@@ -924,7 +924,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
               -> P<ast::Expr> {
         let fn_decl = self.fn_decl(
             ids.iter().map(|id| self.arg(span, *id, self.ty_infer(span))).collect(),
-            self.ty_infer(span));
+            ast::FunctionRetTy::Default(span));
 
         // FIXME -- We are using `span` as the span of the `|...|`
         // part of the lambda, but it probably (maybe?) corresponds to
@@ -970,10 +970,10 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
     }
 
     // FIXME unused self
-    fn fn_decl(&self, inputs: Vec<ast::Arg>, output: P<ast::Ty>) -> P<ast::FnDecl> {
+    fn fn_decl(&self, inputs: Vec<ast::Arg>, output: ast::FunctionRetTy) -> P<ast::FnDecl> {
         P(ast::FnDecl {
             inputs,
-            output: ast::FunctionRetTy::Ty(output),
+            output,
             variadic: false
         })
     }
@@ -1003,7 +1003,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         self.item(span,
                   name,
                   Vec::new(),
-                  ast::ItemKind::Fn(self.fn_decl(inputs, output),
+                  ast::ItemKind::Fn(self.fn_decl(inputs, ast::FunctionRetTy::Ty(output)),
                               ast::Unsafety::Normal,
                               dummy_spanned(ast::Constness::NotConst),
                               Abi::Rust,

src/libsyntax/test.rs

@@ -547,7 +547,7 @@ fn mk_main(cx: &mut TestCtxt) -> P<ast::Item> {
     // pub fn main() { ... }
     let main_ret_ty = ecx.ty(sp, ast::TyKind::Tup(vec![]));
     let main_body = ecx.block(sp, vec![call_test_main]);
-    let main = ast::ItemKind::Fn(ecx.fn_decl(vec![], main_ret_ty),
+    let main = ast::ItemKind::Fn(ecx.fn_decl(vec![], ast::FunctionRetTy::Ty(main_ret_ty)),
                            ast::Unsafety::Normal,
                            dummy_spanned(ast::Constness::NotConst),
                            ::abi::Abi::Rust, ast::Generics::default(), main_body);