Change the dummy type used in DummyResult::raw_ty.

`DummyResult::raw_pat` uses a non-error value. And for `raw_ty`, an
empty tuple works just as well as an error, and doesn't imply anything
about whether errors have been emitted.

Author: nnethercote

compiler/rustc_expand/src/base.rs

@@ -567,10 +567,10 @@ impl DummyResult {
     }
 
     /// A plain dummy type.
-    pub fn raw_ty(sp: Span, is_error: bool) -> P<ast::Ty> {
+    pub fn raw_ty(sp: Span) -> P<ast::Ty> {
         P(ast::Ty {
             id: ast::DUMMY_NODE_ID,
-            kind: if is_error { ast::TyKind::Err } else { ast::TyKind::Tup(ThinVec::new()) },
+            kind: ast::TyKind::Tup(ThinVec::new()),
             span: sp,
             tokens: None,
         })
@@ -611,7 +611,7 @@ impl MacResult for DummyResult {
     }
 
     fn make_ty(self: Box<DummyResult>) -> Option<P<ast::Ty>> {
-        Some(DummyResult::raw_ty(self.span, self.is_error))
+        Some(DummyResult::raw_ty(self.span))
     }
 
     fn make_arms(self: Box<DummyResult>) -> Option<SmallVec<[ast::Arm; 1]>> {