Added ui tests, Limited serde(tag = "...") to structs with named field

Added ui test struct-representation/internally-tagged-unit
Added ui test struct-representation/internally-tagged-tuple
    
Limited the serde(tag = "...") to enums and structs with named field

serde_derive/src/internals/attr.rs

@@ -358,16 +358,30 @@ impl Container {
                     Meta(NameValue(ref m)) if m.ident == "tag" => {
                         if let Ok(s) = get_lit_str(cx, &m.ident, &m.ident, &m.lit) {
                             match item.data {
-                                syn::Data::Enum(_) |
+                                syn::Data::Enum(_) => {
-                                syn::Data::Struct(_, ..) => {
                                     internal_tag.set(&m.ident, s.value());
                                 }
+                                syn::Data::Struct(syn::DataStruct{ref fields, ..}) => {
+                                    match *fields {
+                                        syn::Fields::Named(_) => {
+                                            internal_tag.set(&m.ident, s.value());
+                                        },
+                                        syn::Fields::Unnamed(_) | syn::Fields::Unit => {
+                                            cx.error_spanned_by(
+                                                fields,
+                                                "#[serde(tag = \"...\")] can only be used on enums \
+                                                and structs with named fields",
+                                            );
+                                        }
+                                    }
+                                }
                                 syn::Data::Union(syn::DataUnion {
                                     ref union_token, ..
                                 }) => {
                                     cx.error_spanned_by(
                                         union_token,
-                                        "#[serde(tag = \"...\")] can only be used on enums",
+                                        "#[serde(tag = \"...\")] can only be used on enums \
+                                        and structs with named fields",
                                     );
                                 }
                             }

test_suite/tests/test_macros.rs

@@ -1411,7 +1411,6 @@ fn test_internally_tagged_struct() {
             Token::StructEnd,
         ],
     );
-
 }
 
 #[test]

test_suite/tests/ui/struct-representation/internally-taged-tuple.rs

@@ -0,0 +1,11 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(tag = "type")]
+struct S (
+    u8,
+    u8
+);
+
+fn main() {}

test_suite/tests/ui/struct-representation/internally-taged-tuple.stderr

@@ -0,0 +1,12 @@
+error: #[serde(tag = "...")] can only be used on enums and structs with named fields
+ --> $DIR/internally-taged-tuple.rs:6:10
+  |
+6 |   struct S (
+  |  __________^
+7 | |     u8,
+8 | |     u8
+9 | | );
+  | |_^
+
+error: aborting due to previous error
+

test_suite/tests/ui/struct-representation/internally-tagged-unit.rs

@@ -0,0 +1,8 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)]
+#[serde(tag = "type")]
+struct U;
+
+fn main() {}

test_suite/tests/ui/struct-representation/internally-tagged-unit.stderr

@@ -0,0 +1,8 @@
+error: #[serde(tag = "...")] can only be used on enums and structs with named fields
+ --> $DIR/internally-tagged-unit.rs:4:10
+  |
+4 | #[derive(Serialize)]
+  |          ^^^^^^^^^
+
+error: aborting due to previous error
+