libsyntax: don't allow enum structs with no fields

Unit-like structs are written as `struct Foo;`, but we erroneously
accepted `struct Foo();` and took it to mean the same thing. Now we
don't, so use the `struct Foo;` form!

[breaking-change]

src/libsyntax/parse/parser.rs

@@ -4116,6 +4116,11 @@ impl<'a> Parser<'a> {
                 };
                 spanned(lo, p.span.hi, struct_field_)
             });
+            if fields.len() == 0 {
+                self.fatal(format!("unit-like struct definition should be \
+                                    written as `struct {};`",
+                                   token::get_ident(class_name)).as_slice());
+            }
             self.expect(&token::SEMI);
         } else if self.eat(&token::SEMI) {
             // It's a unit-like struct.