Don't print "private fields" on empty tuple structs

Test for presence rather than absence

Remove redundant tests

Issues in those parts will likely be caught by other parts of the test suite.

src/librustdoc/html/render/print_item.rs

@@ -1501,8 +1501,10 @@ fn print_tuple_struct_fields<'a, 'cx: 'a>(
     s: &'a [clean::Item],
 ) -> impl fmt::Display + 'a + Captures<'cx> {
     display_fn(|f| {
-        if s.iter()
-            .all(|field| matches!(*field.kind, clean::StrippedItem(box clean::StructFieldItem(..))))
+        if !s.is_empty()
+            && s.iter().all(|field| {
+                matches!(*field.kind, clean::StrippedItem(box clean::StructFieldItem(..)))
+            })
         {
             return f.write_str("/* private fields */");
         }
@@ -2275,9 +2277,11 @@ fn render_struct_fields(
         }
         Some(CtorKind::Fn) => {
             w.write_str("(");
-            if fields.iter().all(|field| {
-                matches!(*field.kind, clean::StrippedItem(box clean::StructFieldItem(..)))
-            }) {
+            if !fields.is_empty()
+                && fields.iter().all(|field| {
+                    matches!(*field.kind, clean::StrippedItem(box clean::StructFieldItem(..)))
+                })
+            {
                 write!(w, "/* private fields */");
             } else {
                 for (i, field) in fields.iter().enumerate() {

tests/rustdoc/issue-118180-empty-tuple-struct.rs

@@ -0,0 +1,9 @@
+// @has issue_118180_empty_tuple_struct/enum.Enum.html
+pub enum Enum {
+    // @has - '//*[@id="variant.Empty"]//h3' 'Empty()'
+    Empty(),
+}
+
+// @has issue_118180_empty_tuple_struct/struct.Empty.html
+// @has - '//pre/code' 'Empty()'
+pub struct Empty();