Rollup merge of #84390 - m-ou-se:make-debug-non-exhaustive-without-fields-a-little-bit-less-verbose, r=kennytm

Format `Struct { .. }` on one line even with `{:#?}`.

The result of `debug_struct("A").finish_non_exhaustive()` before this change:
```
A {
    ..
}
```
And after this change:
```
A { .. }
```

If there's any fields, the result stays unchanged:
```
A {
    field: value,
    ..
}

library/core/src/fmt/builders.rs

@@ -188,28 +188,19 @@ pub fn field(&mut self, name: &str, value: &dyn fmt::Debug) -> &mut Self {
     #[stable(feature = "debug_non_exhaustive", since = "1.53.0")]
     pub fn finish_non_exhaustive(&mut self) -> fmt::Result {
         self.result = self.result.and_then(|_| {
-            // Draw non-exhaustive dots (`..`), and open brace if necessary (no fields).
-            if self.is_pretty() {
-                if !self.has_fields {
-                    self.fmt.write_str(" {\n")?;
-                }
-                let mut slot = None;
-                let mut state = Default::default();
-                let mut writer = PadAdapter::wrap(&mut self.fmt, &mut slot, &mut state);
-                writer.write_str("..\n")?;
-            } else {
-                if self.has_fields {
-                    self.fmt.write_str(", ..")?;
+            if self.has_fields {
+                if self.is_pretty() {
+                    let mut slot = None;
+                    let mut state = Default::default();
+                    let mut writer = PadAdapter::wrap(&mut self.fmt, &mut slot, &mut state);
+                    writer.write_str("..\n")?;
+                    self.fmt.write_str("}")
                 } else {
-                    self.fmt.write_str(" { ..")?;
+                    self.fmt.write_str(", .. }")
                 }
-            }
-            if self.is_pretty() {
-                self.fmt.write_str("}")?
             } else {
-                self.fmt.write_str(" }")?;
+                self.fmt.write_str(" { .. }")
             }
-            Ok(())
         });
         self.result
     }

library/core/tests/fmt/builders.rs

@@ -105,12 +105,7 @@ fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
         }
 
         assert_eq!("Foo { .. }", format!("{:?}", Foo));
-        assert_eq!(
-            "Foo {
-    ..
-}",
-            format!("{:#?}", Foo)
-        );
+        assert_eq!("Foo { .. }", format!("{:#?}", Foo));
     }
 
     #[test]