Fix unstable f-string formatting for expressions containing a trailing comma (#15545)

crates/ruff_python_formatter/src/builders.rs

@@ -206,6 +206,20 @@ impl<'fmt, 'ast, 'buf> JoinCommaSeparatedBuilder<'fmt, 'ast, 'buf> {
 
     pub(crate) fn finish(&mut self) -> FormatResult<()> {
         self.result.and_then(|()| {
+            // Don't add a magic trailing comma when formatting an f-string expression
+            // that always must be flat because the `expand_parent` forces enclosing
+            // groups to expand, e.g. `print(f"{(a,)} ")` would format the f-string in
+            // flat mode but the `print` call gets expanded because of the `expand_parent`.
+            if self
+                .fmt
+                .context()
+                .f_string_state()
+                .can_contain_line_breaks()
+                == Some(false)
+            {
+                return Ok(());
+            }
+
             if let Some(last_end) = self.entries.position() {
                 let magic_trailing_comma = has_magic_trailing_comma(
                     TextRange::new(last_end, self.sequence_end),