Treat parameters-with-newline as empty in function formatting (#7550)

## Summary

If a function has no parameters (and no comments within the parameters'
`()`), we're supposed to wrap the return annotation _whenever_ it
breaks. However, our `empty_parameters` test didn't properly account for
the case in which the parameters include a newline (but no other
content), like:

```python
def get_dashboards_hierarchy(
) -> Dict[Type['BaseDashboard'], List[Type['BaseDashboard']]]:
    """Get hierarchy of dashboards classes.

    Returns:
        Dict of dashboards classes.
    """
    dashboards_hierarchy = {}
```

This PR fixes that detection. Instead of lexing, it now checks if the
parameters itself is empty (or if it contains comments).

Closes https://github.com/astral-sh/ruff/issues/7457.

crates/ruff_python_ast/src/nodes.rs

@@ -2135,6 +2135,15 @@ impl Parameters {
         }
         false
     }
+
+    /// Returns `true` if the [`Parameters`] is empty.
+    pub fn is_empty(&self) -> bool {
+        self.posonlyargs.is_empty()
+            && self.args.is_empty()
+            && self.kwonlyargs.is_empty()
+            && self.vararg.is_none()
+            && self.kwarg.is_none()
+    }
 }
 
 /// An alternative type of AST `arg`. This is used for each function argument that might have a default value.

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/return_annotation.py

@@ -75,6 +75,12 @@ def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> Set[
 ]:
     ...
 
+def xxxxxxxxxxxxxxxxxxxxxxxxxxxx(
+) -> Set[
+    "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+]:
+    ...
+
 def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> (
     Set[
         "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"]

crates/ruff_python_formatter/src/statement/stmt_function_def.rs

@@ -1,9 +1,7 @@
-use crate::comments::format::empty_lines_before_trailing_comments;
 use ruff_formatter::write;
-use ruff_python_ast::{Parameters, StmtFunctionDef};
+use ruff_python_ast::StmtFunctionDef;
-use ruff_python_trivia::{SimpleTokenKind, SimpleTokenizer};
-use ruff_text_size::Ranged;
 
+use crate::comments::format::empty_lines_before_trailing_comments;
 use crate::comments::SourceComment;
 use crate::expression::maybe_parenthesize_expression;
 use crate::expression::parentheses::{Parentheses, Parenthesize};
@@ -147,29 +145,30 @@ fn format_function_header(f: &mut PyFormatter, item: &StmtFunctionDef) -> Format
                     [return_annotation.format().with_options(Parentheses::Always)]
                 )?;
             } else {
+                let parenthesize = if parameters.is_empty() && !comments.has(parameters.as_ref()) {
+                    // If the parameters are empty, add parentheses if the return annotation
+                    // breaks at all.
+                    Parenthesize::IfBreaksOrIfRequired
+                } else {
+                    // Otherwise, use our normal rules for parentheses, which allows us to break
+                    // like:
+                    // ```python
+                    // def f(
+                    //     x,
+                    // ) -> Tuple[
+                    //     int,
+                    //     int,
+                    // ]:
+                    //     ...
+                    // ```
+                    Parenthesize::IfBreaks
+                };
                 write!(
                     f,
                     [maybe_parenthesize_expression(
                         return_annotation,
                         item,
-                        if empty_parameters(parameters, f.context().source()) {
+                        parenthesize
-                            // If the parameters are empty, add parentheses if the return annotation
-                            // breaks at all.
-                            Parenthesize::IfBreaksOrIfRequired
-                        } else {
-                            // Otherwise, use our normal rules for parentheses, which allows us to break
-                            // like:
-                            // ```python
-                            // def f(
-                            //     x,
-                            // ) -> Tuple[
-                            //     int,
-                            //     int,
-                            // ]:
-                            //     ...
-                            // ```
-                            Parenthesize::IfBreaks
-                        },
                     )]
                 )?;
             }
@@ -179,25 +178,3 @@ fn format_function_header(f: &mut PyFormatter, item: &StmtFunctionDef) -> Format
 
     group(&format_inner).fmt(f)
 }
-
-/// Returns `true` if [`Parameters`] is empty (no parameters, no comments, etc.).
-fn empty_parameters(parameters: &Parameters, source: &str) -> bool {
-    let mut tokenizer = SimpleTokenizer::new(source, parameters.range())
-        .filter(|token| !matches!(token.kind, SimpleTokenKind::Whitespace));
-
-    let Some(lpar) = tokenizer.next() else {
-        return false;
-    };
-    if !matches!(lpar.kind, SimpleTokenKind::LParen) {
-        return false;
-    }
-
-    let Some(rpar) = tokenizer.next() else {
-        return false;
-    };
-    if !matches!(rpar.kind, SimpleTokenKind::RParen) {
-        return false;
-    }
-
-    true
-}

crates/ruff_python_formatter/tests/snapshots/format@statement__return_annotation.py.snap

@@ -81,6 +81,12 @@ def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> Set[
 ]:
     ...
 
+def xxxxxxxxxxxxxxxxxxxxxxxxxxxx(
+) -> Set[
+    "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+]:
+    ...
+
 def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> (
     Set[
         "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"]
@@ -365,6 +371,14 @@ def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> (
     ...
 
 
+def xxxxxxxxxxxxxxxxxxxxxxxxxxxx() -> (
+    Set[
+        "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
+    ]
+):
+    ...
+
+
 def xxxxxxxxxxxxxxxxxxxxxxxxxxxx(
     x
 ) -> Set[