Make Parameters an optional field on ExprLambda (#6669)

## Summary

If a lambda doesn't contain any parameters, or any parameter _tokens_
(like `*`), we can use `None` for the parameters. This feels like a
better representation to me, since, e.g., what should the `TextRange` be
for a non-existent set of parameters? It also allows us to remove
several sites where we check if the `Parameters` is empty by seeing if
it contains any arguments, so semantically, we're already trying to
detect and model around this elsewhere.

Changing this also fixes a number of issues with dangling comments in
parameter-less lambdas, since those comments are now automatically
marked as dangling on the lambda. (As-is, we were also doing something
not-great whereby the lambda was responsible for formatting dangling
comments on the parameters, which has been removed.)

Closes https://github.com/astral-sh/ruff/issues/6646.

Closes https://github.com/astral-sh/ruff/issues/6647.

## Test Plan

`cargo test`

crates/ruff_python_formatter/src/comments/placement.rs

@@ -162,9 +162,14 @@ fn handle_enclosed_comment<'a>(
     locator: &Locator,
 ) -> CommentPlacement<'a> {
     match comment.enclosing_node() {
-        AnyNodeRef::Parameters(arguments) => {
-            handle_parameters_separator_comment(comment, arguments, locator)
-                .or_else(|comment| handle_bracketed_end_of_line_comment(comment, locator))
+        AnyNodeRef::Parameters(parameters) => {
+            handle_parameters_separator_comment(comment, parameters, locator).or_else(|comment| {
+                if are_parameters_parenthesized(parameters, locator.contents()) {
+                    handle_bracketed_end_of_line_comment(comment, locator)
+                } else {
+                    CommentPlacement::Default(comment)
+                }
+            })
         }
         AnyNodeRef::Arguments(_) | AnyNodeRef::TypeParams(_) => {
             handle_bracketed_end_of_line_comment(comment, locator)
@@ -1544,6 +1549,13 @@ fn is_first_statement_in_alternate_body(statement: AnyNodeRef, has_body: AnyNode
     }
 }
 
+/// Returns `true` if the parameters are parenthesized (as in a function definition), or `false` if
+/// not (as in a lambda).
+fn are_parameters_parenthesized(parameters: &Parameters, contents: &str) -> bool {
+    // A lambda never has parentheses around its parameters, but a function definition always does.
+    contents[parameters.range()].starts_with('(')
+}
+
 /// Counts the number of empty lines in `contents`.
 fn max_empty_lines(contents: &str) -> u32 {
     let mut newlines = 0u32;

crates/ruff_python_formatter/src/expression/expr_lambda.rs

@@ -1,13 +1,14 @@
-use crate::comments::{dangling_node_comments, SourceComment};
+use ruff_formatter::prelude::{space, text};
+use ruff_formatter::{write, Buffer, FormatResult};
+use ruff_python_ast::node::AnyNodeRef;
+use ruff_python_ast::ExprLambda;
+
+use crate::comments::{dangling_comments, SourceComment};
 use crate::context::PyFormatContext;
 use crate::expression::parentheses::{NeedsParentheses, OptionalParentheses};
 use crate::other::parameters::ParametersParentheses;
 use crate::AsFormat;
 use crate::{FormatNodeRule, PyFormatter};
-use ruff_formatter::prelude::{space, text};
-use ruff_formatter::{write, Buffer, FormatResult};
-use ruff_python_ast::node::AnyNodeRef;
-use ruff_python_ast::ExprLambda;
 
 #[derive(Default)]
 pub struct FormatExprLambda;
@@ -20,13 +21,12 @@ impl FormatNodeRule<ExprLambda> for FormatExprLambda {
             body,
         } = item;
 
+        let comments = f.context().comments().clone();
+        let dangling = comments.dangling(item);
+
         write!(f, [text("lambda")])?;
 
-        if !parameters.args.is_empty()
-            || !parameters.posonlyargs.is_empty()
-            || parameters.vararg.is_some()
-            || parameters.kwarg.is_some()
-        {
+        if let Some(parameters) = parameters {
             write!(
                 f,
                 [
@@ -38,21 +38,15 @@ impl FormatNodeRule<ExprLambda> for FormatExprLambda {
             )?;
         }
 
-        write!(
-            f,
-            [
-                text(":"),
-                space(),
-                body.format(),
-                // It's possible for some `Arguments` of `lambda`s to be assigned dangling comments.
-                //
-                // a = (
-                //     lambda  # Dangling
-                //     : 1
-                // )
-                dangling_node_comments(parameters.as_ref())
-            ]
-        )
+        write!(f, [text(":")])?;
+
+        if dangling.is_empty() {
+            write!(f, [space()])?;
+        } else {
+            write!(f, [dangling_comments(dangling)])?;
+        }
+
+        write!(f, [body.format()])
     }
 
     fn fmt_dangling_comments(

crates/ruff_python_formatter/src/other/parameters.rs

@@ -7,8 +7,8 @@ use ruff_python_trivia::{SimpleToken, SimpleTokenKind, SimpleTokenizer};
 use ruff_text_size::{TextRange, TextSize};
 
 use crate::comments::{
-    dangling_open_parenthesis_comments, leading_comments, leading_node_comments, trailing_comments,
-    CommentLinePosition, SourceComment,
+    dangling_comments, dangling_open_parenthesis_comments, leading_comments, leading_node_comments,
+    trailing_comments, CommentLinePosition, SourceComment,
 };
 use crate::context::{NodeLevel, WithNodeLevel};
 use crate::expression::parentheses::empty_parenthesized;
@@ -242,7 +242,7 @@ impl FormatNodeRule<Parameters> for FormatParameters {
             + usize::from(kwarg.is_some());
 
         if self.parentheses == ParametersParentheses::Never {
-            write!(f, [group(&format_inner)])
+            write!(f, [group(&format_inner), dangling_comments(dangling)])
         } else if num_parameters == 0 {
             // No parameters, format any dangling comments between `()`
             write!(f, [empty_parenthesized("(", dangling, ")")])