Preserve required parentheses in lambda bodies (#22747)

Summary
--

This PR fixes the issues revealed in #22744 by adding an additional
branch to
the lambda body formatting that checks if the body `needs_parentheses`
before
falling back on the `Parentheses::Never` case. I also updated the
`ExprNamed::needs_parentheses` implementation to match the one from
#8465.

Test Plan
--

New test based on the failing cases in #22744. I also checked out #22744
and
checked that the tests pass after applying the changes from this PR.

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/lambda.py

@@ -804,3 +804,12 @@ transform = lambda left, right: ibis.timestamp("2017-04-01").cast(dt.date).betwe
         x
     )
 )
+
+lambda x: (
+    x := 1
+)
+
+(
+    lambda  # dangling header comment
+    : (x := 1)
+)

crates/ruff_python_formatter/src/expression/expr_lambda.rs

@@ -145,6 +145,7 @@ impl FormatNodeRule<ExprLambda> for FormatExprLambda {
         let fmt_body = FormatBody {
             body,
             dangling_header_comments,
+            needs_parentheses: body.needs_parentheses(item.into(), f.context()),
         };
 
         match self.layout {
@@ -262,6 +263,7 @@ struct FormatBody<'a> {
     /// )
     /// ```
     dangling_header_comments: &'a [SourceComment],
+    needs_parentheses: OptionalParentheses,
 }
 
 impl Format<PyFormatContext<'_>> for FormatBody<'_> {
@@ -269,6 +271,7 @@ impl Format<PyFormatContext<'_>> for FormatBody<'_> {
         let FormatBody {
             dangling_header_comments,
             body,
+            needs_parentheses,
         } = self;
 
         let body = *body;
@@ -438,6 +441,14 @@ impl Format<PyFormatContext<'_>> for FormatBody<'_> {
         else if has_own_parentheses(body, f.context()).is_some() {
             body.format().fmt(f)
         }
+        // Include parentheses for cases that always require them, such as named expressions:
+        //
+        // ```py
+        // lambda x: (y := x + 1)
+        // ```
+        else if matches!(needs_parentheses, OptionalParentheses::Always) {
+            body.format().with_options(Parentheses::Always).fmt(f)
+        }
         // Finally, for expressions without their own parentheses, use
         // `parenthesize_if_expands` to add parentheses around the body, only if it expands
         // across multiple lines. The `Parentheses::Never` here also removes unnecessary

crates/ruff_python_formatter/src/expression/expr_named.rs

@@ -66,6 +66,7 @@ impl NeedsParentheses for ExprNamed {
             || parent.is_stmt_delete()
             || parent.is_stmt_for()
             || parent.is_stmt_function_def()
+            || parent.is_expr_lambda()
         {
             OptionalParentheses::Always
         } else {

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

@@ -1,6 +1,5 @@
 ---
 source: crates/ruff_python_formatter/tests/fixtures.rs
-input_file: crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/lambda.py
 ---
 ## Input
 ```python
@@ -810,6 +809,15 @@ transform = lambda left, right: ibis.timestamp("2017-04-01").cast(dt.date).betwe
         x
     )
 )
+
+lambda x: (
+    x := 1
+)
+
+(
+    lambda  # dangling header comment
+    : (x := 1)
+)
 ```
 
 ## Output
@@ -1649,6 +1657,13 @@ transform = (
         x
     )
 )
+
+lambda x: (x := 1)
+
+(
+    lambda:  # dangling header comment
+    (x := 1)
+)
 ```
 
 
@@ -2567,4 +2582,14 @@ transform = (
          x
      )
  )
+@@ -837,6 +854,7 @@
+ lambda x: (x := 1)
+ 
+ (
+-    lambda:  # dangling header comment
+-    (x := 1)
++    lambda: (  # dangling header comment
++        x := 1
++    )
+ )
 ```