keep lambda parameters on a single line

crates/ruff_python_formatter/src/expression/expr_lambda.rs

@@ -1,4 +1,4 @@
-use ruff_formatter::write;
+use ruff_formatter::{RemoveSoftLinesBuffer, write};
 use ruff_python_ast::AnyNodeRef;
 use ruff_python_ast::ExprLambda;
 use ruff_text_size::Ranged;
@@ -7,6 +7,7 @@ use crate::comments::dangling_comments;
 use crate::expression::parentheses::{NeedsParentheses, OptionalParentheses};
 use crate::other::parameters::ParametersParentheses;
 use crate::prelude::*;
+use crate::preview::is_force_single_line_lambda_parameters_enabled;
 
 #[derive(Default)]
 pub struct FormatExprLambda;
@@ -37,12 +38,25 @@ impl FormatNodeRule<ExprLambda> for FormatExprLambda {
                 write!(f, [dangling_comments(dangling_before_parameters)])?;
             }
 
+            // Try to keep the parameters on a single line, unless there are intervening comments.
+            if is_force_single_line_lambda_parameters_enabled(f.context())
+                && !comments.contains_comments(parameters.as_ref().into())
+            {
+                let mut buffer = RemoveSoftLinesBuffer::new(f);
+                write!(
+                    buffer,
+                    [parameters
+                        .format()
+                        .with_options(ParametersParentheses::Never)]
+                )?;
+            } else {
                 write!(
                     f,
                     [parameters
                         .format()
                         .with_options(ParametersParentheses::Never)]
                 )?;
+            }
 
             write!(f, [token(":")])?;
 

crates/ruff_python_formatter/src/preview.rs

@@ -52,3 +52,12 @@ pub(crate) const fn is_avoid_parens_for_long_as_captures_enabled(
 ) -> bool {
     context.is_preview()
 }
+
+/// Returns `true` if the
+/// [`force_single_line_lambda_parameters`](https://github.com/astral-sh/ruff/pull/21385) preview
+/// style is enabled.
+pub(crate) const fn is_force_single_line_lambda_parameters_enabled(
+    context: &PyFormatContext,
+) -> bool {
+    context.is_preview()
+}

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

@@ -676,3 +676,65 @@ class C:
             else storage.Bucket(mock_service, destination_bucket_name)
         )
 ```
+
+
+## Preview changes
+```diff
+--- Stable
++++ Preview
+@@ -280,9 +280,7 @@
+ ]  # Trailing
+ # Trailing
+ 
+-lambda self, araa, kkkwargs=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
+-    *args, **kwargs
+-), e=1, f=2, g=2: d
++lambda self, araa, kkkwargs=aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs), e=1, f=2, g=2: d
+ 
+ 
+ # Regression tests for https://github.com/astral-sh/ruff/issues/8179
+@@ -291,9 +289,9 @@
+         c,
+         d,
+         e,
+-        f=lambda self,
+-        *args,
+-        **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs),
++        f=lambda self, *args, **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
++            *args, **kwargs
++        ),
+     )
+ 
+ 
+@@ -302,15 +300,7 @@
+         c,
+         d,
+         e,
+-        f=lambda self,
+-        araa,
+-        kkkwargs,
+-        aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,
+-        args,
+-        kwargs,
+-        e=1,
+-        f=2,
+-        g=2: d,
++        f=lambda self, araa, kkkwargs, aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa, args, kwargs, e=1, f=2, g=2: d,
+         g=10,
+     )
+ 
+@@ -320,9 +310,10 @@
+         c,
+         d,
+         e,
+-        f=lambda self,
+-        *args,
+-        **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(*args, **kwargs) + 1,
++        f=lambda self, *args, **kwargs: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa(
++            *args, **kwargs
++        )
++        + 1,
+     )
+ 
+ 
+```