Fix unstable with-items formatting (#10274)

## Summary

Fixes #10267

The issue with the current formatting is that the formatter flips
between the `SingleParenthesizedContextManager` and
`ParenthesizeIfExpands` or `SingleWithTarget` because the layouts use
incompatible formatting ( `SingleParenthesizedContextManager`:
`maybe_parenthesize_expression(context)` vs `ParenthesizeIfExpands`:
`parenthesize_if_expands(item)`, `SingleWithTarget`:
`optional_parentheses(item)`.

The fix is to ensure that the layouts between which the formatter flips
when adding or removing parentheses are the same. I do this by
introducing a new `SingleWithoutTarget` layout that uses the same
formatting as `SingleParenthesizedContextManager` if it has no target
and prefer `SingleWithoutTarget` over using `ParenthesizeIfExpands` or
`SingleWithTarget`.

## Formatting change

The downside is that we now use `maybe_parenthesize_expression` over
`parenthesize_if_expands` for expressions where
`can_omit_optional_parentheses` returns `false`. This can lead to stable
formatting changes. I only found one formatting change in our ecosystem
check and, unfortunately, this is necessary to fix the instability (and
instability fixes are okay to have as part of minor changes according to
our versioning policy)

The benefit of the change is that `with` items with a single context
manager and without a target are now formatted identically to how the
same expression would be formatted in other clause headers.

## Test Plan

I ran the ecosystem check locally

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

@@ -304,6 +304,17 @@
     with anyio.CancelScope(shield=True) if get_running_loop() else contextlib.nullcontext():
         pass
 
+if True:
+    with anyio.CancelScope(shield=True) if get_running_loop() else contextlib.nullcontext() as c:
+        pass
 
 with Child(aaaaaaaaa, bbbbbbbbbbbbbbb, cccccc), Document(aaaaa, bbbbbbbbbb, ddddddddddddd):
     pass
+
+# Regression test for https://github.com/astral-sh/ruff/issues/10267
+with (
+    open(
+        "/etc/hosts"  # This is an incredibly long comment that has been replaced for sanitization
+    )
+):
+    pass

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/with_39.py

@@ -85,4 +85,8 @@
 with (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa + bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb as c):
     pass
 
-
+with (  # outer comment
+    CtxManager1(),
+    CtxManager2(),
+):
+    pass

crates/ruff_python_formatter/src/other/with_item.rs

@@ -22,6 +22,23 @@ pub enum WithItemLayout {
     /// This layout is used independent of the target version.
     SingleParenthesizedContextManager,
 
+    /// A with item that is the `with`s only context manager and it has no `target`.
+    ///
+    /// ```python
+    /// with a + b:
+    ///     ...
+    /// ```
+    ///
+    /// In this case, use [`maybe_parenthesize_expression`] to get the same formatting as when
+    /// formatting any other statement with a clause header.
+    ///
+    /// This layout is only used for Python 3.9+.
+    ///
+    /// Be careful that [`Self::SingleParenthesizedContextManager`] and this layout are compatible because
+    /// removing optional parentheses or adding parentheses will make the formatter pick the opposite layout
+    /// the second time the file gets formatted.
+    SingleWithoutTarget,
+
     /// This layout is used when the target python version doesn't support parenthesized context managers and
     /// it's either a single, unparenthesized with item or multiple items.
     ///
@@ -106,7 +123,8 @@ impl FormatNodeRule<WithItem> for FormatWithItem {
                 }
             }
 
-            WithItemLayout::SingleParenthesizedContextManager => {
+            WithItemLayout::SingleParenthesizedContextManager
+            | WithItemLayout::SingleWithoutTarget => {
                 write!(
                     f,
                     [maybe_parenthesize_expression(

crates/ruff_python_formatter/src/statement/stmt_with.rs

@@ -73,6 +73,11 @@ impl FormatNodeRule<StmtWith> for FormatStmtWith {
                                 optional_parentheses(&single.format()).fmt(f)
                             }
 
+                            WithItemsLayout::SingleWithoutTarget(single) => single
+                                .format()
+                                .with_options(WithItemLayout::SingleWithoutTarget)
+                                .fmt(f),
+
                             WithItemsLayout::SingleParenthesizedContextManager(single) => single
                                 .format()
                                 .with_options(WithItemLayout::SingleParenthesizedContextManager)
@@ -150,15 +155,35 @@ enum WithItemsLayout<'a> {
     ///
     /// In this case, prefer keeping the parentheses around the context expression instead of parenthesizing the entire
     /// with item.
+    ///
+    /// Ensure that this layout is compatible with [`Self::SingleWithoutTarget`] because removing the parentheses
+    /// results in the formatter taking that layout when formatting the file again
     SingleParenthesizedContextManager(&'a WithItem),
 
+    /// The with statement's only item has no target.
+    ///
+    /// ```python
+    /// with a + b:
+    ///     ...
+    /// ```
+    ///
+    /// In this case, use [`maybe_parenthesize_expression`] to format the context expression
+    /// to get the exact same formatting as when formatting an expression in any other clause header.
+    ///
+    /// Only used for Python 3.9+
+    ///
+    /// Be careful that [`Self::SingleParenthesizedContextManager`] and this layout are compatible because
+    /// adding parentheses around a [`WithItem`] will result in the context expression being parenthesized in
+    /// the next formatting pass.
+    SingleWithoutTarget(&'a WithItem),
+
     /// It's a single with item with a target. Use the optional parentheses layout (see [`optional_parentheses`])
     /// to mimic the `maybe_parenthesize_expression` behavior.
     ///
     /// ```python
     /// with (
-    ///     a + b
-    /// ) as b:
+    ///     a + b as b
+    /// ):
     ///     ...
     /// ```
     ///
@@ -275,7 +300,9 @@ impl<'a> WithItemsLayout<'a> {
 
         Ok(match with.items.as_slice() {
             [single] => {
-                if can_omit_optional_parentheses(&single.context_expr, context) {
+                if single.optional_vars.is_none() {
+                    Self::SingleWithoutTarget(single)
+                } else if can_omit_optional_parentheses(&single.context_expr, context) {
                     Self::SingleWithTarget(single)
                 } else {
                     Self::ParenthesizeIfExpands

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

@@ -310,9 +310,20 @@ if True:
     with anyio.CancelScope(shield=True) if get_running_loop() else contextlib.nullcontext():
         pass
 
+if True:
+    with anyio.CancelScope(shield=True) if get_running_loop() else contextlib.nullcontext() as c:
+        pass
 
 with Child(aaaaaaaaa, bbbbbbbbbbbbbbb, cccccc), Document(aaaaa, bbbbbbbbbb, ddddddddddddd):
     pass
+
+# Regression test for https://github.com/astral-sh/ruff/issues/10267
+with (
+    open(
+        "/etc/hosts"  # This is an incredibly long comment that has been replaced for sanitization
+    )
+):
+    pass
 ```
 
 ## Outputs
@@ -661,11 +672,22 @@ if True:
     ) if get_running_loop() else contextlib.nullcontext():
         pass
 
+if True:
+    with anyio.CancelScope(
+        shield=True
+    ) if get_running_loop() else contextlib.nullcontext() as c:
+        pass
 
 with Child(aaaaaaaaa, bbbbbbbbbbbbbbb, cccccc), Document(
     aaaaa, bbbbbbbbbb, ddddddddddddd
 ):
     pass
+
+# Regression test for https://github.com/astral-sh/ruff/issues/10267
+with open(
+    "/etc/hosts"  # This is an incredibly long comment that has been replaced for sanitization
+):
+    pass
 ```
 
 
@@ -1052,13 +1074,23 @@ if True:
     ):
         pass
 
+if True:
+    with (
+        anyio.CancelScope(shield=True)
+        if get_running_loop()
+        else contextlib.nullcontext() as c
+    ):
+        pass
 
 with (
     Child(aaaaaaaaa, bbbbbbbbbbbbbbb, cccccc),
     Document(aaaaa, bbbbbbbbbb, ddddddddddddd),
 ):
     pass
-```
-
-
 
+# Regression test for https://github.com/astral-sh/ruff/issues/10267
+with open(
+    "/etc/hosts"  # This is an incredibly long comment that has been replaced for sanitization
+):
+    pass
+```

crates/ruff_python_formatter/tests/snapshots/format@statement__with_39.py.snap

@@ -91,7 +91,11 @@ if True:
 with (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa + bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb as c):
     pass
 
-
+with (  # outer comment
+    CtxManager1(),
+    CtxManager2(),
+):
+    pass
 ```
 
 ## Outputs
@@ -217,7 +221,10 @@ with (
     aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa + bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb as c
 ):
     pass
-```
-
-
 
+with (  # outer comment
+    CtxManager1(),
+    CtxManager2(),
+):
+    pass
+```