Prefer splitting right hand side of assignments

crates/ruff_python_formatter/resources/test/fixtures/ruff/statement/assignment_split_value_first.options.json

@@ -0,0 +1,5 @@
+[
+  {
+    "preview": "enabled"
+  }
+]

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

@@ -0,0 +1,71 @@
+# Don't parenthesize the value because the target's trailing comma forces it to split.
+a[
+    aaaaaaa,
+    b,
+] = cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc #  comment
+
+# Parenthesize the value, but don't duplicate the comment.
+a[
+    aaaaaaa,
+    b
+] = cccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc #  comment
+
+# Format both as flat, but don't loos the comment.
+a[
+    aaaaaaa,
+    b
+] = bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb #  comment
+
+#######################################################
+# Test the case where a parenthesized value now fits:
+a[
+    aaaaaaa,
+    b
+] = (
+    cccccccc #  comment
+)
+
+# Doesn't use `BestFit` because the target always breaks because of the trailing comma
+a[
+    aaaaaaa,
+    b,
+] = (
+    cccccccc #  comment
+)
+
+# Doesn't use `BestFit` because the target always breaks because of the trailing comma
+# The group breaks because of its comments
+a[
+    aaaaaaa,
+    b
+] = (
+    # leading comment
+    b
+) = (
+    cccccccc #  comment
+)
+
+
+a[bbbbbbbbbbbbbbbbbb] = ccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccccc
+
+# Does not double-parenthesize tuples
+(
+    first_item,
+    second_item,
+) = some_looooooooong_module.some_loooooog_function_name(
+    first_argument, second_argument, third_argument
+)
+
+
+# Preserve parentheses around the first target
+(
+    req["ticket"]["steps"]["step"][0]["tasks"]["task"]["fields"]["field"][
+        "access_request"
+    ]["destinations"]["destination"][0]["ip_address"]
+) = dst
+
+(
+    req["ticket"]["steps"]["step"][0]["tasks"]["task"]["fields"]["field"][
+        "access_request"
+    ]["destinations"]["destination"][0]["ip_address"]
+) += dst

crates/ruff_python_formatter/src/expression/mod.rs

@@ -952,7 +952,7 @@ impl OwnParentheses {
 /// Differs from [`has_own_parentheses`] in that it returns [`OwnParentheses::NonEmpty`] for
 /// parenthesized expressions, like `(1)` or `([1])`, regardless of whether those expression have
 /// their _own_ parentheses.
-fn has_parentheses(expr: &Expr, context: &PyFormatContext) -> Option<OwnParentheses> {
+pub(crate) fn has_parentheses(expr: &Expr, context: &PyFormatContext) -> Option<OwnParentheses> {
     let own_parentheses = has_own_parentheses(expr, context);
 
     // If the node has its own non-empty parentheses, we don't need to check for surrounding

crates/ruff_python_formatter/src/preview.rs

@@ -17,3 +17,10 @@ pub(crate) const fn is_hug_parens_with_braces_and_square_brackets_enabled(
 ) -> bool {
     context.is_preview()
 }
+
+/// Returns `true` if the [`prefer_splitting_right_hand_side_of_assignments`](https://github.com/astral-sh/ruff/issues/6975) preview style is enabled.
+pub(crate) const fn is_prefer_splitting_right_hand_side_of_assignments_enabled(
+    context: &PyFormatContext,
+) -> bool {
+    context.is_preview()
+}

crates/ruff_python_formatter/src/statement/stmt_ann_assign.rs

@@ -2,8 +2,12 @@ use ruff_formatter::write;
 use ruff_python_ast::StmtAnnAssign;
 
 use crate::comments::{SourceComment, SuppressionKind};
+use crate::expression::has_parentheses;
 use crate::prelude::*;
-use crate::statement::stmt_assign::FormatStatementsLastExpression;
+use crate::preview::is_prefer_splitting_right_hand_side_of_assignments_enabled;
+use crate::statement::stmt_assign::{
+    AnyAssignmentOperator, FormatAssignmentValue, FormatStatementsLastExpression,
+};
 use crate::statement::trailing_semicolon;
 
 #[derive(Default)]
@@ -19,21 +23,33 @@ impl FormatNodeRule<StmtAnnAssign> for FormatStmtAnnAssign {
             simple: _,
         } = item;
 
-        write!(
-            f,
-            [target.format(), token(":"), space(), annotation.format(),]
-        )?;
+        write!(f, [target.format(), token(":"), space()])?;
 
         if let Some(value) = value {
-            write!(
-                f,
-                [
-                    space(),
-                    token("="),
-                    space(),
-                    FormatStatementsLastExpression::new(value, item)
-                ]
-            )?;
+            if is_prefer_splitting_right_hand_side_of_assignments_enabled(f.context())
+                && has_parentheses(annotation, f.context()).is_some()
+            {
+                FormatAssignmentValue {
+                    before_operator: annotation,
+                    operator: AnyAssignmentOperator::Assign,
+                    value,
+                    statement: item.into(),
+                }
+                .fmt(f)?;
+            } else {
+                write!(
+                    f,
+                    [
+                        annotation.format(),
+                        space(),
+                        token("="),
+                        space(),
+                        FormatStatementsLastExpression::new(value, item)
+                    ]
+                )?;
+            }
+        } else {
+            annotation.format().fmt(f)?;
         }
 
         if f.options().source_type().is_ipynb()