Add support for star-unpacking of comprehensions

Author: charliermarsh

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/list_comp_py315.options.json

@@ -0,0 +1,5 @@
+[
+  {
+    "target_version": "3.15"
+  }
+]

crates/ruff_python_formatter/resources/test/fixtures/ruff/expression/list_comp_py315.py

@@ -0,0 +1,12 @@
+[*x for x in y]
+
+[
+    *  # comment between * and x
+    x
+    for x in y
+]
+
+[
+    *values
+    for values in some_really_long_collection_name_that_should_force_wrapping
+]

crates/ruff_python_formatter/tests/snapshots/format@expression__list_comp_py315.py.snap

@@ -0,0 +1,46 @@
+---
+source: crates/ruff_python_formatter/tests/fixtures.rs
+---
+## Input
+```python
+[*x for x in y]
+
+[
+    *  # comment between * and x
+    x
+    for x in y
+]
+
+[
+    *values
+    for values in some_really_long_collection_name_that_should_force_wrapping
+]
+```
+
+## Outputs
+### Output 1
+```
+indent-style               = space
+line-width                 = 88
+indent-width               = 4
+quote-style                = Double
+line-ending                = LineFeed
+magic-trailing-comma       = Respect
+docstring-code             = Disabled
+docstring-code-line-width  = "dynamic"
+preview                    = Disabled
+target_version             = 3.15
+source_type                = Python
+```
+
+```python
+[*x for x in y]
+
+[
+    # comment between * and x
+    *x
+    for x in y
+]
+
+[*values for values in some_really_long_collection_name_that_should_force_wrapping]
+```

crates/ruff_python_parser/resources/inline/err/starred_list_comp_py314.py

@@ -0,0 +1,2 @@
+# parse_options: {"target-version": "3.14"}
+[*x for x in y]

crates/ruff_python_parser/resources/inline/ok/starred_list_comp_py315.py

@@ -0,0 +1,3 @@
+# parse_options: {"target-version": "3.15"}
+[*x for x in y]
+[*factor.dims for factor in bases]

crates/ruff_python_parser/resources/invalid/expressions/list/comprehension.py

@@ -1,6 +1,3 @@
-# Iterable unpacking not allowed
-[*x for x in y]
-
 # Invalid target
 [x for 1 in y]
 [x for 'a' in y]
@@ -12,9 +9,14 @@
 [x for x in yield y]
 [x for x in yield from y]
 [x for x in lambda y: y]
+[**x for x in [{1: 2}]]
+[*x, for x in y]
+[*x, *y for x in y]
 
 # Invalid if
 [x for x in data if *y]
 [x for x in data if yield y]
 [x for x in data if yield from y]
-[x for x in data if lambda y: y]
+[x for x in data if lambda y: y]
+[*x if x else y for x in z]
+[x if x else *y for x in z]

crates/ruff_python_parser/src/error.rs

@@ -837,6 +837,28 @@ pub enum UnsupportedSyntaxErrorKind {
     /// [PEP 646]: https://peps.python.org/pep-0646/#change-2-args-as-a-typevartuple
     StarAnnotation,
 
+    /// Represents the use of iterable unpacking inside a list comprehension
+    /// before Python 3.15.
+    ///
+    /// ## Examples
+    ///
+    /// Before Python 3.15, list comprehensions could not use iterable
+    /// unpacking in their element expression:
+    ///
+    /// ```python
+    /// [*x for x in y]  # SyntaxError
+    /// ```
+    ///
+    /// Starting with Python 3.15, [PEP 798] allows iterable unpacking within
+    /// list comprehensions:
+    ///
+    /// ```python
+    /// [*x for x in y]
+    /// ```
+    ///
+    /// [PEP 798]: https://peps.python.org/pep-0798/
+    IterableUnpackingInListComprehension,
+
     /// Represents the use of tuple unpacking in a `for` statement iterator clause before Python
     /// 3.9.
     ///
@@ -979,6 +1001,9 @@ impl Display for UnsupportedSyntaxError {
                 "Cannot use star expression in index"
             }
             UnsupportedSyntaxErrorKind::StarAnnotation => "Cannot use star annotation",
+            UnsupportedSyntaxErrorKind::IterableUnpackingInListComprehension => {
+                "Cannot use iterable unpacking in a list comprehension"
+            }
             UnsupportedSyntaxErrorKind::UnparenthesizedUnpackInFor => {
                 "Cannot use iterable unpacking in `for` statements"
             }
@@ -1051,6 +1076,9 @@ impl UnsupportedSyntaxErrorKind {
                 Change::Added(PythonVersion::PY311)
             }
             UnsupportedSyntaxErrorKind::StarAnnotation => Change::Added(PythonVersion::PY311),
+            UnsupportedSyntaxErrorKind::IterableUnpackingInListComprehension => {
+                Change::Added(PythonVersion::PY315)
+            }
             UnsupportedSyntaxErrorKind::UnparenthesizedUnpackInFor => {
                 Change::Added(PythonVersion::PY39)
             }

crates/ruff_python_parser/src/parser/expression.rs

@@ -1989,10 +1989,19 @@ impl<'src> Parser<'src> {
             TokenKind::Async | TokenKind::For => {
                 // Parenthesized starred expression isn't allowed either but that is
                 // handled by the `parse_parenthesized_expression` method.
+
+                // test_ok starred_list_comp_py315
+                // # parse_options: {"target-version": "3.15"}
+                // [*x for x in y]
+                // [*factor.dims for factor in bases]
+
+                // test_err starred_list_comp_py314
+                // # parse_options: {"target-version": "3.14"}
+                // [*x for x in y]
                 if first_element.is_unparenthesized_starred_expr() {
-                    self.add_error(
-                        ParseErrorType::IterableUnpackingInComprehension,
-                        &first_element,
+                    self.add_unsupported_syntax_error(
+                        UnsupportedSyntaxErrorKind::IterableUnpackingInListComprehension,
+                        first_element.range(),
                     );
                 }