[syntax-errors] Positional-only parameters before Python 3.8 (#16481)

Summary
--

Detect positional-only parameters before Python 3.8, as marked by the
`/` separator in a parameter list.

Test Plan
--
Inline tests.

crates/ruff_python_parser/src/error.rs

@@ -449,6 +449,32 @@ pub enum UnsupportedSyntaxErrorKind {
     Match,
     Walrus,
     ExceptStar,
+    /// Represents the use of a [PEP 570] positional-only parameter before Python 3.8.
+    ///
+    /// ## Examples
+    ///
+    /// Python 3.8 added the `/` syntax for marking preceding parameters as positional-only:
+    ///
+    /// ```python
+    /// def foo(a, b, /, c): ...
+    /// ```
+    ///
+    /// This means `a` and `b` in this case can only be provided by position, not by name. In other
+    /// words, this code results in a `TypeError` at runtime:
+    ///
+    /// ```pycon
+    /// >>> def foo(a, b, /, c): ...
+    /// ...
+    /// >>> foo(a=1, b=2, c=3)
+    /// Traceback (most recent call last):
+    ///   File "<python-input-3>", line 1, in <module>
+    ///     foo(a=1, b=2, c=3)
+    ///     ~~~^^^^^^^^^^^^^^^
+    /// TypeError: foo() got some positional-only arguments passed as keyword arguments: 'a, b'
+    /// ```
+    ///
+    /// [PEP 570]: https://peps.python.org/pep-0570/
+    PositionalOnlyParameter,
     /// Represents the use of a [type parameter list] before Python 3.12.
     ///
     /// ## Examples
@@ -487,6 +513,9 @@ impl Display for UnsupportedSyntaxError {
             UnsupportedSyntaxErrorKind::Match => "Cannot use `match` statement",
             UnsupportedSyntaxErrorKind::Walrus => "Cannot use named assignment expression (`:=`)",
             UnsupportedSyntaxErrorKind::ExceptStar => "Cannot use `except*`",
+            UnsupportedSyntaxErrorKind::PositionalOnlyParameter => {
+                "Cannot use positional-only parameter separator"
+            }
             UnsupportedSyntaxErrorKind::TypeParameterList => "Cannot use type parameter lists",
             UnsupportedSyntaxErrorKind::TypeAliasStatement => "Cannot use `type` alias statement",
             UnsupportedSyntaxErrorKind::TypeParamDefault => {
@@ -509,6 +538,7 @@ impl UnsupportedSyntaxErrorKind {
             UnsupportedSyntaxErrorKind::Match => PythonVersion::PY310,
             UnsupportedSyntaxErrorKind::Walrus => PythonVersion::PY38,
             UnsupportedSyntaxErrorKind::ExceptStar => PythonVersion::PY311,
+            UnsupportedSyntaxErrorKind::PositionalOnlyParameter => PythonVersion::PY38,
             UnsupportedSyntaxErrorKind::TypeParameterList => PythonVersion::PY312,
             UnsupportedSyntaxErrorKind::TypeAliasStatement => PythonVersion::PY312,
             UnsupportedSyntaxErrorKind::TypeParamDefault => PythonVersion::PY313,

crates/ruff_python_parser/src/parser/statement.rs

@@ -3050,6 +3050,21 @@ impl<'src> Parser<'src> {
                         // first time, otherwise it's a user error.
                         std::mem::swap(&mut parameters.args, &mut parameters.posonlyargs);
                         seen_positional_only_separator = true;
+
+                        // test_ok pos_only_py38
+                        // # parse_options: {"target-version": "3.8"}
+                        // def foo(a, /): ...
+
+                        // test_err pos_only_py37
+                        // # parse_options: {"target-version": "3.7"}
+                        // def foo(a, /): ...
+                        // def foo(a, /, b, /): ...
+                        // def foo(a, *args, /, b): ...
+                        // def foo(a, //): ...
+                        parser.add_unsupported_syntax_error(
+                            UnsupportedSyntaxErrorKind::PositionalOnlyParameter,
+                            slash_range,
+                        );
                     }
 
                     last_keyword_only_separator_range = None;