Add a lint forbidding PEP-570 syntax (#10660)

Author: AlexWaygood

tests/check_new_syntax.py

@@ -26,7 +26,29 @@ def visit_If(self, node: ast.If) -> None:
                 )
             self.generic_visit(node)
 
+    class PEP570Finder(ast.NodeVisitor):
+        def __init__(self) -> None:
+            self.lineno: int | None = None
+
+        def _visit_function(self, node: ast.FunctionDef | ast.AsyncFunctionDef) -> None:
+            old_lineno = self.lineno
+            self.lineno = node.lineno
+            self.generic_visit(node)
+            self.lineno = old_lineno
+
+        visit_FunctionDef = visit_AsyncFunctionDef = _visit_function
+
+        def visit_arguments(self, node: ast.arguments) -> None:
+            if node.posonlyargs:
+                assert isinstance(self.lineno, int)
+                errors.append(
+                    f"{path}:{self.lineno}: PEP-570 syntax cannot be used in typeshed yet. "
+                    f"Prefix parameter names with `__` to indicate positional-only parameters"
+                )
+            self.generic_visit(node)
+
     IfFinder().visit(tree)
+    PEP570Finder().visit(tree)
     return errors