Include underlying error when AST safety check parsing fails (#2693)

CHANGES.md

@@ -5,6 +5,8 @@
 ### _Black_
 
 - Improve error message for invalid regular expression (#2678)
+- Improve error message when parsing fails during AST safety check by embedding the
+  underlying SyntaxError (#2693)
 - Fix mapping cases that contain as-expressions, like `case {"key": 1 | 2 as password}`
   (#2686)
 - No longer color diff headers white as it's unreadable in light themed terminals

src/black/__init__.py

@@ -1305,7 +1305,7 @@ def assert_equivalent(src: str, dst: str, *, pass_num: int = 1) -> None:
         src_ast = parse_ast(src)
     except Exception as exc:
         raise AssertionError(
-            "cannot use --safe with this file; failed to parse source file."
+            f"cannot use --safe with this file; failed to parse source file: {exc}"
         ) from exc
 
     try:

tests/test_black.py

@@ -1584,6 +1584,16 @@ def test_for_handled_unexpected_eof_error(self) -> None:
 
         exc_info.match("Cannot parse: 2:0: EOF in multi-line statement")
 
+    def test_equivalency_ast_parse_failure_includes_error(self) -> None:
+        with pytest.raises(AssertionError) as err:
+            black.assert_equivalent("a«»a  = 1", "a«»a  = 1")
+
+        err.match("--safe")
+        # Unfortunately the SyntaxError message has changed in newer versions so we
+        # can't match it directly.
+        err.match("invalid character")
+        err.match(r"\(<unknown>, line 1\)")
+
 
 class TestCaching:
     def test_cache_broken_file(self) -> None: