FormattedExcinfo.get_source: avoid crash when line number is out-of-bounds/negative

pytest could crash given pathological AST position attributes, which shouldn't happen when testing real Python code, but could happen when testing AST produced by e.g. Hylang.

Another example of the failure is in the nightly CI for the JAX project: https://github.com/google/jax/actions/runs/4607513902/jobs/8142126075

Co-authored-by: Bruno Oliveira <nicoddemus@gmail.com>
Co-authored-by: Jake VanderPlas <jakevdp@google.com>

AUTHORS

@@ -163,6 +163,7 @@ Ionuț Turturică
 Itxaso Aizpurua
 Iwan Briquemont
 Jaap Broekhuizen
+Jake VanderPlas
 Jakob van Santen
 Jakub Mitoraj
 James Bourbeau

changelog/10840.improvement.rst

@@ -0,0 +1 @@
+pytest should no longer crash on AST with pathological position attributes, for example testing AST produced by `Hylang <https://github.com/hylang/hy>__`.

src/_pytest/_code/code.py

@@ -743,11 +743,13 @@ class FormattedExcinfo:
     ) -> List[str]:
         """Return formatted and marked up source lines."""
         lines = []
-        if source is None or line_index >= len(source.lines):
+        if source is not None and line_index < 0:
+            line_index += len(source)
+        if source is None or line_index >= len(source.lines) or line_index < 0:
+            # `line_index` could still be outside `range(len(source.lines))` if
+            # we're processing AST with pathological position attributes.
             source = Source("???")
             line_index = 0
-        if line_index < 0:
-            line_index += len(source)
         space_prefix = "    "
         if short:
             lines.append(space_prefix + source.lines[line_index].strip())

testing/code/test_excinfo.py

@@ -461,6 +461,24 @@ class TestFormattedExcinfo:
         assert lines[0] == "|   def f(x):"
         assert lines[1] == "        pass"
 
+    def test_repr_source_out_of_bounds(self):
+        pr = FormattedExcinfo()
+        source = _pytest._code.Source(
+            """\
+            def f(x):
+                pass
+            """
+        ).strip()
+        pr.flow_marker = "|"  # type: ignore[misc]
+
+        lines = pr.get_source(source, 100)
+        assert len(lines) == 1
+        assert lines[0] == "|   ???"
+
+        lines = pr.get_source(source, -100)
+        assert len(lines) == 1
+        assert lines[0] == "|   ???"
+
     def test_repr_source_excinfo(self) -> None:
         """Check if indentation is right."""
         try: