FormattedExcinfo.get_source: Avoid a crash when source.lines == 0

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 @@ def get_source(
     ) -> 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 @@ def f(x):
         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: