Run compile() against tests/eval_files to catch syntax mistakes

ast.parse() accepts some code the bytecode compiler rejects (e.g.
`x = lambda: await foo()`, `return` outside a function), so a bad
test fixture could silently slip past the plugin's ast-based checks.

Add a new parametrized test that runs compile() on every
tests/eval_files/*.py. A handful of existing fixtures intentionally
contain such constructs to exercise edge cases; mark those with a new
`# NOCOMPILE` magic marker so the new test skips them while still
leaving the rest of the suite guarding against accidents.

Fixes #268

Author: claude

docs/changelog.rst

@@ -10,6 +10,7 @@ Unreleased
 - :ref:`ASYNC910 <async910>` and :ref:`ASYNC911 <async911>` now accept ``__aenter__`` / ``__aexit__`` methods when the partner method provides the checkpoint, or when only one of the two is defined on a class that inherits from another class (charitably assuming the partner is inherited and contains a checkpoint). `(issue #441) <https://github.com/python-trio/flake8-async/issues/441>`_
 - :ref:`ASYNC300 <async300>` no longer triggers when the result of ``asyncio.create_task()`` is returned from a function. `(issue #398) <https://github.com/python-trio/flake8-async/issues/398>`_
 - Document using ruff's ``lint.external`` setting to preserve ``# noqa: ASYNC...`` comments when running ruff alongside flake8-async. `(issue #310) <https://github.com/python-trio/flake8-async/issues/310>`_
+- Tests: each ``tests/eval_files/*.py`` fixture is now checked with ``compile()`` so that accidental semantic errors (e.g. ``lambda: await foo()``) in fixtures are caught. Files that intentionally contain such constructs are marked with ``# NOCOMPILE``. `(issue #268) <https://github.com/python-trio/flake8-async/issues/268>`_
 
 25.7.1
 ======

tests/autofix_files/async124.py

@@ -3,6 +3,9 @@
 
 # ARG --enable=ASYNC124,ASYNC910,ASYNC911
 # ARG --no-checkpoint-warning-decorator=custom_disabled_decorator
+# NOCOMPILE: foo_nested_sync contains `await` in a sync nested function, which is
+# a SyntaxError the bytecode compiler catches but ast.parse accepts. It's only here
+# to make sure the plugin doesn't crash on such code.
 
 # 910/911 will also autofix async124, in the sense of adding a checkpoint. This is perhaps
 # not what the user wants though, so this would be a case in favor of making 910/911 not

tests/eval_files/async103_104_py311.py

@@ -4,6 +4,9 @@
 ASYNC104: cancelled-not-raised
 """
 
+# NOCOMPILE: `return` inside `except*` is a SyntaxError in 3.11+ but we still want
+# to exercise the plugin against it via ast.parse.
+
 # ARG --enable=ASYNC103,ASYNC104
 
 try:

tests/eval_files/async104.py

@@ -1,4 +1,6 @@
 # ARG --enable=ASYNC103,ASYNC104
+# NOCOMPILE: contains `return` outside a function so the bytecode compiler rejects
+# the file, but ast.parse accepts it and the plugin flags it as ASYNC104.
 try:
     ...
 # raise different exception

tests/eval_files/async124.py

@@ -3,6 +3,9 @@
 
 # ARG --enable=ASYNC124,ASYNC910,ASYNC911
 # ARG --no-checkpoint-warning-decorator=custom_disabled_decorator
+# NOCOMPILE: foo_nested_sync contains `await` in a sync nested function, which is
+# a SyntaxError the bytecode compiler catches but ast.parse accepts. It's only here
+# to make sure the plugin doesn't crash on such code.
 
 # 910/911 will also autofix async124, in the sense of adding a checkpoint. This is perhaps
 # not what the user wants though, so this would be a case in favor of making 910/911 not

tests/test_flake8_async.py

@@ -244,6 +244,12 @@ class MagicMarkers:
     # eval file is written using this library, so no substitution is required
     BASE_LIBRARY: str = "trio"
 
+    # File intentionally contains constructs that are syntactically valid for
+    # `ast.parse` but rejected by the bytecode compiler (e.g. `return` outside a
+    # function, `await` in a sync nested function). Used to skip the compile()
+    # sanity check in test_eval_files_compile.
+    NOCOMPILE: bool = False
+
     def library_no_error(self, library: str) -> bool:
         return {
             "anyio": self.ANYIO_NO_ERROR,
@@ -545,6 +551,19 @@ def visit_AsyncFor(self, node: ast.AsyncFor):
         return self.replace_async(node, ast.For, node.target, node.iter)
 
 
+# ast.parse() is lenient and accepts some code that the bytecode compiler will
+# reject (e.g. `x = lambda: await foo()` or `return` outside a function). Running
+# compile() on each eval file catches accidental syntax errors in test fixtures
+# that would otherwise silently slip past the plugin's ast-based checks.
+@pytest.mark.parametrize(("test", "path"), test_files, ids=[f[0] for f in test_files])
+def test_eval_files_compile(test: str, path: Path):
+    check_version(test)
+    content = path.read_text()
+    if find_magic_markers(content).NOCOMPILE:
+        pytest.skip("file intentionally does not compile (has # NOCOMPILE marker)")
+    compile(content, str(path), "exec")
+
+
 @pytest.mark.parametrize(("test", "path"), test_files, ids=[f[0] for f in test_files])
 def test_noerror_on_sync_code(test: str, path: Path):
     if any(e in test for e in error_codes_ignored_when_checking_transformed_sync_code):