Ignore nodes without standalone render in whereis

[kafkasl]

I got an exception using whereis is non-trivial code example (exception at the bottom).

The problem is that not every LibCST node can be rendered on its own. Some nodes are deliberately incomplete because they rely on their parent to fill in a detail at render time. The clearest example is Annotation:

• In def f(x: int), the annotation renders as : int
• In def f() -> int, the annotation renders as -> int

The Annotation node itself doesn't store whether it's a : or a -> — the parent (Param vs FunctionDef) supplies that "indicator" during codegen. Rendered in isolation, there's a MaybeSentinel.DEFAULT where the indicator should be, and LibCST raises CSTCodegenError rather than guess.

The fix is simply to skip the nodes that raise an exception because they can't rendered. Given that whereis is asking "does this node's text equal/contain the fragment?" A node that has no standalone text can never match and returning false is the right answer.

Traceback (most recent call last):

  File ~/ws/.venv/lib/python3.12/site-packages/IPython/core/interactiveshell.py:3748 in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)

  Cell In[50], line 6
    whereis(src, '5b00de99', contains=True)

  File ~/ws/remold/remold/__init__.py:127 in whereis
    return {frag: [p for p,n in sorted(((p,n) for p,n in _walk(mod) if p and _hit(n, frag)), key=lambda o: -len(o[0]))]

  File ~/ws/remold/remold/__init__.py:127 in <genexpr>
    return {frag: [p for p,n in sorted(((p,n) for p,n in _walk(mod) if p and _hit(n, frag)), key=lambda o: -len(o[0]))]

  File ~/ws/remold/remold/__init__.py:123 in _hit
    c = _render(n).strip()

  File ~/ws/.venv/lib/python3.12/site-packages/libcst/_nodes/module.py:136 in code_for_node
    node._codegen(state)

  File ~/ws/.venv/lib/python3.12/site-packages/libcst/_nodes/base.py:300 in _codegen
    self._codegen_impl(state, **kwargs)

  File ~/ws/.venv/lib/python3.12/site-packages/libcst/_nodes/expression.py:1928 in _codegen_impl
    raise CSTCodegenError(

  File <string>
    
CSTCodegenError: Must specify a concrete default_indicator if default used on indicator.