Fix serializing list subclasses with external storage - #1157
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Fixes msgspec#1156. The new `ms_list_is_plain` function checks whether a `list` subclass stores its items in the internal list storage, and if not, copies the items out through the sequence protocol before serializing. This ensures that `list` subclasses are not serialized to an empty array.
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I don't think special casing lists here is the solution, and the current behaviour is documented.
I left a more detailed response here: #1156 (comment)
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Fixes #1156.
The new
ms_list_is_plainfunction checks whether alistsubclass stores its items in the internal list storage, and if not, copies the items out through the sequence protocol before serializing. This ensures thatlistsubclasses are not serialized to an empty array.