Inheritable kw_only for Struct (e.g., kw_only_default=True) #841

Description

Description

Currently, kw_only=True on msgspec.Struct does not inherit to subclasses.
I'm building a pydantic-like wrapper over msgspec, and I want users to define models without repeating kw_only=True:

class BaseModel(Struct, kw_only=True):
    ...

class User(BaseModel):
    age: int = Field(...)
    name: str

Today, users must write:

class User(BaseModel, kw_only=True):
    age: int = Field(...)
    name: str

which hurts usability.

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Proposal

Add an option like kw_always=True that enables kw_only automatically for subclasses, without needing to re-specify it.

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Why it’s needed
• Easier migration for pydantic users
• Cleaner API
• Avoids complex workarounds (like dynamically generating inner structs)

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Current workaround

I tried subclassing StructMeta but failed.
My only working solution is generating a dynamic InnerStruct at runtime, which complicates type handling, nested structures, and serialization.

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Impact

Supporting kw_only_default =True would significantly improve framework and library development on top of msgspec.

Thanks for considering!