Enhancement: use `msgspec` tag field `ClassVar` annotations in OpenAPI specs

[umarbutler]

Summary

When constructing a msgspec.Struct that has a tag field that is annotated using ClassVar, the annotations of that field are not included in the OpenAPI spec automatically generated by Litestar.

This effectively makes it impossible to document discriminator fields in Litestar-generated OpenAPI specs.

I would like to request that annotations inside ClassVars on discriminator/tag fields be included in OpenAPI specs.

Basic Example

Currently, given the below spec, Litestar will not include the annotations on the version field in the automatically generated OpenAPI spec.

"""
This is a minimal reproducible example (MRE) of a Litestar bug. To run the app, create a folder named `_mre`, place this file in it and name it `app.py`, and then run, from the parent directory of `_mre`, `python -m uvicorn _mre.app:app --reload --port 2020`.

Visit http://localhost:2020/openapi.json to see the OpenAPI schema.
"""

from typing import Literal, ClassVar, Annotated

import msgspec

from litestar import get, Request, Litestar


class Resp(msgspec.Struct, tag="1", tag_field="version"):
    version: ClassVar[Annotated[Literal["1"], msgspec.Meta(description="The version of the struct.")]] = "1"
    foo: str = "bar"


@get("/")
async def home() -> Resp:
    return Resp()


@get(path=["/openapi.json"], include_in_schema=False, sync_to_thread=True)
def get_openapi(request: Request) -> dict:
    schema = request.app.openapi_schema

    return schema.to_schema()


app = Litestar(
    [home, get_openapi],
)

if __name__ == "__main__":
    import uvicorn

    try:
        uvicorn.run(
            app="_mre:app",
            host="localhost",
            port=2020,
            reload=True,
        )

    except Exception:
        uvicorn.run(
            app=app,
            host="localhost",
            port=2020,
        )

Drawbacks and Impact

N/A

Unresolved questions

N/A