Add invalid schema placeholder

[sydney-runkle]

Progress on pydantic/pydantic#10299

Sort of reminds me of #619, maybe we want to change this implementation to reflect some of those constraints?

[codspeed-hq]

CodSpeed Performance Report

Merging #1469 will not alter performance

_{Comparing invalid-schema (95626d6) with main (e3eff5c)}

Summary

✅ 155 untouched benchmarks

[davidhewitt]

What's the current error message if you try to create a SchemaValidator containing one of these? Should it be improved?

[sydney-runkle]

What's the current error message if you try to create a SchemaValidator containing one of these? Should it be improved?

Good question. Definitely could be improved, though seems maybe ok for now?

Traceback (most recent call last):
  File "/Users/programming/pydantic_work/pydantic-core/test.py", line 3, in <module>
    sv = SchemaValidator(core_schema.invalid_schema())
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
pydantic_core._pydantic_core.SchemaError: Unknown schema type: "invalid"

[sydney-runkle]

@davidhewitt, I can easily edit the error message in mod.rs. What would you prefer?

[davidhewitt]

That placeholder seems reasonable. How about adding a BuildValidator implementation that's guaranteed to error (with something like "invalid schemas can never be instantiated")?