Safety fixes and docs for vtab #415
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This will prevent the macro generating a warning in edition 2024
Contrary to the previous docs, the instances passed to these functions are *not* initialized by the caller. Rather, the called function is responsible for writing into uninitialized memory.
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The vtab methods are called with pointers to uninitialized blocks of memory:
The docs previously seemed to falsely indicate that these blocks were actually initialized: clarify the docs.
It's UB to read uninitialzed memory, and better to write it with ptr::write. In particular, less trivial code that follows the example can corrupt memory if it uses the patterns shown in the example.
Use unsafe blocks inside unsafe fns in the example, because this will be a warning in Rust 2024. In particular, do this to avoid a warning from the duckdb_entrypoint macro.
The Rust types used in the examples don't need layout compatibility with C, and can simply be Rust structs using native strings.
Improves #414