Assertion failure in `post_return()` for functions returning `error-context`

[kavoc-brown]

Summary

wit-bindgen panics with "assertion failed: sig.retptr" when generating bindings for a WIT world that exports a function returning error-context.

Environment

• wit-bindgen version: 0.47.0
• Affected generator: Rust
• OS: Ubuntu 22.04, x86-64

Steps to Reproduce

1. Create a minimal WIT file

   Save the following as test.wit:

   package poc:demo;
   
   world retptr-bug {
     export get-error: func() -> error-context;
   }

2. Run wit-bindgen

   wit-bindgen rust --out-dir /tmp/test test.wit

3. Observe the panic

   thread 'main' panicked at /home/runner/work/wit-bindgen/wit-bindgen/crates/core/src/abi.rs:1316:9
   assertion failed: sig.retptr
   note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
   

[alexcrichton]

Thanks! I'd recommend avoiding the error-context type as it's not ready for general use yet. Good to still have this on file though.