And what pattern would you recommend if you needed to allocate?

It's worth every effort to avoid situations where a function creates extra clean up responsibilities to the caller only on error conditions.

If I really needed a large error payload, too big to fit on the stack, I'd probably want to do something like this:

  const errbuf = try alloc.alloc(u8, 1024*1024*1024);
  module.setErrorBuf(&errbuf)
  defer {
    module.setErrorBuf(null);
    alloc.free(errbuf);
  }

  var func_diag = diagnostics.OfFunction(module.func){};
  module.func(foo, bar, &func_diag) catch |err| switch (err) {
    error.BigPayload => {
      const payload = func_diag.get(error.BigPayload);

      // The payload can reference the 1MiB of data safely here,
      // and it's cleaned up automatically.
    }
  }

The diagnostic struct could contain a caller-provided allocator field which the callee can use, and a deinit() function on the diagnostic struct which frees everything.