Variant extensions

Another "nice to have" as opposed to being anything particularly crucial. I was considering use of variants as error types, as it seems like the best option to have fine-grained error information as a sort of best practice. Then there might then be some generic base-level errors which are extended by function-specific error handlers.

At a WIT-convenience level it might be useful to support this via variant extensions:

variant base-error {
  unknown-error(string),
  internal-host-error(string),
  validation-error(tuple<string, string>),
  rate-limit-error(tuple<u32, string>)
}

variant create-error extends base-error {
  invalid-param(tuple<string, string>),
  already-exists(string),
  no-backend(tuple<string, u32>)
}
func create(param: string) -> result<_, create-error>

variant update-error extends base-error {
  update-error(tuple<string, string>)
}
func update(param: string, val: string) -> result<_, update-error>

Similarly, enum extensions might be useful.

Some use cases to think through, just posting as a placeholder for now.

Start by reading the issue's proposed WIT examples for variant extensions and the related note about enum extensions. Work through how base and function-specific error variants would compose, including result types and error handlers. Done would require a decided design and specified use cases, but this placeholder names no files, tests, or implementation entry point.