Remove a panic in add_type_definition and add error contexts so resul…

[mhammond]

…ting errors make sense.

With this patch, output from errors looks something like:

  Caused by:
      0: adding function Function { name: "get_maybe_url", module_path: "uniffi_ext_types_lib", is_async: false, arguments: [Argument { name: "url", type_: Optional { inner_type: Custom { module_path: "custom_types", name: "Url", builtin: None } }, by_ref: false, optional: false, default: None }], return_type: Some(Optional { inner_type: Custom { module_path: "custom_types", name: "Url", builtin: None } }), ffi_func: FfiFunction { name: "uniffi_uniffi_ext_types_lib_fn_func_get_maybe_url", is_async: false, arguments: [], return_type: None, has_rust_call_status_arg: true, is_object_free_function: false }, docstring: None, throws: None, checksum_fn_name: "uniffi_uniffi_ext_types_lib_checksum_func_get_maybe_url", checksum: None }
      1: adding type Optional { inner_type: Custom { module_path: "custom_types", name: "Url", builtin: None } }
      2: adding optional type Custom { module_path: "custom_types", name: "Url", builtin: None }
      3: Custom { module_path: "custom_types", name: "Url", builtin: None } must have a resolved builtin

[bendk]

LGTM, I think the context message handling may be could be improved but I'm not sure. Either way this feels like an improvement to me.

[bendk]

Are these getting overridden by the parsing udl file... context? That what it seems like from the trybuild tests, but I'm not sure.

[mhammond]

I don't think so:

[mhammond]

oops:

It's gone from:

error: Failed to generate name: Invalid trait name: PartialEq
to:
error: Failed to generate name: parsing udl : Invalid trait name: PartialEq

which is redundant here, but I don't think is overriding anything.

Interresting note re messages - this has:

.add_known_types(defn.iter_types())
.with_context(|| format!("adding enum {defn:?}"))?;

which imo is nicer in the code, but worse in the errors. For some reason I don't like using the word "error" there:

.add_known_types(defn.iter_types())
.with_context(|| format!("error adding enum {defn:?}"))?;

but I'm torn