Error struct member with default implementation cause rs clients to fail on build

[codypenta]

When an error member has a default value, it will generate errors with smithy-rs client unless you manually specify @required

For example:

// Valid Smithy, fails client creation
@error("client")
structure ReservedGroupUsedError {
    message: String = "Cannot use a reserved group name or any variation of"
}

@error("client")
structure ReservedGroupUsedError {
    @required // This makes the client render again
    message: String = "Cannot use a reserved group name or any variation of"
}

I think the code is question is here

I cloned and made this edit below and it worked for me

// If the message member is optional and wasn't set, we set a generic error message.
if (errorMessageMember != null) {
    if (errorMessageMember.isOptional && !errorMessageMember.hasNonNullDefault()) {
        rust(
            """
            if tmp.message.is_none() {
                tmp.message = _error_message;
            }
            """,
        )
    }
}

Happy to make a PR if this is the right approach

[jdisanti]

To better assess the proposed change, can you provide what the error was before the change?

[codypenta]

            if tmp.message.is_none() {
                tmp.message = _error_message;
            }

In the rust code this would throw is_none() does not exist on type String. And I think there was some code try accessing it as a String but it was an Option<String>

[jdisanti]

I see. I think the proposed change will fix it in some cases, but potentially not all. To make it work in all cases, the NullableIndex must be used to determine if it's wrapped in an Option or not, or better yet, check if the member's symbol is marked as optional (which is determined by the NullableIndex earlier in the codegen). Something like:

if (errorMessageMember != null) {
    val symbol = symbolProvider.toSymbol(errorMessageMember)
    if (symbol.is_optional()) {
        ...

Feel free to open a PR.