httpPayload does not work on union shapes

[david-perez]

$version: "1.0"

namespace com.amazonaws.simple

use aws.protocols#restJson1

@restJson1
service SimpleService {
    operations: [
        HttpPayloadUnion,
// No AWS protocol supports this: https://github.com/awslabs/smithy/blob/3a0d0233b2cbffebae3e14459f8aea2e83978c72/smithy-aws-traits/src/main/java/software/amazon/smithy/aws/traits/protocols/ProtocolHttpPayloadValidator.java#L99-L100
//      HttpPayloadList,
//      HttpPayloadMap,
    ],
}

@http(uri: "/http-payload-union", method: "POST")
operation HttpPayloadUnion {
    input: HttpPayloadUnionInputOutput,
    output: HttpPayloadUnionInputOutput,
}

@http(uri: "/http-payload-list", method: "POST")
operation HttpPayloadList {
    input: HttpPayloadListInputOutput,
    output: HttpPayloadListInputOutput,
}

@http(uri: "/http-payload-map", method: "POST")
operation HttpPayloadMap {
    input: HttpPayloadMapInputOutput,
    output: HttpPayloadMapInputOutput,
}

structure HttpPayloadUnionInputOutput {
    @httpPayload
    unionShape: UnionShape
}

structure HttpPayloadListInputOutput {
    @httpPayload
    listShape: ListShape
}

structure HttpPayloadMapInputOutput {
    @httpPayload
    mapShape: MapShape
}

union UnionShape {
    listVariant: ListShape,
    mapVariant: MapShape
}

list ListShape {
    member: String
}

map MapShape {
    key: String,
    value: String
}

Generates in operation_ser.rs:

pub fn serialize_payload_http_payload_union_output(
    payload: &std::option::Option<crate::model::UnionShape>,
) -> Result<std::vec::Vec<u8>, aws_smithy_http::operation::BuildError> {
    let payload = match payload.as_ref() {
        Some(t) => t,
        None => return Ok(),     ■■ supplied 0 arguments
    };
    Ok(
        crate::operation_ser::serialize_member_com_amazonaws_simple_synthetic_http_payload_union_output_union_shape(payload)?
    )
}

Relevant generator code:

https://github.com/awslabs/smithy-rs/blob/ca2c7883e7c252b7686314b404ecc11d408460f5/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/HttpBoundProtocolPayloadGenerator.kt#L257-L281

The httpPayload trait:

can be applied to structure members that target a string, blob, structure, union, document, map, or list.

However, we're only handling string, blob, document, and structure shapes.

Of note is that no AWS protocol supports list or map, but HttpBoundProtocolPayloadGenerator is protocol-agnostic (in that it handles all protocols that support HTTP binding traits). That's something to take into account if/when we ever support a non-AWS protocol where binding to list or map shapes is allowed.

When we fix this bug, we should ensure to include an else branch to that when that aborts code generation with an informative message, and contribute HTTP protocol tests to awslabs/smithy.

[david-perez]

I think we're blocked on smithy-lang/smithy#1462.