[aws_lambda_events] No longer able to construct response types a lambda may return as a response (they are `non_exhaustive`)

[dcormier]

A number of the types in this crate are responses that a lambda might return. With the recent change to mark structs as non_exhaustive, these types can no longer be constructed. Some examples:

• SqsBatchResponse (and its child types) are returned from an SQS lambda
• AppSyncLambdaAuthorizerResponse (and its child types) are returned from an AppSync authorizer lambda.
• ApiGatewayCustomAuthorizerResponse (and it's child types) are returned from an API Gateway authorizer lambda.

I'm sure there are other response types in this crate that lambdas would construct, but these are the ones I'm currently constructing and am no longer able to after they've been marked non_exhaustive.

[jlizen]

Hi @dcormier , the intended API now when directly constructing would be to use ..Default::default() to guard against unknown fields. Does that work for you? I do think it would be good to have some examples demonstrating this...

These were changed because it was constantly forcing semver bumps every update and was forcing consumers to either update their code or be frozen in time.

[dcormier]

Modifying an existing rust example from AWS's docs to this:

async fn function_handler(event: LambdaEvent<SqsEvent>) -> Result<SqsBatchResponse, Error> {
    let mut batch_item_failures = Vec::new();
    for record in event.payload.records {
        match process_record(&record).await {
            Ok(_) => (),
            Err(_) => batch_item_failures.push(BatchItemFailure {
                item_identifier: record.message_id.unwrap(),
                ..Default::default()
            }),
        }
    }

    Ok(SqsBatchResponse {
        batch_item_failures,
        ..Default::default()
    })
}

Results in this:

error[E0639]: cannot create non-exhaustive struct using struct expression
  --> src/main.rs:18:48
   |
18 |               Err(_) => batch_item_failures.push(BatchItemFailure {
   |  ________________________________________________^
19 | |                 item_identifier: record.message_id.unwrap(),
20 | |                 ..Default::default()
21 | |             }),
   | |_____________^

error[E0639]: cannot create non-exhaustive struct using struct expression
  --> src/main.rs:25:8
   |
25 |       Ok(SqsBatchResponse {
   |  ________^
26 | |         batch_item_failures,
27 | |         ..Default::default()
28 | |     })
   | |_____^

For more information about this error, try `rustc --explain E0639`.

Instead, you have to abandon struct literals entirely, which is less than ideal:

async fn function_handler(event: LambdaEvent<SqsEvent>) -> Result<SqsBatchResponse, Error> {
    let mut batch_item_failures = Vec::new();
    for record in event.payload.records {
        match process_record(&record).await {
            Ok(_) => (),
            Err(_) => {
                let mut item = BatchItemFailure::default();
                item.item_identifier = record.message_id.unwrap();

                batch_item_failures.push(item)
            }
        }
    }

    let mut response = SqsBatchResponse::default();
    response.batch_item_failures = batch_item_failures;

    Ok(response)
}

[jlizen]

Ah, my mistake. This is running afoul of: rust-lang/lang-team#143 , which doesn't seem to have much traction.

It sounds like at minimum we should update the official example to compile properly (@PartiallyUntyped ).

But, agreed that abandoning struct literals is not ideal. This is an awkward situation, since we definitely can't maintain the library with reasonable stability without #[non_exhaustive]

Perhaps some sort of non-default feature flag like features = ["experimental-non-exhaustive"] that has lesser semver guarantees?

[PartiallyUntyped]

We provide examples with the latest behaviour here:

• basic-s3-thumbnail
• basic-error-handling
• advanced-sqs-partial-batch-failures
• basic-s3-object-lambda-thumbnail

I will be updating the aws docs shortly as well.