Format of CloudFormation template that uses Fn::Transform yields invalid template

[tomirio619]

I have the following CloudFormation snippet called input.yml:

Resources:
  RestApi:
    Type: AWS::ApiGateway::RestApi
    Properties:
      ApiKeySourceType: HEADER
      Body:
        Fn::Transform:
          Name: AWS::Include
          Parameters:
            Location: s3://<bucket-name>/<object-key>
      Description: API
      EndpointConfiguration:
        Types:
          - PRIVATE
      FailOnWarnings: true
      Name: My API

When I run $ rain fmt input.yml > output.yml, the contents of output.yml are:

AWSTemplateFormatVersion: "2010-09-09"

Resources:
  RestApi:
    Type: AWS::ApiGateway::RestApi
    Properties:
      ApiKeySourceType: HEADER
      Body: !Transform
        Name: AWS::Include
        Parameters:
          Location: s3://<bucket-name>/<object-key>
      Description: My API
      EndpointConfiguration:
        Types:
          - PRIVATE
      FailOnWarnings: true
      Name: API

If I deploy this formatted template in CloudFormation, deployment fails with the following message:

An error occurred (ValidationError) when calling the ValidateTemplate operation: Template format error: YAML not well-formed. 

Unfortunately, I cannot make use of S3BodyLocation (instead of Body), as our OpenAPI specification contains CloudFormation functions which are not substituted when used with S3BodyLocation (see this issue).
If used, the following message is shown for the CREATE_FAILED status:

Unable to parse API definition because of a malformed integration at path /an/example/path. (Service: AmazonApiGateway; Status Code: 400; Error Code: BadRequestException; Request ID: REDACTED; Proxy: null)

The AWS::Include transform documentation could be related, as it mentions the following:

We don't currently support using shorthand notations for YAML snippets.

[stilvoid]

Thanks for this! After some digging, it seems the Transform function just doesn't support the YAML tag-style notation. I will prevent Fn::Transform from being modified by rain fmt in the next release.

[stilvoid]

Fixed in dd5001e

[tomirio619]

Great, thanks!