[Rust Server] Support types with additional properties

[richardwhiuk]

PR checklist

• Read the contribution guidelines.

• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.

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  • @frol, @farcaller, @bjgill

Description of the PR

This adds support for types which only contain additional properties, which is a common use case. It doesn't handle the thornier case of additional properties within a type with defined properties, which is allowed by the OpenAPI spec, but is more dubious API design in practice (as the API is mixing schema-ed and un-schema-ed data).

Note, this will also only fix the case where the type is defined (e.g. string) - if it's untyped, it remains unsupported.

This thus partially fixes#318

An example of something this will support is already defined in the sample API:

 additionalPropertiesObject:
description: An additionalPropertiesObject
type: object
additionalProperties:
type: string

[richardwhiuk]

Oh, this is technically a breaking change, because objects of this form were previously constructible (although with no properties in them) as AdditionalPropertiesObject {} and the equivalent creation is AdditionalPropertiesObject(HashMap::new()) but that's basically unavoidable.

[bjgill]

Neat.

Agree that we don't need to worry much about the technically breaking change.

[wing328]

@richardwhiuk thanks for the PR, which has been included in v4.2.0 release: https://twitter.com/oas_generator/status/1189824932345069569