[Rust] Complex/optional parameter encodings are not handled correctly

Description

I am dealing with a Swagger 2.0 API which needs to accept an optional parameter consisting of an unexploded comma-separated list of pipe-separated lists of values. The Rust mustache templates really don't handle any of this, so I can't use the generated code: it doesn't even compile.

Swagger-codegen version

Current git master, but seems to apply to all versions.

Swagger declaration file content or url

https://gist.github.com/BartMassey/b58a73520d5ca1b66a6486d4fb345fc6

Command line used for generation

java -jar swagger-codegen-cli.jar generate \
     -i swagger-params.json -l rust -o demo

Steps to reproduce

Generate the Rust API as described above. cd to demo/ and cargo build. Note that the code does not compile. Inspect the code to see that it is not generating anything usable for the parameter.

Related issues/PRs

See issues #8169 and #8171 for more information on my use case.

Suggest a fix/enhancement

rust/api.mustache needs to be expanded to handle the full set of parameter possibilities. I'm not quite sure how to do this, but I might give it a try at some point if I can't get help here.

Start with rust/api.mustache and the Swagger declaration linked in the issue. Run the stated swagger-codegen generation command, then cargo build in demo/ to reproduce the compilation failure and inspect the generated parameter handling. Done means the generated Rust project builds and handles the described optional, unexploded comma- and pipe-separated parameter.