[BUG] [rust] Field access is broken

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

For paths composed of fields of a structure, the generated Rust code does not compile:

error: invalid format string: field access isn't supported
   --> src/apis/amizone_service_api.rs:174:64
    |
174 | ..."{}/api/v1/class_schedule/{date.year}/{date.month}/{date.day}", local_var_configuration.base_p...
    |                               ^^^^^^^^^ not supported in format string

openapi-generator version

6.5.0

OpenAPI declaration file content or url

https://github.com/ditsuke/go-amizone/blob/1f6779de9d19268d069255a5465187ac429db839/server/proto/v1/amizone.proto

Generation Details

openapi-generator-cli generate -g rust -o rust -i amizone.proto

Steps to reproduce

1. Generate client code.
2. cargo build to try building it.

Related issues/PRs

Suggest a fix

As suggested by the rust compiler, use positional arguments instead of interpolation for fields. Less readable but works just as well for field access.

/cc @farcaller

Start with the Rust generator entry point used by openapi-generator-cli generate -g rust, using the linked amizone.proto as the reproduction input. Generate the client and run cargo build to inspect the field-access format string failure. Done means the generated Rust client compiles successfully for this input.