[rust-server] Fix client serialization of enum parameters in form request bodies

[holdeg]

Fixes #24366.

Issue restatement

For endpoints with request bodies of type form (e.g. application/x-www-form-urlencoded) and enum properties, the generated Rust client invoked format!("{:?}", param_foo) where param_foo is the model generated in models.rs for the enum property. {:?} is the debug formatter, which uses the derived impl Debug on the enum model, which resolves to the name of the enum variant (as per the rust docs) - however, we should instead be serializing this with the impl Display on the enum model, which produces the defined allowable values from the API definition. The old incorrect serialization did not obey the OpenAPI spec and caused any server policing the value of the enum to reject requests created by clients created using this tool.

Fix

Check if a parameter is enum-like using getIsEnumOrRef() when deciding how to format it via x-format-string, and instead format enums with {} (which will invoke their impl Display).

This is

• safe because all models impl Display
• correct, because all enums have allowedValues in their impl Display, populated from the openapi definition, and this is what should be serialized.

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---

Summary by cubic

Fix Rust server generator to serialize enum values in application/x-www-form-urlencoded bodies using Display, including enums inside arrays (required and optional), so clients send allowed OpenAPI values. Also standardizes example server logging to use {:?} for all params and regenerates samples.

• Bug Fixes
  • Enum-like form params now use {} via getIsEnumOrRef() checks on both params and array items, not {:?}.
  • Added test verifying required and optional array-of-enum form params use Display (no Debug) serialization.

^{Written for commit 2136948. Summary will update on new commits.}

[holdeg]

Tagging rust-server technical committee for review: @frol @farcaller @richardwhiuk @paladinzh @jacob-pro @dsteeley

[cubic-dev-ai]

No issues found across 9 files

_{Re-trigger cubic}

[holdeg]

This is failing because petstore-with-fake-endpoints-models-for-testing generates some code using the format string, which does not work correctly for enum models wrapped in Options. I think that's improper so taking a look at how it should do that better.

[holdeg]

OK, I've modified the example-server-operation.mustache file to no longer use x-format-string - it was the only other user of this variable. I think this is proper (and doesn't represent a problem for servers).

[dsteeley]

OK, I've modified the example-server-operation.mustache file to no longer use x-format-string - it was the only other user of this variable. I think this is proper (and doesn't represent a problem for servers).

There were previously scenarios where example-server-operation.mustache using x-format-string would log out using {} rather than {:?}. This represents a change in behaviour here but given it's examples and logging I think that's acceptable.

[dsteeley]

Not expecting to see this change in the PR.

[holdeg]

I think this was because at the point I branched off master, we had dac3977 but not the followup 5d0a050 (which my commit was reimplementing). I've rebased so this should disappear.

[dsteeley]

Considering this case where we have an array of enums, which I think is valid.
In that scenario I think we would want all the items in the Array to be formatted with the Display impl as well?

Annoyingly, and apologies, I don't think there's an existing test for this case.

Example:

  statuses:
    type: array
    items:
      type: string
      enum: [available, pending, sold]

Results in

  if let Some(param_statuses) = param_statuses {
  // style=form,explode=true
  for param_statuses in param_statuses {
  params.push(("statuses",
      format!("{:?}", param_statuses)   <-- This is still bugged
  ));
  }
  }

[holdeg]

I agree - I think this is valid OpenAPI which is still being rendered incorrectly. I've amended by checking if array items are enum-like (and if so, changing their format strings).

[holdeg]

I've squashed all the commits into one.

[dsteeley]

Changes look good to me. Thanks for fixing this issue up.
Over to @wing328 for final pass and merge.