[Rust Reqwest] Fixes Enums in Query Parameters via Causing Compilation Failure

[winrid]

Fixes #22280
Fixes our sdk not compiling: https://github.com/fastcomments/fastcomments-rust

@dsteeley @jacob-pro

I had to add tests because existing tests did not cover this scenario.

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[winrid]

Hey folks @frol @farcaller @richardwhiuk @paladinzh anyone willing to review this?

[thesn10]

Unfortunately this PR introduces a pretty serious runtime bug that's probably breaking enum query parameters for everyone using the generated clients.

The problem is that serde_json::to_string() is designed for JSON serialization, so it wraps values in quotes. When you have something like:

pub enum SortOrder {
    #[serde(rename = "asc")]
    Ascending,
    #[serde(rename = "desc")]
    Descending,
}

You'd expect a URL like ?sortOrder=asc, but what you actually get is ?sortOrder=%22asc%22. Those %22 are URL-encoded quotes - the server literally receives "asc" as the value instead of just asc, which fails validation and breaks the API call.

This is because serde_json::to_string() produces JSON strings that must be quoted according to the spec. It's the right tool for JSON bodies, but completely wrong for query parameters which should be plain strings:

let sort = SortOrder::Ascending;
serde_json::to_string(&sort).unwrap()  // "\"asc\"" - wrong!
sort.to_string()                        // "asc" - correct

I get that the original compilation error needed fixing, but this change essentially trades a compile-time error for a runtime bug that silently breaks every API call using enum query parameters. Our production client is completely broken because of this.

This PR really needs to be reverted. The original type inference issue is real and needs a proper solution, but serde_json::to_string() is fundamentally incompatible with query parameter encoding.

Would it be possible to revert this and then we can figure out the right approach to fix the compilation issue without breaking query parameters?

@winrid @wing328

[winrid]

@thesn10 let me see if I can address this ASAP

[winrid]

I have a fix I am testing.

[winrid]

@thesn10 fix is in this PR: #22683

test to prevent this from regressing again: https://github.com/OpenAPITools/openapi-generator/pull/22683/files#diff-154b2d8c634a8e04936d2bb95522716bcf8502b6e226d035f48a87d88a4218c2R119

[thesn10]

@winrid wow, thanks for fixing it that fast. I really appreciate your effort!

[winrid]

@thesn10 well I was very embarrassed! :)