fix(rust): interpolate unknown path parameters without their JSON quotes

[wiebren]

Description

Linear ticket: n/a — found while running one OpenAPI document through fern's SDK generators
and comparing what each one produces.

An unknown-typed path parameter is generated as &serde_json::Value and interpolated into
the path with Display — which prints JSON. A string value keeps its quotes, so the quotes
travel into the URL and every call to the endpoint fails:

pub async fn get(
    &self,
    domain_name: &serde_json::Value,
    ...
) -> Result<Domain, ApiError> {
    self.http_client.execute_request(
        Method::GET,
        &format!("domains/{}", domain_name),   // -> domains/"example.com"

GET /domains/%22example.com%22   (expected: GET /domains/example.com)

OpenAPI documents commonly hit this: a path parameter whose schema carries constraints but no
type —

in: path
name: domainName
required: true
schema:
  maxLength: 255
  minLength: 3
  pattern: "^..."

— imports as unknown. Other generators shrug this off because their unknown is the
language's plain value (unknown in typescript, Any in python) and interpolating the
runtime string produces the string; rust's serde_json::Value is the one representation
whose Display re-encodes to JSON.

Root cause

getPathParameterExpression in generators/rust/sdk/src/generators/SubClientGenerator.ts
returns the parameter name unchanged for the unknown type reference, so format!("{}", value)
uses serde_json::Value's JSON Display.

Changes Made

• For an unknown path parameter, interpolate
  value.as_str().map(ToString::to_string).unwrap_or_else(|| value.to_string()): the string
  content when the value is a string, the JSON encoding otherwise (numbers and booleans print
  bare, which is what a path expects).
• Add the rust-unknown-path-parameter test definition (language-prefixed, so only the rust
  generators run it): an endpoint with an unknown path parameter.

Testing

• Verified against a live socket with the generated fixture crate, calling
  client.get(&serde_json::json!("abc"), None) and capturing the request line:

	request line
before	GET /resources/%22abc%22 HTTP/1.1
after	GET /resources/abc HTTP/1.1

• cargo build + cargo test pass on the new fixture (86 tests).
• pnpm seed test --generator rust-sdk on path-parameterized fixtures (path-parameters,
  imdb) — no output changes; no committed fixture has an unknown path parameter, so only
  the new fixture's output is added.
• pnpm turbo run test --filter @fern-api/rust-sdk — all unit tests pass.

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Generated with Claude Code

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[nitpickybot]

AI Review Summary

Single-line generator fix that unwraps serde_json::Value strings before interpolating them into paths, plus a new seed fixture. The approach is sound; the main gap is that path parameters are still not URL-encoded (a value containing / or ? still escapes the path segment), but that's pre-existing behavior beyond this diff. No blocking issues found.

• 🔵 1 suggestion(s)

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[nitpickybot]

🔵 suggestion

as_str().map(ToString::to_string) allocates twice for the string case (&str -> String). Minor, but map(str::to_owned) is the same length and one allocation. Also worth noting: like every other path param here, the result is still not percent-encoded, so a value containing / or ? will break out of the segment — pre-existing, but this change makes string values reachable where they previously produced quoted-and-encoded garbage.

[devin-ai-integration]

Devin Review found 1 potential issue.

1 flag not posted on this PR by your GitHub settings — view it in Devin Review. (Configure)

[devin-ai-integration]

🔴 Unknown aliases break generated SDK builds

When a path parameter aliases unknown, getPathParameterExpression bypasses the new conversion and passes the alias to format!. Generated aliases lack Display, so the entire SDK fails to compile.

Prompt for agents

Extend path-parameter serialization in generators/rust/sdk/src/generators/SubClientGenerator.ts so named aliases resolve recursively before choosing the expression. An alias whose eventual target is unknown must serialize its .0 serde_json::Value like a direct unknown value. Preserve primitive-alias handling and account for alias chains. Add a fixture endpoint using an alias of unknown as a path parameter and verify the generated Rust crate compiles and removes JSON quotes from string values.

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