fix(rust): escape Rust keywords in union constructor names and deconflict unknown

[devin-ai-integration]

Description

Linear ticket: Closes FER-11400

Fixes two compilation errors in generated Rust code for discriminated unions in UnionGenerator.ts:

1. Bug A — Constructor names that are Rust keywords (static, override, etc.) now emit r# raw identifier prefix via escapeRustKeyword.
2. Bug B — The forward-compatible catch-all constructor deconflicts to unknown_value when a real variant already uses the unknown name.

Changes Made

• Route constructor name through this.context.escapeRustKeyword() in generateVariantConstructor
• Add getUnknownConstructorName() helper that returns "unknown_value" when a real variant collides
• Add KeywordUnion regression test (discriminants: static, override, unknown) to the snapshot harness
• Add changelog entry

Generated output for the regression test:

impl KeywordUnion {
    pub fn r#static(value: String) -> Self { ... }
    pub fn r#override(priority: i64) -> Self { ... }
    pub fn unknown(payload: String) -> Self { ... }      // real variant
    pub fn unknown_value(value: serde_json::Value) -> Self { ... } // catch-all
}

• Updated README.md generator (if applicable) — N/A

Testing

• Unit tests added/updated — snapshot test with KeywordUnion type
• Manual testing completed — pnpm --filter @fern-api/rust-model test passes (16/16)

Link to Devin session: https://app.devin.ai/sessions/797a62f254f3410cbe25eab33737157d
Requested by: @jsklan

[devin-ai-integration]

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SDK Generation Benchmark Results

Comparing PR branch against median of 5 nightly run(s) on main (latest: 2026-06-19T05:42:07Z).

Full benchmark table (click to expand)

Generator	Spec	main (generator)	main (E2E)	PR (generator)	Delta
rust-sdk	square	166s (n=5)	177s (n=5)	170s	+4s (+2.4%)

main (generator): generator-only time via --skip-scripts (includes Docker image build, container startup, IR parsing, and code generation — this is the same Docker-based flow customers use via fern generate). main (E2E): full customer-observable time including build/test scripts (nightly baseline, informational). Delta is computed against generator-only baseline.
⚠️ = generation exited with a non-zero exit code (timing may not reflect a successful run).
Baseline from nightly runs on main (latest: 2026-06-19T05:42:07Z). Trigger benchmark-baseline to refresh.
Last updated: 2026-06-20 00:32 UTC

[devin-ai-integration]

Devin Review found 2 potential issues.

[devin-ai-integration]

🚩 Missing model generator changelog entry

The code change is in generators/rust/model/src/union/UnionGenerator.ts, which belongs to the model generator (independently versioned at generators/rust/model/versions.yml). The PR only adds a changelog entry for the SDK generator (generators/rust/sdk/changes/unreleased/fix-union-keyword-escaping.yml). The model generator doesn't use a changes/unreleased/ workflow — its versions.yml is edited directly in PRs. Since the model generator is released independently (as shown by its own version history), this fix would not be reflected in model generator release notes. The SDK changelog covers the SDK release, but users of the standalone model generator would miss this fix in their changelogs.

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[devin-ai-integration]

Intentional — rust-model doesn't have its own changelog/release entry. The SDK depends on the model package ("@fern-api/rust-model": "workspace:*") and publishes them together, so the SDK changelog is the correct place.

[devin-ai-integration]

Devin Review found 0 new potential issues.