The serde_yaml behavioural contract

“Drop-in replacement” is a behavioural claim, so it is held by a test suite, not a README sentence.

Provenance

The corpus began life as an evaluation harness built by a prospective adopter to decide whether noyalib could replace serde_yaml in their build (zudo-front-builder#2787). noyalib 0.0.28 diverged on 11 of its 18 cases and was rejected — the most rigorous public specification of what serde_yaml compatibility actually means. noyalib v0.0.29 vendors that corpus as its own contract suite (crates/noyalib/tests/serde_yaml_contract.rs), with every expectation captured live from serde_yaml 0.9.34+deprecated: the JSON value produced, the error Display string, and the location() line/column/index. All 18 pass.

Enforcement

• The suite runs in the core’s cargo test --all-features — every PR, every platform in the test matrix.
• A differential fuzzer (fuzz_serde_yaml_compat) runs the shim against the real archived serde_yaml on arbitrary inputs, hunting for divergences nobody has written down yet; confirmed legitimate differences graduate into its documented allowlist, everything else is a bug.
• no_std compatibility of the shim is CI-enforced on wasm32 and three bare-metal targets.