Coverage Workflow

The CI enforces zero missed lines and zero missed regions. Use this workflow instead of hunting through scattered tips:

1. First run prek run --all-files and rerun it until all automatic fixes have stabilised and a full pass succeeds without modifying files.
2. Quick status before pushing: run uv run .agents/skills/coverage/coverage-missing.py (cross-platform; needs uv + a Rust toolchain with cargo-llvm-cov). It reruns the coverage suite and prints any uncovered ranges, or explicitly confirms when coverage is complete.
3. If the coverage script itself fails, it prints the failing nextest output (the FAIL lines + panic tails); fix those tests, then rerun. Fall back to a manual cargo nextest run only when you need the full log.
4. If the script reports files, extend CLI/system tests targeting those ranges until the script produces no output.
5. For richer artifacts (HTML/LCOV), see the cargo-llvm-cov docs (HTML isn't easily machine-readable).
6. When coverage points to tricky regions, prefer CLI/system tests in tests/ that drive env!("CARGO_BIN_EXE_ryl") so you exercise the same paths as users.
7. When you need to observe the exact flow through an uncovered branch, run the failing test under rust-lldb (ships with the toolchain). Start with cargo test --no-run and then rust-lldb target/debug/deps/<test-binary> -- <filter args> to set breakpoints on the problematic lines.
8. If cached coverage lingers, clear target/llvm-cov-target and rerun. But a single stubborn uncovered region is usually an unreachable branch, not stale cache — before clearing, confirm the missed line is reachable by some input. An unhit if let else / error arm looks identical to a cache miss but is closed by a test (or an expect), not a rebuild; chasing the cache first has burned multiple rebuilds.

Coverage-Friendly Rust Idioms

• Guard invariants with expect (or an early return Err(...)) when the “else” branch is truly unreachable. Leaving a return in the unreachable path often shows up as a permanent uncovered region even though the condition is ruled out. Reserve assert! for test-only code or cases where a runtime panic is acceptable.
• When walking indices backwards, call checked_sub(1).expect("…") instead of matching on checked_sub; the expect documents the invariant and removes the uncovered None branch that instrumentation reports.
• When collecting spans, store the raw (start, end) and filter once at the end rather than pushing Range conditionally, so LLVM records the conversion branch once.
• Normalize prefix checks with strip_prefix(...).expect(...) when the prefix is already guaranteed; this removes the otherwise-uncovered return path.

Windows/MSVC: ensure the llvm-tools-preview component is installed (already listed in rust-toolchain.toml). Run from a Developer Command Prompt if linker tools go missing.

Common hotspots

• A CLI/system test that sets a subprocess working dir via Command::current_dir can stop the instrumented child writing its .profraw (so its lines read as uncovered). Drive coverage-sensitive dedup/path tests with absolute paths instead of changing cwd.
• Configuration discovery: use the Env abstraction (discover_config_with) and fake envs to hit inline data, explicit files (success and YAML failure), and env-var paths.
• Project configuration search: cover empty inputs, single files without parents, and multiple files in the same directory to trigger dedup logic.
• YAML parsing: drive from_yaml_str through string vs sequence options and ensure rule merges hit both update and insert branches.
• CLI context resolution: pass an empty PathBuf into resolve_ctx to trigger the fallback to ..
• Flow scanners in rules: always reconcile parser byte spans with char_indices() via crate::rules::span_utils to avoid off-by-byte bugs when UTF-8 characters appear.
• Rules using the shared crate::rules::support::mapping_key_walker::Walker to track key/value position must advance it for every node-producing event, including Event::Alias (call Walker::skip_node). An alias in value position (k: *a, or a <<: *base merge) that does not advance the walker desyncs the key/value alternation, so the following key is read as a value and vice-versa. Exercise rules with aliases in both key and value position.
• Resolving a scalar to its typed value (int/bool/null/float/string) is centralised in crate::yaml_dom::scalar (resolve_scalar / resolve_plain_scalar); reuse it rather than reinventing parsing. ryl resolves scalars per the YAML 1.2 core schema everywhere (leading-zero decimal is an int, an empty plain scalar is null, 0x/0o radixes, full bool/null spelling sets); keep that schema choice consistent across rules instead of switching to JSON/1.1 semantics in any single rule.
• Matching a core-schema tag (!!int, !!str, …): use crate::yaml_dom::core_schema_suffix(tag) / is_core_schema(tag), never granit's Tag::is_yaml_core_schema (it inspects only the handle, so a verbatim core tag !<tag:yaml.org,2002:int> slips past it). The shared helpers handle the canonical handle (incl. a resolving %TAG) and the verbatim spelling, but not a %TAG that splits the URI mid-token; to match one type regardless of split point, compare the full resolved URI (handle ++ suffix), as rules::support::merge_key does.