Assisted-by: Claude Opus 4.6 via pi

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----
-name: rust-coverage
-description: Use when measuring code coverage in Rust projects, debugging uncovered lines, generating coverage reports, or setting up LLVM source-based coverage with cargo-llvm-cov or grcov
----
-
-# Rust Source-Based Code Coverage
-
-LLVM source-based coverage via `cargo-llvm-cov`, which wraps rustc's `-C instrument-coverage` flag.
-
-## How It Works
-
-1. **Compile** with `-C instrument-coverage` — rustc inserts counters at coverage-relevant spans.
-2. **Run** the instrumented binary — writes `.profraw` files.
-3. **Merge** raw profiles into `.profdata` with `llvm-profdata merge`.
-4. **Report** with `llvm-cov show` / `llvm-cov export`.
-
-`cargo-llvm-cov` handles all four steps automatically.
-
-## Prerequisites
-
-```bash
-rustup component add llvm-tools       # ships llvm-profdata + llvm-cov
-cargo install cargo-llvm-cov          # convenience wrapper
-```
-
-## Quick Reference
-
-| Goal | Command |
-|------|---------|
-| Run tests + summary | `cargo llvm-cov` |
-| Workspace coverage | `cargo llvm-cov --workspace` |
-| Fail if below threshold | `cargo llvm-cov --fail-under-lines 100` |
-| Text report (per-line) | `cargo llvm-cov --text` |
-| HTML report | `cargo llvm-cov --html --open` |
-| JSON export | `cargo llvm-cov --json` |
-| LCOV export | `cargo llvm-cov --lcov --output-path lcov.info` |
-| Include only specific test | `cargo llvm-cov --test cli` |
-| Branch coverage (unstable) | `cargo llvm-cov --branch` |
-| MC/DC coverage (unstable) | `cargo llvm-cov --mcdc` |
-| Skip report, just instrument | `cargo llvm-cov --no-report` |
-| Report from prior run | `cargo llvm-cov report --html` |
-| Clean coverage artifacts | `cargo llvm-cov clean` |
-| Show env vars it sets | `cargo llvm-cov show-env` |
-
-## Filtering
-
-```bash
-# Only library tests
-cargo llvm-cov --lib
-
-# Only integration tests
-cargo llvm-cov --test cli
-
-# Only specific binary
-cargo llvm-cov --bin ranger
-
-# Exclude files from report
-cargo llvm-cov --ignore-filename-regex 'bin/.*'
-
-# Exclude package from test AND report
-cargo llvm-cov --workspace --exclude some-crate
-
-# Exclude from report but still test
-cargo llvm-cov --workspace --exclude-from-report some-crate
-```
-
-## Reading the Text Report
-
-```bash
-cargo llvm-cov --text
-```
-
-Output shows each source line with execution count:
-
-```
-   46|      1|    fn display_formats_as_iso8601() {    # executed once
-   32|      0|    fn encode_by_ref(                    # never executed
-   10|       |use std::fmt;                            # non-executable (no counter)
-```
-
-Lines with `0` count are uncovered. Lines with no count are non-executable (declarations, imports, comments).
-
-## Show Missing Lines
-
-```bash
-cargo llvm-cov --show-missing-lines
-```
-
-Appends a `Missing Lines` column to the summary table showing exact uncovered line ranges — useful for quickly spotting gaps without reading the full text report.
-
-## cfg(coverage)
-
-When built with `cargo-llvm-cov`, the cfg flags `coverage` and `coverage_nightly` are set. Use them to skip code that shouldn't run under coverage:
-
-```rust
-#[cfg(not(coverage))]
-fn debug_only_thing() { /* ... */ }
-```
-
-Opt out with `--no-cfg-coverage` if these flags interfere.
-
-## grcov (Mozilla)
-
-[grcov](https://github.com/mozilla/grcov) is Mozilla's coverage tool. It processes `.profraw` files (same as `cargo-llvm-cov`) but also handles `.gcda` (GCC), lcov, JaCoCo, and Go coverage. Useful when you need output formats `cargo-llvm-cov` doesn't support (Cobertura, Coveralls, covdir) or when integrating with CI systems like GitLab that expect Cobertura XML.
-
-### Prerequisites
-
-```bash
-rustup component add llvm-tools       # ships llvm-profdata + llvm-cov
-cargo install grcov                   # or download from GitHub releases
-```
-
-### Workflow
-
-```bash
-# 1. Build and test with instrumentation
-export RUSTFLAGS="-Cinstrument-coverage"
-export LLVM_PROFILE_FILE="target/coverage/%p-%m.profraw"
-cargo build
-cargo test
-
-# 2. Generate report with grcov
-grcov target/coverage \
-  --binary-path ./target/debug/ \
-  -s . \
-  -t html \
-  --branch \
-  --ignore-not-existing \
-  --keep-only 'src/*' \
-  -o ./target/debug/coverage/
-```
-
-The report lands in `target/debug/coverage/index.html`.
-
-### Output Types
-
-| `-t` value | Description |
-|------------|-------------|
-| `lcov` (default) | lcov INFO format — upload to Codecov, feed to `genhtml` |
-| `html` | Self-contained HTML report with coverage badges |
-| `cobertura` | Cobertura XML — GitLab CI integration, IDE support |
-| `cobertura-pretty` | Pretty-printed Cobertura XML |
-| `coveralls` | Coveralls JSON format |
-| `coveralls+` | Coveralls with function-level information |
-| `covdir` | Recursive JSON format |
-| `files` | List of covered/uncovered source files |
-| `markdown` | Human-readable markdown |
-
-Multiple output types at once:
-
-```bash
-grcov target/coverage \
-  --binary-path ./target/debug/ \
-  -s . \
-  --output-types html,cobertura \
-  -o ./target/coverage/
-```
-
-### Common Options
-
-```bash
-# LCOV for Codecov upload
-grcov . --binary-path ./target/debug/ -s . -t lcov --branch \
-  --ignore-not-existing -o lcov.info
-
-# Filter to only project source
-grcov . --binary-path ./target/debug/ -s . -t html \
-  --keep-only 'src/*' \
-  --ignore 'src/bin/*' \
-  -o coverage/
-
-# Coveralls upload
-grcov . --binary-path ./target/debug/ -s . -t coveralls \
-  --token "$COVERALLS_TOKEN" > coveralls.json
-
-# Exclude lines with marker comments
-grcov . --binary-path ./target/debug/ -s . -t lcov \
-  --excl-line 'LCOV_EXCL_LINE' \
-  --excl-start 'LCOV_EXCL_START' \
-  --excl-stop 'LCOV_EXCL_STOP'
-```
-
-### When to Use grcov vs cargo-llvm-cov
-
-| Scenario | Prefer |
-|----------|--------|
-| Quick local coverage check | `cargo-llvm-cov` — single command, no env vars |
-| Fail CI on threshold | `cargo-llvm-cov` — `--fail-under-lines` built in |
-| GitLab CI with Cobertura | `grcov` — native Cobertura output |
-| Coveralls integration | `grcov` — native Coveralls format |
-| Multiple output formats at once | `grcov` — `--output-types html,cobertura` |
-| Coverage badges for GitHub Pages | `grcov` — generates badges + `coverage.json` |
-| Mixed language project (Rust + C) | `grcov` — handles `.gcda` + `.profraw` together |
-| Doctests coverage | `cargo-llvm-cov` — `--doctests` flag (nightly) |
-
-## Manual Workflow (Without cargo-llvm-cov or grcov)
-
-For environments where neither wrapper is available:
-
-```bash
-# 1. Build with coverage
-RUSTFLAGS="-C instrument-coverage" cargo test --no-run
-
-# 2. Run tests (produces .profraw files)
-LLVM_PROFILE_FILE="ranger-%p-%m.profraw" cargo test
-
-# 3. Merge profiles
-llvm-profdata merge -sparse *.profraw -o ranger.profdata
-
-# 4. Find the test binary
-BINARY=$(cargo test --no-run --message-format=json 2>/dev/null \
-  | jq -r 'select(.executable) | .executable')
-
-# 5. Generate report
-llvm-cov show "$BINARY" \
-  --instr-profile=ranger.profdata \
-  --format=html \
-  --output-dir=coverage/ \
-  --Xdemangler=rustfilt
-
-# Or export as LCOV
-llvm-cov export "$BINARY" \
-  --instr-profile=ranger.profdata \
-  --format=lcov > lcov.info
-```
-
-Use `rustfilt` as demangler for readable Rust symbol names (`cargo install rustfilt`).
-
-## Common Pitfalls
-
-**Multiple binaries**: `llvm-cov` needs all instrumented binaries passed via `-object`. `cargo-llvm-cov` handles this automatically. Manually, you must pass each binary:
-
-```bash
-llvm-cov show -object bin1 -object bin2 --instr-profile=merged.profdata
-```
-
-**Dead code appears uncovered**: Unreachable code still gets instrumented. If coverage matters, delete dead code rather than excluding it.
-
-**Stale incremental artifacts with grcov**: Incremental compilation reuses old object files with outdated debug info, causing grcov to report phantom uncovered lines. Use a separate `CARGO_TARGET_DIR` for coverage builds so instrumented and non-instrumented artifacts never mix.
-
-**Proc macros and build scripts**: Not instrumented by default. Use `--include-build-script` if needed. When using `--no-rustc-wrapper` with `--target`, proc macros won't show coverage.
-
-**Doctests**: Experimental. Enable with `--doctests` (nightly only).
-
-**Branch coverage**: Requires nightly. `--branch` enables it but the feature is unstable — expect rough edges with match arms and closures.
-
-## This Project
-
-This project uses `just coverage` which builds into an isolated `target/coverage/` directory and runs grcov with covdir output:
-
-```bash
-RUSTFLAGS="-Cinstrument-coverage" CARGO_TARGET_DIR=target/coverage \
-    cargo test --workspace
-grcov target/coverage/profraw \
-    --binary-path ./target/coverage/debug/ -s . -t covdir \
-    --keep-only 'src/**' --ignore 'src/bin/**' \
-    --excl-line 'cov-excl-line' --excl-start 'cov-excl-start' --excl-stop 'cov-excl-stop'
-```
-
-100% line coverage is enforced for library code (`src/`, excluding `src/bin/`). When adding code, make sure every line is exercised by tests.
-
-To find uncovered lines, use the markdown output:
-
-```bash
-grcov target/coverage/profraw \
-    --binary-path ./target/coverage/debug/ -s . -t markdown \
-    --keep-only 'src/**' --ignore 'src/bin/**' \
-    --excl-line 'cov-excl-line' --excl-start 'cov-excl-start' --excl-stop 'cov-excl-stop'
-```
-
-Use `// cov-excl-line` for structurally unreachable code (e.g., `unreachable!()` after exhaustive loops). Use `// cov-excl-start` / `// cov-excl-stop` for blocks. Avoid custom format strings in test `assert!` macros — they create phantom uncovered lines.