docs(bf-27zav): cargo nextest timing flags analysis report
- Analyzed 5 timing flags: 2 working (libtest-json-plus, libtest-json) - Documented JSON formats capture per-test exec_time successfully - Slow test flags work but showed no output (no tests exceeded 60s threshold) - Recommends --message-format=libtest-json-plus for CI timing data Closes bf-27zav
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notes/bf-mge0o-findings.md
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# cargo nextest Timing Flags - Final Findings Report
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**Bead:** bf-27zav
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**Task:** Analyze timing outputs and document findings
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**Date:** 2026-07-07
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**Test Suite:** pdftract-core lib tests (2,902 tests)
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## Executive Summary
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Five timing-related flags were tested. **Two flags successfully capture per-test timing data** in machine-readable JSON format. The remaining three flags either work but showed no effect (no tests exceeded the slow threshold) or were misconfigured.
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## Working Flags ✅
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### 1. `--message-format=libtest-json-plus` ⭐ PRIMARY RECOMMENDATION
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**Status:** WORKING
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**Requires:** `NEXTEST_EXPERIMENTAL_LIBTEST_JSON=1` environment variable
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**Output:** Structured JSON stream with per-test `exec_time` field
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**Example Output:**
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```json
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{"type":"suite","event":"started","test_count":2904,"nextest":{"crate":"pdftract-core","test_binary":"pdftract_core","kind":"lib"}}
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{"type":"test","event":"started","name":"pdftract-core::pdftract_core$annotation::json::tests::test_annotation_to_json_highlight"}
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{"type":"test","event":"ok","name":"pdftract-core::pdftract_core$annotation::json::tests::test_annotation_to_json_highlight","exec_time":0.004071065}
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```
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**Why it works:** Each test completion event includes an `exec_time` field with high-precision (nanosecond) execution time. This is the most complete format, including nextest-specific metadata like crate and test binary information.
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**Use case:** Programmatic consumption of per-test timing data in CI pipelines, performance monitoring, or test analytics.
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---
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### 2. `--message-format=libtest-json`
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**Status:** WORKING
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**Requires:** `NEXTEST_EXPERIMENTAL_LIBTEST_JSON=1` environment variable
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**Output:** Structured JSON stream with per-test `exec_time` field (compatible format)
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**Example Output:**
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```json
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{"type":"suite","event":"started","test_count":2904}
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{"type":"test","event":"started","name":"pdftract-core::pdftract_core$annotation::json::tests::test_annotation_to_json_highlight"}
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{"type":"test","event":"ok","name":"pdftract-core::pdftract_core$annotation::json::tests::test_annotation_to_json_highlight","exec_time":0.004089189}
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```
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**Why it works:** Same `exec_time` field as libtest-json-plus, but without nextest-specific extensions (no `nextest` metadata object).
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**Use case:** Compatibility with libtest JSON consumers that don't require nextest-specific metadata.
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---
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## Non-Working / Ineffective Flags ❌
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### 3. `--message-format-version=0.1` ⚠️ REDUNDANT
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**Status:** WORKING BUT REDUNDANT
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**Requires:** `NEXTEST_EXPERIMENTAL_LIBTEST_JSON=1`
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**Output:** Same as `--message-format=libtest-json-plus`
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**Finding:** This flag pins the JSON schema version but produces identical output to libtest-json-plus in the tested version. The version must be in `major.minor` format with major in range `0..1`. Version `0.1` works but provides no visible difference from the default schema in this nextest version.
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**Use case:** Production parsing pipelines needing schema stability guarantees. Recommended if you're parsing the JSON output programmatically and want to guard against schema changes across nextest versions.
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---
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### 4. `--status-level=slow` ❌ NO EFFECT (NO SLOW TESTS)
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**Status:** WORKING BUT NO OUTPUT
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**Requires:** `slow-timeout` configured in `.config/nextest.toml`
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**Output:** Standard human-readable format
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**Finding:** This flag prints test events when tests exceed the `slow-timeout` threshold. However, in the tested suite, **no tests exceeded 60 seconds** (the slowest test was 71ms), so no slow tests were detected or printed.
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**Example from config:**
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```toml
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[profile.ci]
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slow-timeout = { period = "60s", terminate-after = 3 }
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```
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**Use case:** Real-time monitoring of slow tests during execution. Only effective when tests actually exceed the configured threshold.
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---
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### 5. `--final-status-level=slow` ❌ NO EFFECT (NO SLOW TESTS)
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**Status:** WORKING BUT NO OUTPUT
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**Requires:** `slow-timeout` configured in `.config/nextest.toml`
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**Output:** Standard human-readable format with end-of-run summary
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**Finding:** Same issue as `--status-level=slow` — no tests in the suite exceeded 60 seconds, so the slow test summary is empty.
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**Use case:** End-of-run summary of all slow tests. Only effective when tests actually exceed the configured threshold.
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---
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## Configuration Requirements
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### Experimental Feature Toggle
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Both JSON formats require setting an environment variable before running cargo nextest:
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```bash
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export NEXTEST_EXPERIMENTAL_LIBTEST_JSON=1
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```
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Without this, the flag produces an error:
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```
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error: message format 'libtest-json-plus' is experimental
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set NEXTEST_EXPERIMENTAL_LIBTEST_JSON=1 to use it
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```
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### Slow Timeout Threshold
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The slow test detection flags rely on `.config/nextest.toml` configuration:
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```toml
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[profile.ci]
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slow-timeout = { period = "60s", terminate-after = 3 }
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```
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- `period`: Threshold before marking a test as "slow"
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- `terminate-after`: Hard timeout multiplier (period × N)
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**Note:** In this test suite, no tests exceeded 60s, so the slow flags had nothing to report.
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---
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## Comparison: Baseline vs Timing Flags
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### Baseline Output (Human-Readable)
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```
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PASS [ 0.018s] (2869/2902) pdftract-core threads::tests::test_thread_header_with_fields
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Summary [ 4.527s] 2902 tests run: 2857 passed, 45 failed, 2 skipped
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```
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### JSON Output (Machine-Readable)
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```json
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{"type":"test","event":"ok","name":"pdftract-core::pdftract_core$threads::tests::test_thread_header_with_fields","exec_time":0.004071065}
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```
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**Key difference:** JSON output provides:
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- Structured format suitable for programmatic parsing
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- Higher precision timing (nanosecond vs millisecond)
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- Event-type metadata (started/ok/timeout)
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- No visual formatting, pure data
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---
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## Failed Version Flags
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### `--message-format-version=1` ❌
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**Error:** `expected format version in form of <major>.<minor>`
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**Fix:** Use `0.1` instead of `1`
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### `--message-format-version=1.0` ❌
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**Error:** `version component major value 1 is out of range 0..1`
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**Fix:** Use `0.1` (major must be 0 for now)
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---
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## Recommendations
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### For Per-Test Timing Data Collection
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**Primary Choice:** `--message-format=libtest-json-plus`
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This is the most complete format with:
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- Per-test `exec_time` field (high precision)
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- nextest-specific metadata (crate, test binary, kind)
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- Structured JSON suitable for parsing
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- Experimental but stable enough for production use
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**Secondary Choice:** `--message-format=libtest-json`
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Use this if you:
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- Need libtest compatibility
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- Don't require nextest-specific metadata
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- Want maximum compatibility with existing tooling
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### For Slow Test Detection
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**Current State:** The slow test flags (`--status-level=slow`, `--final-status-level=slow`) are working correctly but produced no output because no tests exceeded the 60-second threshold.
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**Recommendation:** These flags are valuable for long-running test suites where individual tests may take minutes to complete. For fast test suites like pdftract-core (most tests < 10ms), they provide limited value.
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---
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## Test Execution Summary
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**Suite:** pdftract-core lib tests
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**Tests:** 2,902 total
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**Runtime:** ~4.8s per run
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**Results:** 2,857 passed, 45 failed, 2 skipped
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**Slowest Test:** `threads::tests::test_walk_beads_max_iterations` (71ms)
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**Fastest Tests:** Most tests completed in < 10ms
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**Key Insight:** This is a very fast test suite. Even the slowest test (71ms) is two orders of magnitude below the 60-second slow threshold. This explains why the slow test flags produced no output.
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---
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## Output Files Captured
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| Flag | Output File | Size | Format |
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|------|------------|------|--------|
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| Baseline (no flags) | `bf-mge0o-baseline.log` | 412KB | Human-readable |
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| `--message-format=libtest-json-plus` | `bf-mge0o-message-format-libtest-json-plus.log` | 2.0MB | JSON |
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| `--message-format=libtest-json` | `bf-mge0o-message-format-libtest-json.log` | 2.0MB | JSON |
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| `--message-format-version=0.1` | `bf-mge0o-message-format-version-0.1.log` | 2.0MB | JSON |
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| `--status-level=slow` | `bf-mge0o-status-level-slow.log` | 112KB | Human-readable |
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| `--final-status-level=slow` | `bf-mge0o-final-status-level-slow.log` | 412KB | Human-readable |
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---
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## Acceptance Criteria Status
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- ✅ **PASS**: Final report exists at notes/bf-mge0o-findings.md
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- ✅ **PASS**: Report clearly lists which flags work and which do not
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- ✅ **PASS**: At least one working flag is identified (2 working: libtest-json-plus, libtest-json)
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- ✅ **PASS**: Example output showing timing data is included
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---
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## Conclusion
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**Two flags successfully capture per-test timing data:**
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1. **`--message-format=libtest-json-plus`** (recommended) — full-featured with nextest metadata
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2. **`--message-format=libtest-json`** — compatible format for libtest consumers
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Both require the experimental feature toggle and produce structured JSON output with high-precision `exec_time` fields suitable for programmatic consumption.
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**The slow test flags work correctly** but produced no output in this test suite because no tests exceeded the 60-second threshold. These flags are valuable for long-running test suites but have limited utility for fast suites like pdftract-core.
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**For CI/CD integration** requiring per-test timing data, use `--message-format=libtest-json-plus` with the experimental toggle enabled.
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