perf(uv): rebuild fbuild ~14x faster on no-source-change reinstall

[zackees]

Summary

• uv sync --reinstall-package fbuild (and version bumps / lockfile churn that trigger the same path): 14.9s → 1.1s (13.6×). PEP 517 build isolation used to copy the source tree to a temp dir and discard cargo's incremental cache; now CARGO_TARGET_DIR is pinned + a mtime-skip short-circuits cargo entirely when the staged binary is already current.
• Editable installs of fbuild no longer silently use a stale _native.pyd after .rs edits: cache-keys watches crates/**/*.rs, Cargo.{toml,lock}, pyproject.toml, so uv run triggers re-sync on Rust changes. The real-edit rebuild is at the cargo + linker floor (~14s on Windows); pushing further needs a linker swap, which is a separate change.
• No regression on the warm uv run path (~100ms either way).

Numbers

Scenario	Before	After
Forced reinstall, no source change	14.9s	1.1s
Real .rs edit + uv run python --version	15.7s	14.3s
Warm uv run python --version	110ms	100ms

Full methodology + raw ci/bench_uv_run.py JSON in ci/bench-results/REPORT.md.

Files

• setup.py — CARGO_TARGET_DIR pin (separate from <repo>/target/ so dev-CLI soldr cargo runs don't churn the same cache); _staged_binary_is_up_to_date mtime check in BuildWithCargo.run.
• pyproject.toml — [tool.uv] no-build-isolation-package = ["fbuild"], default-groups = ["dev"], setuptools added to dev group (no chicken-and-egg with no-build-isolation), cache-keys so editable installs catch Rust source changes.
• ci/bench_uv_run.py + ci/bench-results/ — benchmark script + raw timings + writeup.

Test plan

• uv sync --reinstall-package fbuild with no source change → measured 1.1s
• Append \n to crates/fbuild-core/src/lib.rs then uv run python --version → measured 14.3s, fresh binary
• Warm uv run python --version → ~100ms
• from fbuild.api import SerialMonitor succeeds against the rebuilt wheel
• Linux/macOS — not exercised locally; CI matrix will catch regressions

🤖 Generated with Claude Code

Summary by CodeRabbit

• Chores
  • Added benchmarking infrastructure and performance reports to measure and track development rebuild efficiency across various workflows.
  • Optimized the build system with persistent caching and incremental build detection to reduce development rebuild times.

[coderabbitai]

Caution

Review failed

The pull request is closed.

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: 8c5386ed-3358-48e7-99d4-9158db031834

📥 Commits

Reviewing files that changed from the base of the PR and between a5baf02 and f8b7160.

⛔ Files ignored due to path filters (1)

• uv.lock is excluded by !**/*.lock

📒 Files selected for processing (7)

• ci/bench-results/README.md
• ci/bench-results/REPORT.md
• ci/bench-results/after_fixes.json
• ci/bench-results/baseline.json
• ci/bench_uv_run.py
• pyproject.toml
• setup.py

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📝 Walkthrough

Walkthrough

Adds a persistent CARGO_TARGET_DIR and staged-binary freshness check to setup.py so BuildWithCargo.run() skips Cargo when the binary is already up-to-date. Updates pyproject.toml to add fbuild/fbuild.api package config, uv cache-keys tied to Rust sources, and a dev dependency group. Introduces ci/bench_uv_run.py to measure uv sync/uv run timing across several dev-loop scenarios, and commits baseline and after-fix JSON results plus a full optimization report.

Changes

uv run Rebuild Speedup

Layer / File(s)	Summary
Cargo fast-path and packaging config setup.py, pyproject.toml	setup.py sets a stable per-user CARGO_TARGET_DIR, adds helpers to enumerate Cargo inputs and check staged-binary mtime, and short-circuits BuildWithCargo.run() when the binary is fresh. pyproject.toml adds a dev dependency group with setuptools>=64, disables build isolation for fbuild, extends uv cache-keys to Rust sources and Cargo manifests, maps fbuild/fbuild.api packages to python/, and ships _native.pyd/_native.so as package data.
Benchmark script ci/bench_uv_run.py	Defines time_command, touch, _bench_env, warm_state, force_rebuild, and measure_scenario helpers. main() runs all scenario groups (warm, Rust/Python/both touched, forced reinstall clean/dirty), writes structured JSON to ci/bench-results/<label>.json, and returns process status.
Benchmark result artifacts and documentation ci/bench-results/README.md, ci/bench-results/REPORT.md, ci/bench-results/baseline.json, ci/bench-results/after_fixes.json	Commits measured timing data for all scenarios before and after the build fixes, a README describing the directory layout and reproduction command, and a REPORT documenting root causes, applied changes, measured results, and remaining optimization items.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• FastLED/fbuild#458: Also modifies setup.py's Rust-build and binary-staging flow, making it directly related to this PR's new _staged_binary_is_up_to_date short-circuit and CARGO_TARGET_DIR pinning.

Poem

🐇 A bunny once waited for Cargo to spin,
Now the binary's cached — no rebuild again!
We touched a .rs, peeked at the mtime,
"Already fresh!" said the script, working fine.
JSON results tell the story complete:
Warm runs go fast, and stale builds retreat. 🥕

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch perf/uv-rebuild-speedup

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