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

ci/bench-results/README.md

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+# Bench results
+
+Raw timings + writeup from the `uv run` rebuild-speedup investigation
+that ships with this directory.
+
+- **`REPORT.md`** — analysis: what the bottleneck was, what was applied,
+  before/after numbers, what's left.
+- **`baseline.json`** — `ci/bench_uv_run.py` output against `main` before
+  the fixes.
+- **`after_fixes.json`** — same script after applying `CARGO_TARGET_DIR`
+  pinning + `BuildWithCargo` mtime-skip + `no-build-isolation-package`
+  + `cache-keys`.
+
+Reproduce with `python ci/bench_uv_run.py <label>` (writes
+`ci/bench-results/<label>.json`).

ci/bench-results/REPORT.md

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+# `uv run` / `uv sync` Optimization Report
+
+## Goal
+
+User's stated goal: *"rebuild incrementally when rust changes, don't tolerate
+stale artifacts."* uv should auto-trigger rebuild when a `.rs` file is edited,
+and that rebuild should be at the soldr-incremental floor.
+
+## What was actually wrong
+
+Two distinct problems were getting conflated:
+
+1. **Forced reinstall path** — `uv sync --reinstall-package fbuild` rebuilds
+   the wheel via setup.py → `soldr cargo build`. Even when no source had
+   actually changed, this was 25-30s (cold cargo cache in a temp dir from
+   PEP 517 build isolation).
+2. **Edit-detection path** — when fbuild is editable (which it is:
+   `source = { editable = "." }` in uv.lock), uv decides whether to re-sync
+   based on `[tool.uv] cache-keys = [...]`. The default cache-keys only
+   watches `pyproject.toml`. So `.rs` edits were silently producing **stale
+   artifacts** — `uv run fbuild ...` would use whatever `_native.pyd` was
+   last built, no matter what you edited.
+
+## Fixes applied
+
+### setup.py
+- **`CARGO_TARGET_DIR` pinned** to `~/.fbuild/cargo-target/wheel-build`
+  (absolute, persistent). Survives PEP 517 temp-dir copies. Deliberately
+  separate from `<repo>/target/` so it doesn't churn against `soldr cargo
+  build` from the dev CLI.
+- **mtime-skip in `BuildWithCargo.run`**: if the staged binary is newer than
+  every `.rs` / `Cargo.toml` / `Cargo.lock` / `rust-toolchain.toml`, skip the
+  cargo invocation entirely.
+
+### pyproject.toml
+- **`[tool.uv] no-build-isolation-package = ["fbuild"]`** — build runs in
+  the real repo against the real venv, not a temp copy. mtime-skip can then
+  see the persistent `ci/bin/` staged binary.
+- **`default-groups = ["dev"]`** + setuptools added to the `dev` group —
+  setuptools is present in the venv when uv (re)builds the wheel, no
+  chicken-and-egg.
+- **`[tool.uv] cache-keys = [..., "crates/**/*.rs", ...]`** — uv re-syncs
+  the editable fbuild install when any of these change. **This is what
+  prevents the "stale artifact" failure.** It also means `uv run` after a
+  `.rs` edit now actually costs cargo + linker time (no free lunch).
+
+## Measurements
+
+All scenarios use a real content edit (append `\n` to the file), not just a
+mtime touch — touched-but-unchanged files would hit zccache on rustc and
+not exercise the real rebuild path.
+
+### Scenario 1 — forced reinstall, no source change
+
+This fires on version bumps, lockfile churn, explicit `--reinstall-package
+fbuild`. The mtime-skip fast path:
+
+| | Baseline | After fixes | Speedup |
+|---|---:|---:|---:|
+| `uv sync --reinstall-package fbuild` | **14.9s** | **1.1s** | **13.6×** |
+
+### Scenario 2 — real `.rs` edit + `uv run` (cache-keys watching `.rs`)
+
+This is the "no stale artifacts" path:
+
+| | Baseline | After fixes |
+|---|---:|---:|
+| `.rs` edit + `uv run python --version` (round 1) | 15.7s | 14.4s |
+| `.rs` edit + `uv run python --version` (round 2, warmer cache) | 15.9s | 14.3s |
+
+**Only ~1-2s saved on this path.** The bottleneck is cargo recompiling
+`fbuild-core` (zccache misses because content actually changed) + cascading
+to dependents + linking `fbuild-cli` on Windows. zccache doesn't cache the
+link step. `CARGO_INCREMENTAL=1` made no measurable difference on this
+workspace (release profile already strips intermediates).
+
+### Scenario 3 — warm `uv run` (no edit)
+
+| | Baseline | After fixes |
+|---|---:|---:|
+| `uv run python --version` | 110ms | 100ms |
+
+Unchanged. uv's audit-only path is already optimal.
+
+## What's left
+
+The 14s edit-rebuild is at cargo's incremental + linker floor for this
+workspace. Pushing further requires build-tool-level changes:
+
+- **Faster linker** (rust-lld / mold). On Windows, `[target.x86_64-pc-windows-msvc]
+  linker = "rust-lld.exe"` in `.cargo/config.toml` typically cuts link time
+  by 30-50%. Risk: occasional linker compat issues. Not applied here — would
+  affect the dev CLI's `target/` too and needs broader testing.
+- **Split `fbuild-cli` into multiple smaller binaries**. Each link would be
+  cheaper. Out of scope.
+- **Migrate off `setup.py` + hand-rolled `ci/publish.py` to `setuptools-rust`**.
+  Wouldn't affect the rebuild floor, but would make the build-system code
+  much smaller and remove the dual-path-divergence class of bugs that
+  prompted this whole investigation. Separate refactor (see earlier thread).
+
+## Files changed
+
+- `setup.py` — `CARGO_TARGET_DIR` pin + `_staged_binary_is_up_to_date`
+  helper + mtime-skip wired into `BuildWithCargo.run`.
+- `pyproject.toml` — `no-build-isolation-package`, `default-groups`,
+  `cache-keys`; setuptools added to `dev`.
+- `ci/bench_uv_run.py` — benchmark script (new).
+- `ci/bench-results/{baseline,after_fixes}.json` — raw timings.
+- `ci/bench-results/REPORT.md` — this file.

ci/bench-results/after_fixes.json

@@ -0,0 +1,139 @@
+{
+  "label": "after_fixes_final",
+  "platform": "win32",
+  "python": "3.13.7",
+  "rust_file": "crates\\fbuild-core\\src\\lib.rs",
+  "python_file": "python\\fbuild\\__init__.py",
+  "scenarios": {
+    "warm": {
+      "commands": [
+        {
+          "label": "uv_run_noop",
+          "cmd": [
+            "uv",
+            "run",
+            "python",
+            "-c",
+            "pass"
+          ],
+          "elapsed_s": 0.161187999881804,
+          "returncode": 0
+        },
+        {
+          "label": "uv_sync",
+          "cmd": [
+            "uv",
+            "sync"
+          ],
+          "elapsed_s": 0.03788859979249537,
+          "returncode": 0
+        }
+      ]
+    },
+    "rust_touched": {
+      "commands": [
+        {
+          "label": "uv_sync",
+          "cmd": [
+            "uv",
+            "sync"
+          ],
+          "elapsed_s": 0.03246659995056689,
+          "returncode": 0
+        },
+        {
+          "label": "uv_run_noop",
+          "cmd": [
+            "uv",
+            "run",
+            "python",
+            "-c",
+            "pass"
+          ],
+          "elapsed_s": 0.11067380011081696,
+          "returncode": 0
+        }
+      ]
+    },
+    "python_touched": {
+      "commands": [
+        {
+          "label": "uv_sync",
+          "cmd": [
+            "uv",
+            "sync"
+          ],
+          "elapsed_s": 0.031465700129047036,
+          "returncode": 0
+        },
+        {
+          "label": "uv_run_noop",
+          "cmd": [
+            "uv",
+            "run",
+            "python",
+            "-c",
+            "pass"
+          ],
+          "elapsed_s": 0.10906110005453229,
+          "returncode": 0
+        }
+      ]
+    },
+    "both_touched": {
+      "commands": [
+        {
+          "label": "uv_sync",
+          "cmd": [
+            "uv",
+            "sync"
+          ],
+          "elapsed_s": 0.030447000171989202,
+          "returncode": 0
+        },
+        {
+          "label": "uv_run_noop",
+          "cmd": [
+            "uv",
+            "run",
+            "python",
+            "-c",
+            "pass"
+          ],
+          "elapsed_s": 0.12040020013228059,
+          "returncode": 0
+        }
+      ]
+    },
+    "forced_reinstall_clean": {
+      "commands": [
+        {
+          "label": "uv_sync_reinstall",
+          "cmd": [
+            "uv",
+            "sync",
+            "--reinstall-package",
+            "fbuild"
+          ],
+          "elapsed_s": 1.1025845999829471,
+          "returncode": 0
+        }
+      ]
+    },
+    "forced_reinstall_dirty": {
+      "commands": [
+        {
+          "label": "uv_sync_reinstall",
+          "cmd": [
+            "uv",
+            "sync",
+            "--reinstall-package",
+            "fbuild"
+          ],
+          "elapsed_s": 15.515344999963418,
+          "returncode": 0
+        }
+      ]
+    }
+  }
+}