From d9b732ad25f285b701c2d9097840403cddc48d0c Mon Sep 17 00:00:00 2001
From: zackees <z@zackees.com>
Date: Sun, 21 Jun 2026 22:58:56 -0700
Subject: [PATCH] perf(build): rust-lld linker on Windows + opt-3 deps in dev
 profile (#2 items 2+3)

Two cargo-config knobs ported from FastLED/fbuild#744 that together
collapse the local Rust-edit iteration loop without any release-build
regression:

1. `[profile.dev.package."*"] opt-level = 3` in Cargo.toml. Cargo
   compiles each upstream crate once and caches the opt-level-3
   artifact, so runtime hot paths (pyo3, serde, etc.) stay at
   release-grade perf while first-party crates compile unoptimized for
   fast iteration. This is what makes "default to dev for local
   iteration" safe for downstream consumers; without it, defaulting to
   dev silently regresses runtime perf.

2. `[target.{x86_64,aarch64}-pc-windows-msvc] linker = "rust-lld.exe"`
   in .cargo/config.toml. rust-lld ships with the Rust toolchain (no
   extra install) and links 2-5x faster than MSVC's link.exe on
   link-heavy rebuilds. Escape hatch: RUSTFLAGS="-C linker=link.exe"
   for the rare case rust-lld trips on a foreign object file. POSIX
   builds keep their platform default linker.

Verified `cargo build -p template-cli` succeeds with both changes in
place. fbuild measured ~5x faster Rust-edit rebuild from this pair of
changes on its workspace; will be smaller here because the template's
dep tree is tiny, but the proportional win scales with downstream
consumers as they grow.

Refs zackees/template-python-rust-cmd#2 (items 2 + 3).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
---
 .cargo/config.toml | 16 ++++++++++++++++
 Cargo.toml         | 11 +++++++++++
 2 files changed, 27 insertions(+)

diff --git a/.cargo/config.toml b/.cargo/config.toml
index fcb07c1..7210779 100644
--- a/.cargo/config.toml
+++ b/.cargo/config.toml
@@ -10,3 +10,19 @@ incremental = true
 [profile.test]
 debug = "line-tables-only"
 incremental = false
+
+# rust-lld ships with the Rust toolchain (no extra install) and links
+# 2-5x faster than MSVC's link.exe on link-heavy rebuilds. The dev
+# profile already lives in the hot iteration loop; cutting the link
+# step is the biggest single win there. Set per Windows target so
+# unix/macos builds keep their platform default linker.
+#
+# Escape hatch if rust-lld trips on a foreign object file:
+#   RUSTFLAGS="-C linker=link.exe" cargo build
+#
+# See fbuild#744 and zackees/template-python-rust-cmd#2.
+[target.x86_64-pc-windows-msvc]
+linker = "rust-lld.exe"
+
+[target.aarch64-pc-windows-msvc]
+linker = "rust-lld.exe"
diff --git a/Cargo.toml b/Cargo.toml
index 50543cb..f78d54b 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -17,3 +17,14 @@ homepage = "https://github.com/example/template-python-rust-cmd"
 [workspace.dependencies]
 anyhow = "1"
 pyo3 = { version = "0.23", features = ["extension-module"] }
+
+# Keep third-party crates release-grade even in the dev profile.
+# `cargo build` (no --release) compiles first-party crates unoptimized
+# for fast iteration; third-party deps (pyo3, serde, etc.) are compiled
+# once and cached at opt-level 3 so the runtime hot path stays as fast
+# as a release build. This is what makes "default to dev for local
+# iteration" safe — see fbuild#744 (~5x faster Rust-edit rebuild on
+# fbuild's workspace) and zackees/template-python-rust-cmd#2.
+[profile.dev.package."*"]
+opt-level = 3
+debug = false