perf(build): rust-lld linker on Windows + opt-3 deps in dev profile

.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"

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