Advanced Setup

sccache

sccache is a compiler cache that supports Rust and other languages, and its use helps to decrease the build times of GWR packages under many circumstances.

Installation

sccache can be installed using system package manages or via Cargo:

cargo binstall --disable-telemetry --locked sccache

Configuration

Cargo

Cargo must be configured to use sccache via the rustc-wrapper setting. This can be set via an environment variable or via a configuration file.

To config Cargo globally to use sccache add the following to $HOME/.cargo/config.toml:

[build]
rustc-wrapper = "/Users/<username>/.cargo/bin/sccache"

sccache

By default sccache requires absolute path matches to achieve a cache hit. This can be mitigated by setting SCCACHE_BASEDIRS appropriately.

For example, assume that the GWR repo will be checked out in the directory Volumes/projects/gwr and various worktrees will be created as subdirectories within the Volumes/projects/gwr-wt directory. To configure the basedirs such that multiple checkouts and worktrees of the GWR repo in these directories can all share a cache the following is required:

basedirs = ["/Volumes/projects/", "/Volumes/projects/gwr-wt/"]

On MacOS this config should be written to $HOME/Library/Application\ Support/Mozilla.sccache/config.

Status

To see the current cache status run:

sccache -s

Caveats

Users

Care must be taken when to avoid running Cargo commands as different users when sccache is configured. To avoid using sccache for certain Cargo commands set RUSTC_WRAPPER = "", for example, when using sudo:

sudo RUSTC_WRAPPER="" cargo ...

Agents

Tools such as Codex can run Cargo in environments where sccache is not avaliable resulting it unexpected failures for agents.

To avoid Codex attempting to run Cargo commands with sccache the following can be added to $HOME/.codex/config.toml:

[shell_environment_policy]
set = { RUSTC_WRAPPER = "" }