perf(build): share sccache across worktrees (#2379)

* perf(build): share sccache across worktrees

Signed-off-by: Matthew Grossman <mgrossman@nvidia.com>

* fix(build): make sccache directory overridable

Signed-off-by: Matthew Grossman <mgrossman@nvidia.com>

* fix(build): support pinned mise version

Signed-off-by: Matthew Grossman <mgrossman@nvidia.com>

---------

Signed-off-by: Matthew Grossman <mgrossman@nvidia.com>

Author: matthewgrossman

CONTRIBUTING.md

@@ -195,6 +195,28 @@ openshell --help
 openshell sandbox create -- codex
 ```
 
+### Rust build cache
+
+Mise preserves an existing `SCCACHE_DIR` so each environment can choose where
+to store compiler cache entries. When `SCCACHE_DIR` is unset, OpenShell uses
+the worktree-local `.cache/sccache` directory. To make cache entries available
+to multiple worktrees on a workstation, set the variable to a user-level
+directory before activating mise. For example:
+
+```shell
+export SCCACHE_DIR="$HOME/.cache/openshell/sccache"
+```
+
+CI can select a different directory or configure a remote sccache backend
+without changing the workstation setting. Cargo output remains in each
+worktree's `target/` directory.
+
+OpenShell does not set `SCCACHE_BASEDIRS`. Sccache loads base directories when
+its machine-local daemon starts, but the correct workspace root differs for
+each worktree. Cache reuse therefore depends on the compiler inputs: outputs
+that embed absolute paths, including Rust dependencies in some builds, can
+still miss across worktrees.
+
 ## Main Tasks
 
 These are the primary `mise` tasks for day-to-day development:

mise.toml

@@ -53,11 +53,10 @@ _.file = [".env"]
 KUBECONFIG = "{{config_root}}/kubeconfig"
 UV_CACHE_DIR = "{{config_root}}/.cache/uv"
 
-# Enable sccache for faster Rust builds.
-# Local builds use a disk cache (SCCACHE_DIR); CI sets SCCACHE_MEMCACHED_ENDPOINT
-# via repository variables to use a shared memcached backend instead.
+# Enable sccache for faster Rust builds. Preserve an SCCACHE_DIR selected by the
+# caller so workstations and CI can configure their cache locations separately.
 RUSTC_WRAPPER = "sccache"
-SCCACHE_DIR = "{{config_root}}/.cache/sccache"
+SCCACHE_DIR = "{{ get_env(name='SCCACHE_DIR', default=config_root ~ '/.cache/sccache') }}"
 
 # Shared build constants (overridable via environment).
 # DOCKER_BUILDKIT enables BuildKit for Docker; ignored by podman.