perf(rust): share intermediates via build-dir instead of moving target

target-dir has no path templating and cargo expands neither ~ nor $HOME,
so a committed value could only ever be relative to the checkout. That
limited sharing to sibling worktrees and, because it moved the final
artifacts too, required CARGO_TARGET_DIR pinned in two workflows to keep
the release steps able to locate a built binary.

build-dir does support templating. {cargo-cache-home} resolves to
CARGO_HOME, so one shared location covers every clone and worktree on a
machine rather than only siblings, and it stays portable with no absolute
path committed.

It also only moves intermediates. Final artifacts still land in
<checkout>/target, so the release steps keep working and both workflow
pins are gone: release-claw-server has none at all now, and test.yml
names the shared directory to rust-cache instead, which caches only
workspace target dirs plus the registry and git caches by default.

Measured across two checkouts of the same branch:

  cold build          38.35s   target 227M   shared 1.6G
  second checkout     16.14s   target 227M   shared 2.1G

and a release build against a warm shared directory still produces
target/release/browseros-claw-server-rs, which is the path the release
workflow reads.

The tradeoff against the previous form is ~227M left per checkout rather
than nothing, bought in exchange for machine-wide sharing and no
release-path coupling.

Author: DaniAkash

.cargo/config.toml

@@ -1,14 +1,11 @@
 [build]
-# Put compiler output beside the checkout rather than inside it, so sibling
-# git worktrees of this repo share one target directory instead of each
-# building a full private copy. Resolved relative to this file's parent, so a
-# checkout at <path>/BrowserOS builds into <path>/shared-target.
+# Keep intermediate artifacts outside the checkout so every clone and worktree
+# on a machine shares one copy instead of each rebuilding the dependency graph.
+# {cargo-cache-home} expands to CARGO_HOME, so this is portable: no absolute
+# path, no assumption about where the repo lives. Cargo does not expand ~ or
+# $HOME, and target-dir does not support templating, so build-dir is the only
+# form that gets this.
 #
-# Single-checkout clones are unaffected in practice: they simply build one
-# directory higher than before.
-#
-# CI pins CARGO_TARGET_DIR back to the in-repo path, because a workflow run has
-# exactly one checkout and gains nothing from sharing, while several release
-# steps locate the compiled binary at packages/browseros-agent/target. An env
-# var overrides this file, so anything running cargo in CI must set it.
-target-dir = "../shared-target"
+# Final artifacts still land in <checkout>/target, so anything locating a built
+# binary by path keeps working.
+build-dir = "{cargo-cache-home}/build/browseros"

.github/workflows/release-claw-server.yml

@@ -75,12 +75,6 @@ on:
 permissions:
   contents: write
 
-env:
-  # A committed .cargo/config.toml moves compiler output beside the checkout so
-  # sibling worktrees can share it. A workflow run has a single checkout and gains
-  # nothing from that, and several steps locate build output at the in-repo path,
-  # so pin it back here. An env var takes precedence over the config file.
-  CARGO_TARGET_DIR: ${{ github.workspace }}/packages/browseros-agent/target
 
 concurrency:
   # The legacy key keeps in-flight revisions of the renamed workflow serialized.

.github/workflows/test.yml

@@ -25,11 +25,6 @@ permissions:
   contents: read
 
 env:
-  # A committed .cargo/config.toml moves compiler output beside the checkout so
-  # sibling worktrees can share it. A workflow run has a single checkout and gains
-  # nothing from that, and several steps locate build output at the in-repo path,
-  # so pin it back here. An env var takes precedence over the config file.
-  CARGO_TARGET_DIR: ${{ github.workspace }}/packages/browseros-agent/target
   BROWSEROS_APPIMAGE_URL: https://files.browseros.com/download/BrowserOS.AppImage
 
 concurrency:
@@ -105,6 +100,11 @@ jobs:
         uses: Swatinem/rust-cache@v2
         with:
           workspaces: packages/browseros-agent
+          # Intermediate artifacts live outside the checkout so worktrees can
+          # share them, and rust-cache only saves workspace target dirs plus the
+          # registry and git caches by default. Name the build dir explicitly or
+          # every run would recompile the dependency graph from scratch.
+          cache-directories: ~/.cargo/build/browseros
 
       - name: Install dependencies
         run: bun ci