@@ -1,7 +1,7 @@
 name: Set up Go
 description: >-
-  Installs the Go version from go.mod and caches the module and build caches
-  under a key specific to the calling job.
+  Installs the Go version from go.mod and restores the module and build caches
+  under a key specific to the calling job. Only trunk saves the caches.
 
 inputs:
   cache-key-prefix:
@@ -23,19 +23,39 @@ runs:
         go-version-file: go.mod
         cache: false
 
-    - name: Determine Go cache paths
+    - name: Determine Go cache paths and key
       id: go-cache
       shell: bash
+      env:
+        PREFIX: ${{ inputs.cache-key-prefix }}-${{ runner.os }}-${{ runner.arch }}
+        GO_MOD_HASH: ${{ hashFiles('go.mod') }}
       run: |
+        restore_prefix="${PREFIX}-${ImageOS}-$(go env GOVERSION)-"
         echo "modcache=$(go env GOMODCACHE)" >> "$GITHUB_OUTPUT"
         echo "buildcache=$(go env GOCACHE)" >> "$GITHUB_OUTPUT"
-        echo "version=$(go env GOVERSION)" >> "$GITHUB_OUTPUT"
-        echo "image=${ImageOS}" >> "$GITHUB_OUTPUT"
+        echo "restore-prefix=${restore_prefix}" >> "$GITHUB_OUTPUT"
+        echo "key=${restore_prefix}${GO_MOD_HASH}" >> "$GITHUB_OUTPUT"
 
+    # Only trunk saves, so pull requests don't fill the repository's cache
+    # storage with entries no other branch can restore. Each cache is roughly
+    # 300 MB, or 520 MB for -race builds.
     - name: Cache Go modules and build artifacts
+      if: github.ref == 'refs/heads/trunk'
       uses: actions/cache@caa296126883cff596d87d8935842f9db880ef25 # v5.1.0
       with:
         path: |
           ${{ steps.go-cache.outputs.modcache }}
           ${{ steps.go-cache.outputs.buildcache }}
-        key: ${{ inputs.cache-key-prefix }}-${{ runner.os }}-${{ runner.arch }}-${{ steps.go-cache.outputs.image }}-${{ steps.go-cache.outputs.version }}-${{ hashFiles('go.mod') }}
+        key: ${{ steps.go-cache.outputs.key }}
+
+    # Pull requests that change go.mod fall back to the most recent trunk
+    # cache, which still holds most of the compiled packages they need.
+    - name: Restore Go modules and build artifacts
+      if: github.ref != 'refs/heads/trunk'
+      uses: actions/cache/restore@caa296126883cff596d87d8935842f9db880ef25 # v5.1.0
+      with:
+        path: |
+          ${{ steps.go-cache.outputs.modcache }}
+          ${{ steps.go-cache.outputs.buildcache }}
+        key: ${{ steps.go-cache.outputs.key }}
+        restore-keys: ${{ steps.go-cache.outputs.restore-prefix }}