From 771d10ac6bd9fb66654838665e9bfd6bae5f9bf1 Mon Sep 17 00:00:00 2001
From: Stefan Ceriu <stefan.ceriu@gmail.com>
Date: Tue, 18 Aug 2026 12:50:32 +0300
Subject: [PATCH] Cache CI test run compilations

---
 .github/workflows/unit-tests-enterprise.yml | 15 +++++++++++++++
 .github/workflows/unit-tests.yml            | 15 +++++++++++++++
 Tools/Sources/Commands/CI/RunTests.swift    |  6 ++++++
 3 files changed, 36 insertions(+)

diff --git a/.github/workflows/unit-tests-enterprise.yml b/.github/workflows/unit-tests-enterprise.yml
index d0ad10e7d..27117fb7a 100644
--- a/.github/workflows/unit-tests-enterprise.yml
+++ b/.github/workflows/unit-tests-enterprise.yml
@@ -34,12 +34,27 @@ jobs:
       - name: Setup environment
         run: source ci_scripts/ci_common.sh && setup_github_actions_environment
 
+      - name: Restore compilation cache
+        uses: actions/cache/restore@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ~/Library/Developer/Xcode/CompilationCache.noindex
+          key: compilation-cache-unit-tests-enterprise-${{ github.sha }}
+          restore-keys: compilation-cache-unit-tests-enterprise-
+
       - name: Configure Enterprise
         run: swift run -q pipeline configure-element-pro
 
       - name: Run tests
         run: swift run -q tools ci unit-tests --skip-previews
 
+      - name: Save compilation cache
+        # Only develop populates the cache, all of the other runs simply read from it.
+        if: ${{ !cancelled() && github.event_name == 'push' && github.ref == 'refs/heads/develop' }}
+        uses: actions/cache/save@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ~/Library/Developer/Xcode/CompilationCache.noindex
+          key: compilation-cache-unit-tests-enterprise-${{ github.sha }}
+
       - name: Archive artifacts
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         # We only care about artefacts if the tests fail
diff --git a/.github/workflows/unit-tests.yml b/.github/workflows/unit-tests.yml
index 39d2e2eeb..f72fb550f 100644
--- a/.github/workflows/unit-tests.yml
+++ b/.github/workflows/unit-tests.yml
@@ -29,9 +29,24 @@ jobs:
       - name: Setup environment
         run: source ci_scripts/ci_common.sh && setup_github_actions_environment
 
+      - name: Restore compilation cache
+        uses: actions/cache/restore@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ~/Library/Developer/Xcode/CompilationCache.noindex
+          key: compilation-cache-unit-tests-${{ github.sha }}
+          restore-keys: compilation-cache-unit-tests-
+
       - name: Run tests
         run: swift run -q tools ci unit-tests
 
+      - name: Save compilation cache
+        # Only develop populates the cache, all of the other runs simply read from it.
+        if: ${{ !cancelled() && github.event_name == 'push' && github.ref == 'refs/heads/develop' }}
+        uses: actions/cache/save@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        with:
+          path: ~/Library/Developer/Xcode/CompilationCache.noindex
+          key: compilation-cache-unit-tests-${{ github.sha }}
+
       - name: Archive artifacts
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a # v7.0.1
         # We only care about artefacts if the tests fail
diff --git a/Tools/Sources/Commands/CI/RunTests.swift b/Tools/Sources/Commands/CI/RunTests.swift
index 4f75f131d..7b139b02a 100644
--- a/Tools/Sources/Commands/CI/RunTests.swift
+++ b/Tools/Sources/Commands/CI/RunTests.swift
@@ -139,6 +139,12 @@ struct RunTests: AsyncParsableCommand {
         command += " -destination 'platform=iOS Simulator,name=\(device),OS=\(osVersion),arch=arm64'"
         command += " -resultBundlePath \(resultBundlePath)"
         
+        // Use a dedicated compilation cache for incremental builds on CI (as Xcode's DerivedData
+        // is invalidated by a fresh checkout).
+        command += " COMPILATION_CACHE_ENABLE_CACHING=YES"
+        // Store it outside DerivedData so CI can cache just what it needs.
+        command += " COMPILATION_CACHE_CAS_PATH=$HOME/Library/Developer/Xcode/CompilationCache.noindex"
+        
         // Use xcodebuild's native retry support to re-run only failing tests
         // instead of re-running the entire suite. retries=0 means no retries (single run).
         if retries > 0 {