[Caching] Issue a warning when caching build without explicit module

Issue a warning and disable compile job caching when explicit module is
not enabled. This prevents swift-driver attempting to planning a caching
build using implicit build, which will result into either a build system
crash or bad swift-frontend invocations.

rdar://121206439

Author: cachemeifyoucan

Sources/SwiftDriver/Driver/Driver.swift

@@ -638,8 +638,18 @@ public struct Driver {
                                                                                         diagnosticEngine: diagnosticsEngine,
                                                                                         compilerMode: compilerMode)
 
-    let cachingEnableOverride = parsedOptions.hasArgument(.driverExplicitModuleBuild) && env.keys.contains("SWIFT_ENABLE_CACHING")
-    self.enableCaching = parsedOptions.hasArgument(.cacheCompileJob) || cachingEnableOverride
+    let cachingEnabled = parsedOptions.hasArgument(.cacheCompileJob) || env.keys.contains("SWIFT_ENABLE_CACHING")
+    if cachingEnabled {
+      if !parsedOptions.hasArgument(.driverExplicitModuleBuild) {
+        diagnosticsEngine.emit(.warning("-cache-compile-job cannot be used without explicit module build, turn off caching"),
+                               location: nil)
+        self.enableCaching = false
+      } else {
+        self.enableCaching = true
+      }
+    } else {
+      self.enableCaching = false
+    }
     self.useClangIncludeTree = !parsedOptions.hasArgument(.noClangIncludeTree) && !env.keys.contains("SWIFT_CACHING_USE_CLANG_CAS_FS")
     self.scannerPrefixMap = try Self.computeScanningPrefixMapper(&parsedOptions)
     if let sdkMapping =  parsedOptions.getLastArgument(.scannerPrefixMapSdk)?.asSingle {

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -7605,6 +7605,13 @@ final class SwiftDriverTests: XCTestCase {
       }
     }
   }
+
+  func testCachingBuildOptions() throws {
+    try assertDriverDiagnostics(args: "swiftc", "foo.swift", "-emit-module", "-cache-compile-job") {
+      $1.expect(.warning("-cache-compile-job cannot be used without explicit module build, turn off caching"))
+    }
+    try assertNoDriverDiagnostics(args: "swiftc", "foo.swift", "-emit-module", "-cache-compile-job", "-explicit-module-build")
+  }
 }
 
 func assertString(