Enable cross-import overlays in test targets. (#8158)

This PR enables cross-import overlays when building test targets. Swift
Testing cannot link to most modules because it would/could introduce
circular dependencies that prevent those modules from being tested using
Swift Testing. However, we need to introduce some API that uses types
from Foundation and other modules, which means we need to use the
cross-import overlays feature supported by the compiler.

Cross-import overlays haven't been through Swift Evolution, so we don't
want to enable the feature in the general case. We talked with @beccadax
who agreed that because our use case is relatively unique, enabling
these overlays just for test targets was probably the right approach
until/unless the feature can be fully reviewed and supported.

We're also looking at consulting available information (the dependency
scanner) to determine if there is any dependency on Swift Testing. If
there isn't one, we don't need this flag. That's a "future direction"
for this PR at any rate.

Author: grynspan

Sources/Build/BuildDescription/SwiftModuleBuildDescription.swift

@@ -982,7 +982,15 @@ public final class SwiftModuleBuildDescription {
         if self.isTestTarget {
             // test targets must be built with -enable-testing
             // since its required for test discovery (the non objective-c reflection kind)
-            return ["-enable-testing"]
+            var result = ["-enable-testing"]
+
+            // Test targets need to enable cross-import overlays because Swift
+            // Testing cannot directly link to most other modules and needs to
+            // provide API that works with e.g. Foundation. (Developers can
+            // override this flag by passing -disable-cross-import-overlays.)
+            result += ["-Xfrontend", "-enable-cross-import-overlays"]
+
+            return result
         } else if self.buildParameters.enableTestability {
             return ["-enable-testing"]
         } else {

Tests/BuildTests/BuildPlanTests.swift

@@ -2337,6 +2337,8 @@ final class BuildPlanTests: XCTestCase {
                 "-enable-batch-mode",
                 "-Onone",
                 "-enable-testing",
+                "-Xfrontend",
+                "-enable-cross-import-overlays",
                 .equal(self.j),
                 "-DSWIFT_PACKAGE",
                 "-DDEBUG",