MergeSwiftModule - Building a Swift app in Xcode 6.3 gets stuck in "Merge MyApp.swiftmodule"

Question

Building an app in Xcode 6.3 gets stuck for several minutes (approx. 10mins) on a step Merge MyApp.swiftmodule:

MergeSwiftModule normal x86_64 /Users/.../Library/Developer/Xcode/DerivedData/MyApp-crbdsfrhkvpiynaffvhzzsspiels/Build/Intermediates/MyApp.build/Debug-iphonesimulator/MyApp.build/Objects-normal/x86_64/MyApp.swiftmodule

this wasn't an issue in previous version of Xcode 6.2 and Swift 1.0.

Answer 1

Enabling SWIFT_WHOLE_MODULE_OPTIMIZATION for the target seems to fix this issue.

Source: https://devforums.apple.com/message/1119343#1119343