Unable to load Swift runtime library (image not found)

I updated one of my core frameworks to use Swift as an experiment. It builds fine, the compatibility header has the Swift classes available to the Objective-C classes that depend on them, and everything compiles fine.

However, when launching the application (base SDK and deployment target OS X 10.10) it crashes immediately with:

dyld: Library not loaded: @rpath/libswift_stdlib_core.dylib
  Referenced from: /Users/Craig/Library/Developer/Xcode/DerivedData/MacApp-dxcgahgplwpbjedqnembegifbowj/Build/Products/Debug/Fluffy.framework/Versions/A/Fluffy
  Reason: image not found

Note that MacApp is the application I'm launching, and it uses the Fluffy framework containing Swift code.

The Fluffy project has its Runpath Search Paths set to:

@executable_path/Frameworks

But I have also tried it blank. I have tried restarting Xcode, to no avail. Using Yosemite 14A298i and Xcode beta 4.

Solution

The Runpath Search Paths for my framework (Fluffy) containing the Swift code needed to be updated. It originally contained:

$(inherited)
@executable_path/../Frameworks

But after creating a sample Swift project and comparing the build settings, Xcode was adding:

@loader_path/Frameworks

To the newly-created project. This was missing from my existing framework project. Adding it so the Runpath Search Paths contained these values solved the problem:

$(inherited)
@executable_path/../Frameworks
@loader_path/Frameworks