Just a few weeks ago, Apple open-sourced the Swift compiler and standard library. This is exciting news for the Swift community: not only will Swift development now be done in the open, but the availability of Swift's source makes it possible to port Swift to other platforms.
The Swift codebase is very large (approaching 400k lines of C++ code, including test cases) so an IDE to help you navigate through it would be ideal. I'm partial to the Jetbrains tools, but wasn't sure if I could use them to browse the Swift source tree. I had some difficulty getting CLion to use the CMake scripts in the Swift repo and was about to give up. Fortunately, I noticed in the bottom of Swift's README that there's a script to generate an Xcode project from the Swift repo.
So here's how I generated the Xcode project and was able to work on Swift using AppCode:
1. Create a new parent directory, cd into it and clone the Swift repo:
$ mkdir Swift
$ cd Swift
$ git clone https://github.com/apple/swift.git
2. Prep the Swift repo, per the instructions in the README. This will import other tools that Swift depends on (like cmark and some LLVM tools)
$ cd swift
$ ./utils/update-checkout --clone
3. Generate the Xcode project:
$ utils/build-script -X --skip-build -- --reconfigure
4. Now open AppCode and click File | Open...
5. Find the outer Swift directory you created in step 1 (the one that holds the Swift repo and all its associated tools).
6. Open the build folder, then open Xcode-DebugAssert. You'll see a folder that looks like swift-macosx-x86_64 (the exact name may differ based on your system).
Just open this folder in AppCode, wait for indexing to complete, and you'll be able to browse and Edit the Swift source from AppCode!