Add an Environment Variable to Specify the Toolchain Path

This is particularly useful for Windows which doesn't currently have a
standardized place to install the toolchain to.

Author: gmittert

Sources/Workspace/Destination.swift

@@ -1,6 +1,7 @@
 import TSCBasic
 import Build
 import TSCUtility
+import Foundation
 
 public enum DestinationError: Swift.Error {
     /// Couldn't find the Xcode installation.
@@ -57,16 +58,16 @@ public struct Destination: Encodable {
     private static func hostBinDir(
         originalWorkingDirectory: AbsolutePath? = localFileSystem.currentWorkingDirectory
     ) -> AbsolutePath {
+        if let toolchainDir = ProcessInfo.processInfo.environment["SWIFT_TOOLCHAIN_PATH"] {
+            return AbsolutePath(toolchainDir)
+        }
       #if Xcode
         // For Xcode, set bin directory to the build directory containing the fake
         // toolchain created during bootstraping. This is obviously not production ready
         // and only exists as a development utility right now.
         //
         // This also means that we should have bootstrapped with the same Swift toolchain
         // we're using inside Xcode otherwise we will not be able to load the runtime libraries.
-        //
-        // FIXME: We may want to allow overriding this using an env variable but that
-        // doesn't seem urgent or extremely useful as of now.
         return AbsolutePath(#file).parentDirectory
             .parentDirectory.parentDirectory.appending(components: ".build", hostTargetTriple.tripleString, "debug")
       #else