Adds support for building using swiftly

- when building on Linux the usage of swiftly improves the installation
of Swift toolchains. Updated README and Package.swift to help building
it when using swiftly.

Author: mmllr

Package.swift

@@ -17,6 +17,16 @@ func envEnable(_ key: String, default defaultValue: Bool = false) -> Bool {
     }
 }
 
+func detectIncludePath() -> String {
+    if let path = Context.environment["SWIFT_TOOLCHAIN_PATH"] {
+        return path + "/usr/lib/swift"
+    }
+    let swiftBinPath = Context.environment["_"] ?? "/usr/bin/swift"
+    let swiftBinURL = URL(fileURLWithPath: swiftBinPath)
+    let SDKPath = swiftBinURL.deletingLastPathComponent().deletingLastPathComponent().deletingLastPathComponent().path
+    return SDKPath.appending("/usr/lib/swift")
+}
+
 #if os(macOS)
 // NOTE: #if os(macOS) check is not accurate if we are cross compiling for Linux platform. So we add an env key to specify it.
 let buildForDarwinPlatform = envEnable("OPENSWIFTUI_BUILD_FOR_DARWIN_PLATFORM", default: true)
@@ -34,11 +44,7 @@ let isSPIBuild = envEnable("SPI_BUILD")
 
 let isXcodeEnv = Context.environment["__CFBundleIdentifier"] == "com.apple.dt.Xcode"
 let development = envEnable("OPENATTRIBUTEGRAPH_DEVELOPMENT", default: false)
-
-let swiftBinPath = Context.environment["_"] ?? "/usr/bin/swift"
-let swiftBinURL = URL(fileURLWithPath: swiftBinPath)
-let SDKPath = swiftBinURL.deletingLastPathComponent().deletingLastPathComponent().deletingLastPathComponent().path
-let includePath = SDKPath.appending("/usr/lib/swift")
+let includePath = detectIncludePath()
 
 var sharedCSettings: [CSetting] = [
     .unsafeFlags(["-I", includePath], .when(platforms: .nonDarwinPlatforms)),

README.md

@@ -38,6 +38,7 @@ dependencies: [
 ```
 
 > [!NOTE]
+>
 > - You may need to configure the Swift toolchain header for proper integration of OpenAttributeGraph
 > - By default, OpenAttributeGraphShims will use the private AttributeGraph as its implementation on Apple platforms
 
@@ -49,6 +50,20 @@ For a simpler setup, you can use the prebuilt XCFramework available on the [rele
 
 The current suggested toolchain to build the project is Swift 6.1.2 / Xcode 16.4.
 
+### Usage with ```swiftly````
+
+If you have installed [Swiftly](https://github.com/swiftlang/swiftly) for managing your installed swift toolchains (Linux) set the following variable to your shell:
+
+```bash
+export SWIFT_TOOLCHAIN_PATH="$(swiftly use --print-location)"
+```
+
+```fish
+set -gx SWIFT_TOOLCHAIN_PATH (swiftly use --print-location)
+```
+
+Then run `swift build`
+
 ## License
 
 See LICENSE file - MIT