Add support for building Swift on Android targets

Swift Build currently has support for building C/C++ code on Android; this extends it to Swift.

Concretely, this fixes the position of the version number in the triple passed to swiftc for Android targets, and adds the relevant search paths to find Swift standard libraries and other content in the Swift SDK.

The sysroot still comes from an Android NDK found on the system; the sysroot in the Swift SDK is ignored based on the intended direction around supporting Android targets in Swift.

Author: jakepetroules

Sources/SWBAndroidPlatform/AndroidSDK.swift

@@ -57,10 +57,10 @@ struct AndroidSDK: Sendable {
         }
 
         struct LLVMTriple: Codable {
-            let arch: String
-            let vendor: String
-            let system: String
-            let environment: String
+            var arch: String
+            var vendor: String
+            var system: String
+            var environment: String
 
             var description: String {
                 "\(arch)-\(vendor)-\(system)-\(environment)"

Sources/SWBAndroidPlatform/Plugin.swift

@@ -116,6 +116,37 @@ struct AndroidSDKRegistryExtension: SDKRegistryExtension {
             return []
         }
 
+        let allPossibleTriples = abis.values.flatMap { abi in
+            (max(deploymentTargetRange.min, abi.min_os_version)...deploymentTargetRange.max).map { deploymentTarget in
+                var triple = abi.llvm_triple
+                triple.vendor = "unknown" // Android NDK uses "none", Swift SDKs use "unknown"
+                triple.environment += "\(deploymentTarget)"
+                return triple
+            }
+        }.map(\.description)
+
+        let androidSwiftSDKs = (try? SwiftSDK.findSDKs(
+            targetTriples: allPossibleTriples,
+            fs: context.fs,
+            hostOperatingSystem: context.hostOperatingSystem
+        )) ?? []
+
+        let swiftSettings: [String: PropertyListItem]
+        // FIXME: We need a way to narrow down the list, possibly by passing down a Swift SDK identifier from SwiftPM
+        // The resource path should be the same for all triples in an Android Swift SDK
+        if let androidSwiftSDK = androidSwiftSDKs.only, let swiftResourceDir = Set(androidSwiftSDK.targetTriples.values.map { tripleProperties in androidSwiftSDK.path.join(tripleProperties.swiftResourcesPath) }).only {
+            swiftSettings = [
+                "SWIFT_LIBRARY_PATH": .plString(swiftResourceDir.join("android").str),
+                "SWIFT_RESOURCE_DIR": .plString(swiftResourceDir.str),
+                "SWIFT_TARGET_TRIPLE": .plString("$(CURRENT_ARCH)-unknown-$(SWIFT_PLATFORM_TARGET_PREFIX)$(LLVM_TARGET_TRIPLE_SUFFIX)"),
+                "LIBRARY_SEARCH_PATHS": "$(inherited) $(SWIFT_RESOURCE_DIR)/../$(__ANDROID_TRIPLE_$(CURRENT_ARCH))",
+            ].merging(abis.map {
+                ("__ANDROID_TRIPLE_\($0.value.llvm_triple.arch)", .plString($0.value.triple))
+            }, uniquingKeysWith: { _, new in new })
+        } else {
+            swiftSettings = [:]
+        }
+
         return [(androidSdk.sysroot ?? .root, androidPlatform, [
             "Type": .plString("SDK"),
             "Version": .plString("0.0.0"),
@@ -129,7 +160,7 @@ struct AndroidSDKRegistryExtension: SDKRegistryExtension {
                 // FIXME: Make this configurable in a better way so we don't need to push build settings at the SDK definition level
                 "LLVM_TARGET_TRIPLE_OS_VERSION": .plString("linux"),
                 "LLVM_TARGET_TRIPLE_SUFFIX": .plString("-android$(ANDROID_DEPLOYMENT_TARGET)"),
-            ]),
+            ].merging(swiftSettings, uniquingKeysWith: { _, new in new })),
             "SupportedTargets": .plDict([
                 "android": .plDict([
                     "Archs": .plArray(abis.map { .plString($0.value.llvm_triple.arch) }),