[Darwin] Add external plugin paths into platform specified by PLATFORM_DIR

The driver currently has logic to infer the location of the platform
based on the conventional layout of SDKs within platforms. However,
that doesn't work well when SDKs have been built up in other places.

Introduce support for a second convention, that the PLATFORM_DIR
environment variable can specify the path to the platform. When
present, also add external plugin paths pointing into that platform.

Fixes rdar://138370421.

Author: DougGregor

Sources/SwiftDriver/Toolchains/DarwinToolchain.swift

@@ -456,29 +456,52 @@ public final class DarwinToolchain: Toolchain {
     }
 
     if driver.isFrontendArgSupported(.externalPluginPath) {
-      // Determine the platform path. For simulator platforms, look into the
-      // corresponding device platform instance.
-      let origPlatformPath = VirtualPath.lookup(sdkPath)
-        .parentDirectory
-        .parentDirectory
-        .parentDirectory
-      let platformPath: VirtualPath
-      if let simulatorRange = origPlatformPath.basename.range(of: "Simulator.platform") {
-        let devicePlatform = origPlatformPath.basename.replacingCharacters(in: simulatorRange, with: "OS.platform")
-        platformPath = origPlatformPath.parentDirectory.appending(component: devicePlatform)
-      } else {
-        platformPath = origPlatformPath
-      }
+      // Determine the platform path based on the SDK path.
+      addPluginPaths(
+        forPlatform: VirtualPath.lookup(sdkPath)
+          .parentDirectory
+          .parentDirectory
+          .parentDirectory,
+        commandLine: &commandLine
+      )
 
-      // Default paths for compiler plugins within the platform (accessed via that
-      // platform's plugin server).
-      let platformPathRoot = platformPath.appending(components: "Developer", "usr")
-      commandLine.appendFlag(.externalPluginPath)
-      commandLine.appendFlag("\(platformPathRoot.pluginPath.name)#\(platformPathRoot.pluginServerPath.name)")
+      // If the PLATFORM_DIR environment variable is set, also add plugin
+      // paths into it.
+      if let platformDir = env["PLATFORM_DIR"],
+         let platformPath = try? VirtualPath(path: platformDir) {
+        addPluginPaths(
+          forPlatform: platformPath,
+          commandLine: &commandLine
+        )
+      }
+    }
+  }
 
-      commandLine.appendFlag(.externalPluginPath)
-      commandLine.appendFlag("\(platformPathRoot.localPluginPath.name)#\(platformPathRoot.pluginServerPath.name)")
+  /// Given the platform path (e.g., a path into something like iPhoneOS.platform),
+  /// add external plugin path arguments for compiler plugins that are distributed
+  /// within that path.
+  func addPluginPaths(
+    forPlatform origPlatformPath: VirtualPath,
+    commandLine: inout [Job.ArgTemplate]
+  ) {
+    // For simulator platforms, look into the corresponding device platform instance,
+    // because they share compiler plugins.
+    let platformPath: VirtualPath
+    if let simulatorRange = origPlatformPath.basename.range(of: "Simulator.platform") {
+      let devicePlatform = origPlatformPath.basename.replacingCharacters(in: simulatorRange, with: "OS.platform")
+      platformPath = origPlatformPath.parentDirectory.appending(component: devicePlatform)
+    } else {
+      platformPath = origPlatformPath
     }
+
+    // Default paths for compiler plugins within the platform (accessed via that
+    // platform's plugin server).
+    let platformPathRoot = platformPath.appending(components: "Developer", "usr")
+    commandLine.appendFlag(.externalPluginPath)
+    commandLine.appendFlag("\(platformPathRoot.pluginPath.name)#\(platformPathRoot.pluginServerPath.name)")
+
+    commandLine.appendFlag(.externalPluginPath)
+    commandLine.appendFlag("\(platformPathRoot.localPluginPath.name)#\(platformPathRoot.pluginServerPath.name)")
   }
 }
 

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -7749,7 +7749,12 @@ final class SwiftDriverTests: XCTestCase {
   func pluginPathTest(platform: String, sdk: String, searchPlatform: String) throws {
     let sdkRoot = try testInputsPath.appending(
       components: ["Platform Checks", "\(platform).platform", "Developer", "SDKs", "\(sdk).sdk"])
-    var driver = try Driver(args: ["swiftc", "-typecheck", "foo.swift", "-sdk", VirtualPath.absolute(sdkRoot).name, "-plugin-path", "PluginA", "-external-plugin-path", "Plugin~B#Bexe", "-load-plugin-library", "PluginB2", "-plugin-path", "PluginC", "-working-directory", "/tmp"])
+    var env = ProcessEnv.vars
+    env["PLATFORM_DIR"] = "/tmp/PlatformDir/\(platform).platform"
+    var driver = try Driver(
+      args: ["swiftc", "-typecheck", "foo.swift", "-sdk", VirtualPath.absolute(sdkRoot).name, "-plugin-path", "PluginA", "-external-plugin-path", "Plugin~B#Bexe", "-load-plugin-library", "PluginB2", "-plugin-path", "PluginC", "-working-directory", "/tmp"],
+      env: env
+    )
     guard driver.isFrontendArgSupported(.pluginPath) && driver.isFrontendArgSupported(.externalPluginPath) else {
       return
     }
@@ -7791,6 +7796,18 @@ final class SwiftDriverTests: XCTestCase {
     XCTAssertNotNil(platformLocalPluginPathIndex)
     XCTAssertLessThan(platformPluginPathIndex!, platformLocalPluginPathIndex!)
 
+    // Plugin paths that come from the PLATFORM_DIR environment variable.
+    let envOrigPlatformPath = try AbsolutePath(validating: "/tmp/PlatformDir/\(searchPlatform).platform")
+    let envPlatformPath = envOrigPlatformPath.appending(components: "Developer", "usr")
+    let envPlatformServerPath = envPlatformPath.appending(components: "bin", "swift-plugin-server").pathString
+    let envPlatformPluginPath = envPlatformPath.appending(components: "lib", "swift", "host", "plugins")
+    let envPlatformPluginPathIndex = job.commandLine.firstIndex(of: .flag("\(envPlatformPluginPath)#\(envPlatformServerPath)"))
+    XCTAssertNotNil(envPlatformPluginPathIndex)
+
+    if let platformPluginPathIndex, let envPlatformPluginPathIndex {
+      XCTAssertLessThan(platformPluginPathIndex, envPlatformPluginPathIndex)
+    }
+
     let toolchainPluginPathIndex = job.commandLine.firstIndex(of: .path(.absolute(try driver.toolchain.executableDir.parentDirectory.appending(components: "lib", "swift", "host", "plugins"))))
     XCTAssertNotNil(toolchainPluginPathIndex)