[xcodegen] Infer project root from xcodegen location

Instead of inferring from the build directory location,
infer from the location of swift-xcodegen itself since
we know that's in the swift repo.

Author: hamishknight

utils/swift-xcodegen/Sources/SwiftXcodeGen/Error.swift

@@ -13,7 +13,7 @@
 enum XcodeGenError: Error, CustomStringConvertible {
   case pathNotFound(AbsolutePath)
   case noSwiftBuildDir(AbsolutePath, couldBeParent: Bool)
-  case expectedParent(AbsolutePath)
+  case couldNotInferProjectRoot(reason: String)
 
   var description: String {
     switch self {
@@ -23,8 +23,11 @@ enum XcodeGenError: Error, CustomStringConvertible {
       let base = "no swift build directory found in '\(basePath)'"
       let note = "; did you mean to pass the path of the parent?"
       return couldBeParent ? "\(base)\(note)" : base
-    case .expectedParent(let basePath):
-      return "expected '\(basePath)' to have parent directory"
+    case .couldNotInferProjectRoot(let reason):
+      return """
+        could not infer project root path; \(reason); please manually specify \
+        using '--project-root-dir' instead
+        """
     }
   }
 }

utils/swift-xcodegen/Sources/SwiftXcodeGen/Ninja/NinjaBuildDir.swift

@@ -30,17 +30,20 @@ public final class NinjaBuildDir: Sendable {
     let couldBeParent = buildDir.fileName.hasPrefix("swift-")
     throw XcodeGenError.noSwiftBuildDir(buildDir, couldBeParent: couldBeParent)
   }
-  
-  private static func detectProjectRoot(
-    buildDir: AbsolutePath
-  ) throws -> AbsolutePath {
-    guard let parent = buildDir.parentDir else {
-      throw XcodeGenError.expectedParent(buildDir)
-    }
-    guard let projectDir = parent.parentDir else {
-      throw XcodeGenError.expectedParent(parent)
+
+  // We can infer the project root from the location of swift-xcodegen itself.
+  //                      1     2         3          4           5         6      7
+  // #filePath = <root>/swift/utils/swift-xcodegen/Sources/SwiftXcodeGen/Ninja/NinjaBuildDir.swift
+  private static let inferredProjectRootPath = AbsolutePath(#filePath).dropLast(7)
+
+  private static func detectProjectRoot() throws -> AbsolutePath {
+    let inferredSwiftPath = inferredProjectRootPath.appending(Repo.swift.relativePath)
+    guard inferredSwiftPath.exists else {
+      throw XcodeGenError.couldNotInferProjectRoot(
+        reason: "expected swift repo at '\(inferredSwiftPath)'"
+      )
     }
-    return projectDir
+    return inferredProjectRootPath
   }
 
   public init(at path: AbsolutePath, projectRootDir: AbsolutePath?) throws {
@@ -49,7 +52,7 @@ public final class NinjaBuildDir: Sendable {
     }
     self.path = path
     self.tripleSuffix = try Self.detectTripleSuffix(buildDir: path)
-    self.projectRootDir = try projectRootDir ?? Self.detectProjectRoot(buildDir: path)
+    self.projectRootDir = try projectRootDir ?? Self.detectProjectRoot()
   }
 
   public func buildDir(for repo: Repo) throws -> RepoBuildDir {

utils/swift-xcodegen/Sources/SwiftXcodeGen/Path/PathProtocol.swift

@@ -28,6 +28,12 @@ public extension PathProtocol {
     return Self(result)
   }
 
+  /// Drops the last `n` components, or all components if `n` is greater
+  /// than the number of components.
+  func dropLast(_ n: Int = 1) -> Self {
+    Self(FilePath(root: storage.root, storage.components.dropLast(n)))
+  }
+
   var fileName: String {
     storage.lastComponent?.string ?? ""
   }

utils/swift-xcodegen/Tests/SwiftXcodeGenTest/PathTests.swift

@@ -25,4 +25,16 @@ class PathTests: XCTestCase {
     XCTAssertEqual(AbsolutePath("/foo").parentDir, "/")
     XCTAssertEqual(AbsolutePath("/foo/bar").parentDir, "/foo")
   }
+
+  func testDropLast() throws {
+    XCTAssertEqual(AbsolutePath("/").dropLast(), "/")
+    XCTAssertEqual(AbsolutePath("/foo/bar").dropLast(), "/foo")
+    XCTAssertEqual(AbsolutePath("/foo/bar").dropLast(2), "/")
+    XCTAssertEqual(AbsolutePath("/foo/bar").dropLast(5), "/")
+
+    XCTAssertEqual(RelativePath("").dropLast(), "")
+    XCTAssertEqual(RelativePath("foo/bar").dropLast(), "foo")
+    XCTAssertEqual(RelativePath("foo/bar").dropLast(2), "")
+    XCTAssertEqual(RelativePath("foo/bar").dropLast(5), "")
+  }
 }