Allow relative paths in output file map

Sources/SwiftDriver/Driver/Driver.swift

@@ -247,13 +247,19 @@ public struct Driver {
     let inputFiles = try Self.collectInputFiles(&self.parsedOptions)
     self.inputFiles = inputFiles
 
+    let outputFileMap: OutputFileMap?
     // Initialize an empty output file map, which will be populated when we start creating jobs.
     if let outputFileMapArg = parsedOptions.getLastArgument(.outputFileMap)?.asSingle {
       let path = try AbsolutePath(validating: outputFileMapArg)
-      self.outputFileMap = try .load(file: path, diagnosticEngine: diagnosticEngine)
+      outputFileMap = try .load(file: path, diagnosticEngine: diagnosticEngine)
+    } else {
+      outputFileMap = nil
     }
-    else {
-      self.outputFileMap = nil
+
+    if let workingDirectory = self.workingDirectory {
+      self.outputFileMap = outputFileMap?.resolveRelativePaths(relativeTo: workingDirectory)
+    } else {
+      self.outputFileMap = outputFileMap
     }
 
     // Determine the compilation mode.

Sources/SwiftDriver/Driver/OutputFileMap.swift

@@ -47,6 +47,25 @@ public struct OutputFileMap: Equatable {
     try! existingOutput(inputFile: VirtualPath(path: ""), outputType: outputType)
   }
 
+  public func resolveRelativePaths(relativeTo absPath: AbsolutePath) -> OutputFileMap {
+    let resolvedKeyValues: [(VirtualPath, [FileType : VirtualPath])] = entries.map {
+      let resolvedKey: VirtualPath
+      // Special case for single dependency record, leave it as is
+      if ($0.key == .relative(.init(""))) {
+        resolvedKey = $0.key
+      } else {
+        resolvedKey = $0.key.resolvedRelativePath(base: absPath)
+      }
+      let resolvedValue = $0.value.mapValues {
+        $0.resolvedRelativePath(base: absPath)
+      }
+      return (resolvedKey, resolvedValue)
+    }
+    return OutputFileMap(entries: .init(resolvedKeyValues, uniquingKeysWith: { _,_ in
+      fatalError("Paths collided after resolving")
+    }))
+  }
+
   /// Load the output file map at the given path.
   public static func load(
     file: AbsolutePath,
@@ -156,9 +175,9 @@ fileprivate struct OutputFileMapJSON: Codable {
 
   /// Converts into virtual path entries.
   func toVirtualOutputFileMap() throws -> [VirtualPath : [FileType : VirtualPath]] {
-    Dictionary(uniqueKeysWithValues: try entries.map { input, entry in
+    Dictionary(try entries.map { input, entry in
       (try VirtualPath(path: input), try entry.paths.mapValues(VirtualPath.init(path:)))
-    })
+    }, uniquingKeysWith: { $1 })
   }
 
   /// Converts from virtual path entries
@@ -186,3 +205,10 @@ extension String {
     return self + "." + ext
   }
 }
+
+extension VirtualPath {
+  fileprivate func resolvedRelativePath(base: AbsolutePath) -> VirtualPath {
+    guard case let .relative(relPath) = self else { return self }
+    return .absolute(.init(base, relPath))
+  }
+}

Tests/SwiftDriverTests/SwiftDriverTests.swift

@@ -453,6 +453,44 @@ final class SwiftDriverTests: XCTestCase {
     }
   }
 
+  func testOutputFileMapResolving() throws {
+    // Create sample OutputFileMap:
+
+    let stringyEntries: [String: [FileType: String]] = [
+      "": [.swiftDeps: "foo.build/master.swiftdeps"],
+      "foo.swift" : [
+        .dependencies: "foo.build/foo.d",
+        .object: "foo.build/foo.swift.o",
+        .swiftModule: "foo.build/foo~partial.swiftmodule",
+        .swiftDeps: "foo.build/foo.swiftdeps"
+      ]
+    ]
+    let resolvedStringyEntries: [String: [FileType: String]] = [
+      "": [.swiftDeps: "/foo_root/foo.build/master.swiftdeps"],
+      "/foo_root/foo.swift" : [
+        .dependencies: "/foo_root/foo.build/foo.d",
+        .object: "/foo_root/foo.build/foo.swift.o",
+        .swiftModule: "/foo_root/foo.build/foo~partial.swiftmodule",
+        .swiftDeps: "/foo_root/foo.build/foo.swiftdeps"
+      ]
+    ]
+    func outputFileMapFromStringyEntries(
+      _ entries: [String: [FileType: String]]
+    ) throws -> OutputFileMap {
+      .init(entries: Dictionary(uniqueKeysWithValues: try entries.map { try (
+        VirtualPath(path: $0.key),
+        $0.value.mapValues(VirtualPath.init(path:))
+      )}))
+    }
+    let sampleOutputFileMap =
+      try outputFileMapFromStringyEntries(stringyEntries)
+    let resolvedOutputFileMap = sampleOutputFileMap
+      .resolveRelativePaths(relativeTo: .init("/foo_root"))
+    let expectedOutputFileMap =
+      try outputFileMapFromStringyEntries(resolvedStringyEntries)
+    XCTAssertEqual(expectedOutputFileMap, resolvedOutputFileMap)
+  }
+
   func testResponseFileExpansion() throws {
     try withTemporaryDirectory { path in
       let diags = DiagnosticsEngine()