Allow for relative build record paths

Author: David Ungar

Sources/SwiftDriver/Driver/Driver.swift

@@ -435,6 +435,7 @@ public struct Driver {
     self.buildRecordInfo = BuildRecordInfo(
       actualSwiftVersion: self.frontendTargetInfo.compilerVersion,
       compilerOutputType: compilerOutputType,
+      workingDirectory: self.workingDirectory ?? fileSystem.currentWorkingDirectory,
       diagnosticEngine: diagnosticEngine,
       fileSystem: fileSystem,
       moduleOutputInfo: moduleOutputInfo,

Sources/SwiftDriver/Incremental Compilation/BuildRecordInfo.swift

@@ -30,6 +30,7 @@ import SwiftOptions
   init?(
     actualSwiftVersion: String,
     compilerOutputType: FileType?,
+    workingDirectory: AbsolutePath?,
     diagnosticEngine: DiagnosticsEngine,
     fileSystem: FileSystem,
     moduleOutputInfo: ModuleOutputInfo,
@@ -41,6 +42,7 @@ import SwiftOptions
     guard let buildRecordPath = Self.computeBuildRecordPath(
             outputFileMap: outputFileMap,
             compilerOutputType: compilerOutputType,
+            workingDirectory: workingDirectory,
             diagnosticEngine: diagnosticEngine)
     else {
       return nil
@@ -80,6 +82,7 @@ import SwiftOptions
   private static func computeBuildRecordPath(
     outputFileMap: OutputFileMap?,
     compilerOutputType: FileType?,
+    workingDirectory: AbsolutePath?,
     diagnosticEngine: DiagnosticsEngine
   ) -> VirtualPath? {
     // FIXME: This should work without an output file map. We should have
@@ -93,7 +96,7 @@ import SwiftOptions
       diagnosticEngine.emit(.warning_incremental_requires_build_record_entry)
       return nil
     }
-    return partialBuildRecordPath
+    return partialBuildRecordPath.resolveRelativePath(workingDirectory)
   }
 
   /// Write out the build record.
@@ -175,3 +178,12 @@ import SwiftOptions
     }
   }
 }
+
+extension VirtualPath {
+  fileprivate func resolveRelativePath(_ workingDirectory: AbsolutePath?) -> VirtualPath {
+    switch (workingDirectory, self) {
+    case let (wd?, .relative(relPath)): return .absolute(.init(wd, relPath))
+    default: return self
+    }
+  }
+}