Merge pull request #1088 from compnerd/module-diagnostics

Driver: provide a more suitable default for `emit-module.d`

Author: compnerd

Sources/SwiftDriver/Driver/Driver.swift

@@ -3014,10 +3014,21 @@ extension Driver {
 
     // Emit-module discovered dependencies are always specified as a single-output
     // file
-    if type == .emitModuleDependencies,
-      let singleOutputPath = outputFileMap?.existingOutputForSingleInput(
-           outputType: type) {
-      return singleOutputPath
+    if type == .emitModuleDependencies {
+      if let path = outputFileMap?.existingOutputForSingleInput(outputType: type) {
+        return path
+      }
+
+      // If an explicit path is not provided by the output file map, attempt to
+      // synthesize a path from the master swift dependency path.  This is
+      // important as we may other emit this file at the location where the
+      // driver was invoked, which is normally the root of the package.
+      if let path = outputFileMap?.existingOutputForSingleInput(outputType: .swiftDeps) {
+        return VirtualPath.lookup(path)
+                    .parentDirectory
+                    .appending(component: "\(moduleName).\(type.rawValue)")
+                    .intern()
+      }
     }
 
     // If there is an output argument, derive the name from there.