Unity: No repeated path for internal generated unity files

Fixes: #20002

Source/cmLocalGenerator.cxx

@@ -3148,6 +3148,22 @@ std::string cmLocalGenerator::GetObjectFileNameWithoutTarget(
     }
   }
 
+  // Ensure that for the CMakeFiles/<target>.dir/generated_source_file
+  // we don't end up having:
+  // CMakeFiles/<target>.dir/CMakeFiles/<target>.dir/generated_source_file.obj
+  const char* unitySourceFile = source.GetProperty("UNITY_SOURCE_FILE");
+  const char* pchExtension = source.GetProperty("PCH_EXTENSION");
+  if (unitySourceFile || pchExtension) {
+    if (pchExtension) {
+      customOutputExtension = pchExtension;
+    }
+
+    cmsys::RegularExpression var("(CMakeFiles/[^/]+.dir/)");
+    if (var.find(objectName)) {
+      objectName.erase(var.start(), var.end() - var.start());
+    }
+  }
+
   // Replace the original source file extension with the object file
   // extension.
   bool keptSourceExtension = true;
@@ -3164,19 +3180,6 @@ std::string cmLocalGenerator::GetObjectFileNameWithoutTarget(
       }
     }
 
-    const char* pchExtension = source.GetProperty("PCH_EXTENSION");
-    if (pchExtension) {
-      customOutputExtension = pchExtension;
-
-      // Make sure that for the CMakeFiles/<target>.dir/cmake_pch.h|xx.c|xx
-      // source file, we don't end up having
-      // CMakeFiles/<target>.dir/CMakeFiles/<target>.dir/cmake_pch.h|xx.pch
-      cmsys::RegularExpression var("(CMakeFiles/[^/]+.dir/)");
-      while (var.find(objectName)) {
-        objectName.erase(var.start(), var.end() - var.start());
-      }
-    }
-
     // Remove the source extension if it is to be replaced.
     if (replaceExt || customOutputExtension) {
       keptSourceExtension = false;