cmake: Fix finding corresponding source directory from build directory

When trying to find the corresponding source directory for a build directory,
it is better to just read it out of the CMakeDirectoryInformation.cmake file
found in the CMakeFiles subdirectory of the build directory, instead of
guessing it using the relative paths.

Change-Id: Ie96b3255a27f46bcd7f0ab1566dfb91ac52f3be9
Reviewed-by: Daniel Teske <daniel.teske@theqtcompany.com>

src/plugins/cmakeprojectmanager/cmakeproject.cpp

@@ -1057,10 +1057,28 @@ void CMakeCbpParser::parseBuildTargetOption()
             m_buildTarget.targetType = TargetType(value.toInt());
     } else if (attributes().hasAttribute(QLatin1String("working_dir"))) {
         m_buildTarget.workingDirectory = attributes().value(QLatin1String("working_dir")).toString();
-        QDir dir(m_buildDirectory);
-        QString relative = dir.relativeFilePath(m_buildTarget.workingDirectory);
-        m_buildTarget.sourceDirectory
-                = Utils::FileName::fromString(m_sourceDirectory).appendPath(relative).toString();
+
+        QFile cmakeSourceInfoFile(m_buildTarget.workingDirectory
+                                  + QStringLiteral("/CMakeFiles/CMakeDirectoryInformation.cmake"));
+        if (cmakeSourceInfoFile.open(QIODevice::ReadOnly | QIODevice::Text)) {
+            QTextStream stream(&cmakeSourceInfoFile);
+            const QLatin1String searchSource("SET(CMAKE_RELATIVE_PATH_TOP_SOURCE \"");
+            while (!stream.atEnd()) {
+                const QString lineTopSource = stream.readLine().trimmed();
+                if (lineTopSource.startsWith(searchSource)) {
+                    m_buildTarget.sourceDirectory = lineTopSource.mid(searchSource.size());
+                    m_buildTarget.sourceDirectory.chop(2); // cut off ")
+                    break;
+                }
+            }
+        }
+
+        if (m_buildTarget.sourceDirectory.isEmpty()) {
+            QDir dir(m_buildDirectory);
+            const QString relative = dir.relativeFilePath(m_buildTarget.workingDirectory);
+            m_buildTarget.sourceDirectory
+                    = Utils::FileName::fromString(m_sourceDirectory).appendPath(relative).toString();
+        }
     }
     while (!atEnd()) {
         readNext();