Make the `PREFIX` Parameter of the `qt_add_resources` Optional

The `rcc_PREFIX` will be set to `/` if it is not passed to the
function and it is not defined in `QT_RESOURCE_PREFIX`.

I also removed an unnecessary check of the `rcc_PREFIX`.

[ChangeLog][QtCore][CMake] The `PREFIX` parameter of the
`qt_add_resources` is now optional. If not passed, and
`QT_RESOURCE_PREFIX` is not defined, `/` will be used as the path
prefix.

Fixes: QTBUG-104938
Change-Id: I6524ab5dc54f035272e4c2e3154eb67591efb650
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>

src/corelib/Qt6CoreMacros.cmake

@@ -1781,7 +1781,7 @@ function(_qt_internal_process_resource target resourceName)
     if(NOT rcc_PREFIX)
         get_target_property(rcc_PREFIX ${target} QT_RESOURCE_PREFIX)
         if (NOT rcc_PREFIX)
-            message(FATAL_ERROR "_qt_internal_process_resource() was called without a PREFIX and the target does not provide QT_RESOURCE_PREFIX. Please either add a PREFIX or make the target ${target} provide a default.")
+            set(rcc_PREFIX "/")
         endif()
     endif()
 
@@ -1797,9 +1797,8 @@ function(_qt_internal_process_resource target resourceName)
 
     # <RCC><qresource ...>
     set(qrcContents "<RCC>\n  <qresource")
-    if (rcc_PREFIX)
-        string(APPEND qrcContents " prefix=\"${rcc_PREFIX}\"")
-    endif()
+    string(APPEND qrcContents " prefix=\"${rcc_PREFIX}\"")
+
     if (rcc_LANG)
         string(APPEND qrcContents " lang=\"${rcc_LANG}\"")
     endif()

src/corelib/doc/src/cmake/qt_add_resources.qdoc

@@ -57,7 +57,9 @@ See \l{The Qt Resource System} for a general description of Qt resources.
 \c PREFIX specifies a path prefix under which all files of this resource are
 accessible from C++ code. This corresponds to the XML attribute \c prefix of the
 \c .qrc file format. If \c PREFIX is not given, the target property
-\l{cmake-target-property-QT_RESOURCE_PREFIX}{QT_RESOURCE_PREFIX} is used.
+\l{cmake-target-property-QT_RESOURCE_PREFIX}{QT_RESOURCE_PREFIX} is used. Since
+6.5, \c{PREFIX} is optional. If it is omitted and not specified by
+\c{QT_RESOURCE_PREFIX}, \c{"/"} will be used as the default path prefix.
 
 \c LANG specifies the locale of this resource. This corresponds to the XML
 attribute \c lang of the \c .qrc file format.

tests/auto/cmake/CMakeLists.txt

@@ -158,6 +158,7 @@ if (NOT NO_WIDGETS)
     _qt_internal_test_expect_pass(test_dependent_modules)
     _qt_internal_test_expect_pass("test(needsquoting)dirname")
 endif()
+_qt_internal_test_expect_pass(test_add_resource_prefix BINARY test_add_resource_prefix)
 _qt_internal_test_expect_build_fail(test_add_resource_options)
 _qt_internal_test_expect_build_fail(test_wrap_cpp_options)
 _qt_internal_test_expect_pass(test_platform_defs_include)

tests/auto/cmake/test_add_resource_prefix/CMakeLists.txt

@@ -0,0 +1,25 @@
+# Copyright (C) 2022 The Qt Company Ltd.
+# SPDX-License-Identifier: BSD-3-Clause
+
+cmake_minimum_required(VERSION 3.16)
+
+project(test_add_resource_prefix)
+
+find_package(Qt6 REQUIRED
+    COMPONENTS Core Test
+)
+
+set(CMAKE_AUTOMOC ON)
+
+add_executable(test_add_resource_prefix main.cpp)
+
+# Tests if "/" is being used when PREFIX is missing
+qt_add_resources(test_add_resource_prefix "resources_without_prefix"
+    FILES resource_file.txt)
+
+# Tests if the PREFIX parameter is being respected
+qt_add_resources(test_add_resource_prefix "resources_with_prefix"
+    PREFIX "/resources"
+    FILES resource_file.txt)
+
+target_link_libraries(test_add_resource_prefix PRIVATE Qt::Core Qt::Test)

tests/auto/cmake/test_add_resource_prefix/main.cpp

@@ -0,0 +1,26 @@
+// Copyright (C) 2022 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only WITH Qt-GPL-exception-1.0
+
+#include <QFile>
+#include <QtTest>
+
+class TestAddResourcePrefix : public QObject
+{
+    Q_OBJECT
+private slots:
+    void resourceInDefaultPathExists();
+    void resourceInGivenPathExists();
+};
+
+void TestAddResourcePrefix::resourceInDefaultPathExists()
+{
+    QVERIFY(QFile::exists(":/resource_file.txt"));
+}
+
+void TestAddResourcePrefix::resourceInGivenPathExists()
+{
+    QVERIFY(QFile::exists(":/resources/resource_file.txt"));
+}
+
+QTEST_MAIN(TestAddResourcePrefix)
+#include "main.moc"

tests/auto/cmake/test_add_resource_prefix/resource_file.txt

@@ -0,0 +1 @@
+Ken sent me.