Backport cppstd_flag from Conan 2.x (conan-io#15731)

Add wrapper for cppstd_std with conanfile

Signed-off-by: Uilian Ries <[email protected]>

conan/tools/build/__init__.py

@@ -3,3 +3,4 @@
 from conan.tools.build.cppstd import check_min_cppstd, valid_min_cppstd, default_cppstd, \
     supported_cppstd
 from conan.tools.build.stdcpp_library import stdcpp_library
+from conan.tools.build.flags import cppstd_flag

conan/tools/build/flags.py

@@ -0,0 +1,16 @@
+from conan.tools._compilers import cppstd_flag as cppstd_flag_settings
+
+
+def cppstd_flag(conanfile):
+    """
+    Returns flags specific to the C++ standard based on the ``conanfile.settings.compiler``,
+    ``conanfile.settings.compiler.version`` and ``conanfile.settings.compiler.cppstd``.
+    It also considers when using GNU extension in ``settings.compiler.cppstd``, reflecting it in the
+    compiler flag. Currently, it supports GCC, Clang, AppleClang, MSVC, Intel, MCST-LCC.
+    In case there is no ``settings.compiler`` or ``settings.cppstd`` in the profile, the result will
+    be an **empty string**.
+    :param conanfile: The current recipe object. Always use ``self``.
+    :return: ``str`` with the standard C++ flag used by the compiler. e.g. "-std=c++11", "/std:c++latest"
+    """
+    settings = conanfile.settings
+    return cppstd_flag_settings(settings)

conans/test/unittests/client/build/cpp_std_flags_test.py

@@ -1,9 +1,11 @@
 import unittest
 
 from conans.client.build.cppstd_flags import cppstd_default
-from conans.test.utils.mocks import MockSettings
+from conans.test.utils.mocks import MockSettings, ConanFileMock
 from conans.tools import cppstd_flag
 
+from conan.tools.build import cppstd_flag as cppstd_flag_conanfile
+
 
 def _make_cppstd_flag(compiler, compiler_version, cppstd=None, compiler_base=None):
     settings = MockSettings({"compiler": compiler,
@@ -399,3 +401,12 @@ def test_mcst_lcc_cppstd_flag(self):
         self.assertEqual(_make_cppstd_flag("mcst-lcc", "1.25", "14", "gcc"), "-std=c++14")
         self.assertEqual(_make_cppstd_flag("mcst-lcc", "1.25", "17", "gcc"), "-std=c++17")
         self.assertEqual(_make_cppstd_flag("mcst-lcc", "1.25", "20", "gcc"), "-std=c++2a")
+
+    def test_cppstd_flag_conanfile(self):
+        """The conan.tools.build.cppstd_flag should work when passing a ConanFile instance
+        """
+        conanfile = ConanFileMock()
+        conanfile.settings = MockSettings({"compiler": "gcc",
+                                          "compiler.version": "9",
+                                          "compiler.cppstd": "17"})
+        self.assertEqual(cppstd_flag_conanfile(conanfile), "-std=c++17")