Add XcodeToolchain.build_settings for settings without a dedicated conf

conan/tools/apple/xcodetoolchain.py

@@ -24,6 +24,7 @@ class XcodeToolchain:
         {cflags}
         {cppflags}
         {ldflags}
+        {extra}
         """)
 
     _agreggated_xconfig = textwrap.dedent("""\
@@ -45,6 +46,7 @@ def __init__(self, conanfile):
         sharedlinkflags = self._conanfile.conf.get("tools.build:sharedlinkflags", default=[], check_type=list)
         exelinkflags = self._conanfile.conf.get("tools.build:exelinkflags", default=[], check_type=list)
         self._global_ldflags = sharedlinkflags + exelinkflags
+        self.build_settings = {}
 
     def generate(self):
         check_duplicated_generator(self, self._conanfile)
@@ -115,7 +117,8 @@ def _agreggated_xconfig_filename(self):
 
     @property
     def _check_if_extra_flags(self):
-        return self._global_cflags or self._global_cxxflags or self._global_ldflags or self._global_defines
+        return (self._global_cflags or self._global_cxxflags or self._global_ldflags
+                or self._global_defines or self.build_settings)
 
     @property
     def _flags_xcconfig_content(self):
@@ -126,7 +129,9 @@ def _flags_xcconfig_content(self):
         cflags = "OTHER_CFLAGS{} = $(inherited) {}".format(condition, " ".join(self._global_cflags)) if self._global_cflags else ""
         cppflags = "OTHER_CPLUSPLUSFLAGS{} = $(inherited) {}".format(condition, " ".join(self._global_cxxflags)) if self._global_cxxflags else ""
         ldflags = "OTHER_LDFLAGS{} = $(inherited) {}".format(condition, " ".join(self._global_ldflags)) if self._global_ldflags else ""
-        ret = self._flags_xconfig.format(defines=defines, cflags=cflags, cppflags=cppflags, ldflags=ldflags)
+        extra = "\n".join("{}{} = {}".format(k, condition, v) for k, v in self.build_settings.items())
+        ret = self._flags_xconfig.format(defines=defines, cflags=cflags, cppflags=cppflags, ldflags=ldflags,
+                                         extra=extra)
         return ret
 
     @property

test/integration/toolchains/apple/test_xcodetoolchain.py

@@ -93,6 +93,28 @@ def test_toolchain_flags():
     assert '#include "conan_global_flags.xcconfig"' in conan_global_file
 
 
+@pytest.mark.skipif(platform.system() != "Darwin", reason="Only for MacOS")
+def test_toolchain_build_settings():
+    client = TestClient()
+    conanfile = textwrap.dedent("""
+        from conan import ConanFile
+        from conan.tools.apple import XcodeToolchain
+        class Demo(ConanFile):
+            settings = "os", "arch", "compiler", "build_type"
+            def generate(self):
+                tc = XcodeToolchain(self)
+                tc.build_settings["OTHER_SWIFT_FLAGS"] = "$(inherited) -cxx-interoperability-mode=default"
+                tc.generate()
+        """)
+    client.save({"conanfile.py": conanfile})
+    client.run("install . -s build_type=Release -s arch=x86_64")
+    filename = _get_filename("Release", "x86_64", None)
+    condition = _condition("Release", "x86_64", None)
+
+    conan_global_flags_props = client.load("conan_global_flags{}.xcconfig".format(filename))
+    assert "OTHER_SWIFT_FLAGS{} = $(inherited) -cxx-interoperability-mode=default".format(condition) in conan_global_flags_props
+
+
 @pytest.mark.skipif(platform.system() != "Darwin", reason="Only for MacOS")
 def test_flags_generated_if_only_defines():
     # https://github.com/conan-io/conan/issues/16422