interpreter: Add warning function

docs/markdown/Reference-manual.md

@@ -847,6 +847,14 @@ The keyword arguments for this are the same as for [`executable`](#executable) w
 
 This function prints its argument to stdout.
 
+### warning()
+
+``` meson
+    void warning(text)
+```
+
+This function prints its argument to stdout prefixed with WARNING:.
+
 ### project()
 
 ``` meson

docs/markdown/snippets/warning_function

@@ -0,0 +1,6 @@
+# Added warning function
+
+This function prints its argument to the console prefixed by "WARNING:" in
+yellow color. A simple example:
+
+warning('foo is deprecated, please use bar instead')

mesonbuild/interpreter.py

@@ -1443,6 +1443,7 @@ class Interpreter(InterpreterBase):
                            'join_paths': self.func_join_paths,
                            'library': self.func_library,
                            'message': self.func_message,
+                           'warning': self.func_warning,
                            'option': self.func_option,
                            'project': self.func_project,
                            'run_target': self.func_run_target,
@@ -1869,8 +1870,7 @@ to directly access options of other subprojects.''')
     def func_add_languages(self, node, args, kwargs):
         return self.add_languages(args, kwargs.get('required', True))
 
-    @noKwargs
-    def func_message(self, node, args, kwargs):
+    def get_message_string_arg(self, node):
         # reduce arguments again to avoid flattening posargs
         (posargs, _) = self.reduce_arguments(node.args)
         if len(posargs) != 1:
@@ -1886,8 +1886,18 @@ to directly access options of other subprojects.''')
         else:
             raise InvalidArguments('Function accepts only strings, integers, lists and lists thereof.')
 
+        return argstr
+
+    @noKwargs
+    def func_message(self, node, args, kwargs):
+        argstr = self.get_message_string_arg(node)
         mlog.log(mlog.bold('Message:'), argstr)
 
+    @noKwargs
+    def func_warning(self, node, args, kwargs):
+        argstr = self.get_message_string_arg(node)
+        mlog.warning(argstr)
+
     @noKwargs
     def func_error(self, node, args, kwargs):
         self.validate_arguments(args, 1, [str])