Remove indirection for OptionKey.as_root() et al.

Removes one function call and a few comparisons.

mesonbuild/options.py

@@ -252,19 +252,19 @@ def evolve(self,
                          subproject if subproject != _BAD_VALUE else self.subproject, # None is a valid value so it can'the default value in method declaration.
                          machine if machine is not None else self.machine)
 
-    def as_root(self) -> 'OptionKey':
+    def as_root(self) -> OptionKey:
         """Convenience method for key.as_root()."""
-        if self.subproject is None or self.subproject == '':
+        if not self.subproject:
             return self
-        return self.evolve(subproject='')
+        return OptionKey(self.name, subproject='', machine=self.machine)
 
-    def as_build(self) -> 'OptionKey':
+    def as_build(self) -> OptionKey:
         """Convenience method for key.as_build()."""
-        return self.evolve(machine=MachineChoice.BUILD)
+        return OptionKey(self.name, self.subproject, machine=MachineChoice.BUILD)
 
-    def as_host(self) -> 'OptionKey':
+    def as_host(self) -> OptionKey:
         """Convenience method for key.as_host()."""
-        return self.evolve(machine=MachineChoice.HOST)
+        return OptionKey(self.name, self.subproject, machine=MachineChoice.HOST)
 
     def has_module_prefix(self) -> bool:
         return '.' in self.name
@@ -833,7 +833,7 @@ def get_value_object_for(self, key: 'T.Union[OptionKey, str]') -> AnyOptionType:
             return potential
         else:
             if potential is None:
-                parent_key = key.evolve(subproject=None)
+                parent_key = OptionKey(key.name, subproject=None, machine=key.machine)
                 if parent_key not in self.options:
                     raise KeyError(f'Tried to access nonexistant project parent option {parent_key}.')
                 return self.options[parent_key]