Function keeps_its_value 

fn keeps_its_value(hir: &Hir<'_>, variable: VariableId) -> bool

Whether the variable cannot change between the delegation and the callback guard. An intervening call, including one inside a recursively delegated override, can reenter and mutate a state variable after the mint reads it but before the guard does. A local, a parameter, a constant or an immutable cannot be moved that way.