Macro find_trait_mut 

macro_rules! find_trait_mut {
    ($component:expr, $trait:ty) => { ... };
}

The borrowing rules for components are the standard rust rules: mutable references are exclusive references. But they apply to individual objects within a component so it’s possible to simultaneously get two mutable references to two different objects within a component but not two mutable references to the same object (this is checked at runtime).