Function from_mut 

pub fn from_mut<'a, T: By<'a, Mut>>(by_mut: &'a mut T) -> T::Type

Safe, zero-cost cast from &'a mut T to <T as By<'a, Mut>>::Type.

Rust’s type system does not always know that &'a mut T is <T as By<'a, Mut>>::Type for all 'a. This function safely converts from the former to the latter.