Simplify assert_assignable
Simplify assert_assignable

Assignments are no longer used to implicitly cast between &mut T and &T

graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust