Set sift=true only when PeekMut yields a mutable reference
Set sift=true only when PeekMut yields a mutable reference

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