Fix compilation on rustc 1.20.0-nightly (067971139 2017-07-02)
Fix compilation on rustc 1.20.0-nightly (067971139 2017-07-02)

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