rustup: fix build with rustc 1.31.0-nightly (cae6efc37 2018-10-27)
rustup: fix build with rustc 1.31.0-nightly (cae6efc37 2018-10-27)

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