Fix Itertools::flatten() warning
Fix Itertools::flatten() warning

Tracking issue: https://github.com/rust-lang/rust/issues/48919

inejge/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,rust-lang/rustup,inejge/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,nodakai/rustup.rs,rust-lang/rustup