Fix warnings on the latest nightly

[taiki-e]

This fixes warnings due to new clippy lints added by rust-lang/rust-clippy#3766.

[taiki-e]

Oh, there also seem to be warnings due to incomplete_features became lint by rust-lang/rust#63095.

[taiki-e]

😕

$ rustup target add thumbv7m-none-eabi
info: downloading component 'rust-std' for 'thumbv7m-none-eabi'
error: component download failed for rust-std-thumbv7m-none-eabi
info: caused by: could not download file from 'https://static.rust-lang.org/dist/2019-08-02/rust-std-nightly-thumbv7m-none-eabi.tar.xz' to '/home/travis/.rustup/downloads/ac33d8fd5a4b68735a9762b26807ab273e8f155557002c5388dddd5e22335cb7.partial'
info: caused by: failed to make network request
info: caused by: https://static.rust-lang.org/dist/2019-08-02/rust-std-nightly-thumbv7m-none-eabi.tar.xz: timed out
The command "rustup target add thumbv7m-none-eabi" failed and exited with 1 during .

[cramertj]

Great, thanks!