Suppress a "unused variable" warning.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Suppress warnings on 32bit platforms.

On 32bit platforms, int is the same as i32, so 0xffffffff is "out of
range." Annotating variables as u32 fixes the problems.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #14186 : omasanori/rust/suppress-warnings, r=alexcrichton