rust: fix build errors on darwin
I'm not all _that_ versed in Rust, but I think the root cause is that
some constants are u16 rather than u32 on Darwin. I checked that the
code still compiles on the latest Ubuntu.
Differential Revision: https://phab.mercurial-scm.org/D11955
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true