Compare https://docs.rs/libc/0.2.182/libc/enum.c_void.html with https://doc.rust-lang.org/std/ffi/enum.c_void.html - the former has
Equivalent to C’s void type when used as a [pointer].
while the later has
Equivalent to C’s void type when used as a pointer.
The link should be fixed in the docs.rs documentation for the crate