Sidestep ICE in `FieldPlacement::count` by not calling it when count will not fit in host's usize.
Sidestep ICE in `FieldPlacement::count` by not calling it when count will not fit in host's usize.

aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust