Appears to be trying to build for wasm, which doesn't work because the wasm32-unknown-unknown target isn't built by default in lang/rust. Fallout report: https://portsfallout.com/fallout/1215597/
It should be fixed after https://cgit.freebsd.org/ports/commit/?id=c13471c0db85fe9c4f3f9e8c9f803aa71c986068