Is everyone genuinely liking this!
This is, IMHO, not a good style.
Isn't something like this much clearer?
// Add `as_cstr()` to `NixPath` trait first
let some_or_null_cstr = |v| v.map(NixPath::as_cstr)
.unwrap_or(Ok(std::ptr::null()));
// `Option::or_null_cstr()` for `OptionᐸTᐳ`
// where `T: NixPath` would make this even better
let source_cstr = some_or_null_cstr(&source)?;
let target_cstr = target.as_cstr()?;
let fs_type_cstr = some_or_null_cstr(&fs_type)?;
let data_cstr = some_or_null_cstr(&data)?;
let res = unsafe { .. };
Edit: using alternative chars to circumvent broken Lemmy sanitization.