Given that reverse-depends is broken for packages only in proposed, best effort is:
$ apt rdepends '?source-package("^(rust-num|rust-num-rational)$")'
<librust-num+i128-dev>
<librust-num-dev>
<librust-num+num-bigint-dev>
<librust-num+rand-dev>
<librust-num+serde-dev>
<librust-num+std-dev>
<librust-num-rational+bigint-dev>
<librust-num-rational-dev>
<librust-num-rational+bigint-std-dev>
<librust-num-rational+default-dev>
<librust-num-rational+std-dev>
<librust-num-rational+i128-dev>
<librust-num-rational+serde-dev>
Given that reverse-depends is broken for packages only in proposed, best effort is: package( "^(rust- num|rust- num-rational) $")' num+i128- dev> num+num- bigint- dev> num+rand- dev> num+serde- dev> num+std- dev> num-rational+ bigint- dev> num-rational- dev> num-rational+ bigint- std-dev> num-rational+ default- dev> num-rational+ std-dev> num-rational+ i128-dev> num-rational+ serde-dev>
$ apt rdepends '?source-
<librust-
<librust-num-dev>
<librust-
<librust-
<librust-
<librust-
<librust-
<librust-
<librust-
<librust-
<librust-
<librust-
<librust-