Debian Build-Depends on non-package binary

I am developing a Rust application and I want to provide packages for Debian. I have something similar to this configuration under debian/controls

Source: com.github.yourname.yourrepo
Section: x11
Priority: extra
Maintainer: Someone
Build-Depends: cargo,
               debhelper (>= 9),
               libgtk-3-dev,
               libglib2.0-dev
Standards-Version: 3.9.3

Package: com.github.yourname.yourrepo
Architecture: any
Depends: ${misc:Depends}, ${shlibs:Depends}
Description: SomeApp
Some app developed in Rust

But when I try to build it with debuild it complains that I don't have cargo. However, if I type cargo in my terminal it is present in the system. I guess the reason is that I didn't install the cargo package. And I don't want to force people to install it either, if they already have cargo. Is there a way to enforce having a dependency that is not a Debian package?