For my package to compile, I need rustc and cargo. How do i tell the package to install that, in order to have it for the ubuntu build process?

You must always declare whatever you need in your source package's Build-Depends field. Launchpad does not preinstall anything for you, aside from a minimal set of packages that are generally required for most builds. See this section of the Debian Policy Manual, which we follow:

  https://www.debian.org/doc/debian-policy/ch-source.html#s-pkg-relations

Ok thanks cjwatson,

I have added the Build-Depends line with rustc cargo debcargo

I can install 'debcargo' locally on my Linux Mint.
But the launchpad build system does not have it.

Is it possible to configure some other package source, to get debcargo?

`The following packages have unmet dependencies:
 sbuild-build-depends-main-dummy : Depends: debcargo but it is not installable
`
https://launchpadlibrarian.net/663457735/buildlog_ubuntu-bionic-amd64.grassfeeder_0.1.11_BUILDING.txt.gz

debcargo is in Ubuntu as of jammy, but not in bionic (see https://launchpad.net/ubuntu/+source/rust-debcargo).

By default, Launchpad will fetch build-dependencies from the corresponding series of Ubuntu and from your own PPA. You can use the "Edit PPA dependencies" option on your PPA to arrange for it to use build-dependencies from another PPA as well.

There's a search facility at the bottom of https://launchpad.net/ubuntu/+source/rust-debcargo which you can use to look for PPAs where somebody might have backported debcargo to bionic. The Launchpad team cannot help you with the specifics of this - we can't possibly be experts in everyone's build systems - but you can often make things work this way if you don't want to deal with backporting things you need yourself.