Checking for substitutes

[Konrad Hinsen]

Hi Guix,

I wonder if there is any way to check if substitutes are available for a
given set of packages (e.g. a single package with its dependencies, or a
complete manifest).

The context is optimizing package updates: if I know everything I need
is available as a substitute, I can do an update immediately, but if big
packages need to be built locally, I prefer to wait until my computer is
idle.

Ideally, I'd like "guix package" to have an option –only-substitutes
that will stop if local builds are required, providing a list of the
packages that must be built.

Konrad.