Making it possible to set preferences for a non-installed package?

[oschulz]

This came up in the context of CUDA.jl:

JuliaGPU/CUDA.jl#1922 (comment)

To use a non-artifact/system-wide CUDA installation, the "official" way it to use

CUDA.set_runtime_version!("local")

but that results in a multi-GB download of the CUDA runtime artifacts, only to then switch to not using them. As an alternative, one can use

Preferences.set_preferences!(CUDA_Runtime_jll, "version" => "local")

But to do that, CUDA_Runtime_jll needs to be loaded first - since it's a low-level package, it's much better of in the [extras] section of the current project then in it's direct dependencies. But it its now a direct dependency, it can't be loaded directly, so we don't have access to the CUDA_Runtime_jll object. But what about using the UUID of CUDA_Runtime_jll? That'll would do the trick:

Preferences.set_preferences!(Base.UUID("76a88914-d11a-5bdc-97e0-2f5a05c973a2"), "version" => "local")

it's set's the preference and would add CUDA_Runtime_jll to [extras] automatically. But it's not that easy because what happens is

julia> Preferences.set_preferences!(Base.UUID("76a88914-d11a-5bdc-97e0-2f5a05c973a2"), "version" => "local")
ERROR: Cannot set preferences of an unknown package that is not loaded!

It does work, however, if CUDA_Runtime_jll is already in the [extras] section of the current project. So it's a bit of a chicken-and-egg problem, and the only way seems to be adding CUDA_Runtime_jll to [extras] manually. That makes it hard to provide simple set-up commands/scripts for systems to users, especially if they don't have much Julia experience yet (tutorial scenarios, etc.). Unfortunately there seems to be no user-accessible functionality in Pkg that adds a dependency to [extras] instead of [deps].

Maybe we could add allow

Preferences.set_preferences!("CUDA_Runtime_jll", "version" => "local")

as well? It could warn that the package hasn't been installed yet and add it to [extras] (and set the preference(s), of course).

Or maybe this is already possible in some other way with Preferences?

CC @vchuravy