Error from `set_preferences!` after `activate --temp`

[mcabbott]

Following the method documented here https://juliadiff.org/ForwardDiff.jl/stable/user/advanced/#Fixing-NaN/Inf-Issues aiming to set preferences before loading a package, gives the following error:

(@v1.11) pkg> activate --temp
  Activating new project at `/var/folders/yq/4p2zwd614y59gszh7y9ypyhh0000gn/T/jl_g5YtBV`

julia> using Preferences, UUIDs

julia> set_preferences!(UUID("f6369f11-7733-5829-9624-2563aa707210"), "nansafe_mode" => true)
ERROR: Cannot set preferences of an unknown package that is not loaded!
Stacktrace:
 [1] error(s::String)
   @ Base ./error.jl:35
 [2] set_preferences!(u::UUID, prefs::Pair{String, Bool}; export_prefs::Bool, active_project_only::Bool, kwargs::@Kwargs{})
   @ Preferences ~/.julia/packages/Preferences/enLcN/src/Preferences.jl:281
 [3] set_preferences!(u::UUID, prefs::Pair{String, Bool})
   @ Preferences ~/.julia/packages/Preferences/enLcN/src/Preferences.jl:223
 [4] top-level scope
   @ REPL[3]:1

julia> using ForwardDiff

julia> sqrt(ForwardDiff.Dual(0.0, 0.0)) 
Dual{Nothing}(0.0,NaN)

Same error with a string, which literally contains what needs to be written to LocalPreferences.toml, no need to look up UUIDs:

(@v1.11) pkg> activate --temp
  Activating new project at `/var/folders/yq/4p2zwd614y59gszh7y9ypyhh0000gn/T/jl_VyTFmo`

julia> using Preferences

julia> set_preferences!("ForwardDiff", "nansafe_mode" => true)
ERROR: Cannot set preferences of an unknown package that is not loaded!

I'm not sure you regard this as a bug, but something like this seems the obvious way to try out some preference option with a package. By "try out" I mean I want a temporary environment so that I'm sure it is not making any changes globally which I'll have to figure out how to reverse later. I think that loading the package and then calling set_preferences!(ForwardDiff, "nansafe_mode" => true) can't do that, as it needs a restart, and then I will get a new temporary environment.

Is there a way? Can it be documented somewhere easy to find?

[mcabbott]

After much messing around, it emerges that you can write to a temporary environment... but you have to explicitly add packages to it, rather than rely on them being visible from the global environment.

(@v1.11) pkg> activate --temp
  Activating new project at `/var/folders/yq/4p2zwd614y59gszh7y9ypyhh0000gn/T/jl_XNB4Fb`

julia> using Preferences  # available from global environment, no problem

(jl_XNB4Fb) pkg> add ForwardDiff
   Resolving package versions...
    Updating `/private/var/folders/yq/4p2zwd614y59gszh7y9ypyhh0000gn/T/jl_XNB4Fb/Project.toml`
  [f6369f11] + ForwardDiff v1.0.1
...

julia> set_preferences!("ForwardDiff", "nansafe_mode" => true)

julia> using ForwardDiff  # also available from global environment, but that's not enough for set_preferences!

julia> sqrt(ForwardDiff.Dual(0.0, 0.0))  # this is now using the preference
Dual{Nothing}(0.0,0.0)

If you like this can be closed as a duplicate of #27 . But really elaborate docs aren't what's missing, the readme could just explain the most basic use.

Also:

• There is no validation of the preferences, e.g. set_preferences!("ForwardDiff", "junk" => true) runs fine. That's OK but why is it necessary to be strict about a package name? What's written to LocalPreferences.toml is just text.

• Perhaps the error message above should not say "package that is not loaded!" since that isn't the problem.