When working on a Julia package, we can state that a certain dependency is not user-facing by adding it with ]add --extra Foo. This puts Foo in the [extras] section of the Project.toml.
However, while testing this I encountered a strange problem:
julia --project=.
(Bar) pkg> add --extra JuliaFormatter
julia> using JuliaFormatter
│ Package JuliaFormatter not found, but a package named JuliaFormatter is available from a registry.
│ Install package?
│ (Bar) pkg> add JuliaFormatter
└ (y/n/o) [y]:
How do I activate my project with it's extras enabled?
[extras] is useless on its own. You also have to specify [targets]. extras is used to specify dependencies for a subdirectory, where the package itself doesn't use these dependencies. In the example TOML below:
[extras]
Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
[targets]
test = ["Test"]
Test is specified as a dependency for the subdirectory, target, test/.
This is documented in https://pkgdocs.julialang.org/v1/creating-packages/#target-based-test-specific-dependencies.
You can specify arbitrarily many dependencies for the test and build targets:
[extras]
Aqua = "4c88cf16-eb10-579e-8560-4a9242c79595"
JET = "c3a54625-cd67-489e-a8e7-0a5a0ff4e31b"
Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
XUnit = "3e3c03f2-1a94-11e9-2981-050a4ca824ab"
[targets]
test = ["Aqua", "JET", "XUnit", "Test"]
but the feature isn't generic enough to support arbitrary targets. Refer to this feature request which describes precisely what you need, and this related feature proposal.