Recover from unavailable targets

[cocoa-xu]

Adding support for specifying how to recover from unavailable targets using the make_precompiler_unavailable_target config key (probably needs a shorter name for this). Allowed values are :compile and :ignore. Defaults to :compile.

It is also possible to pass in a 2-arity function to make_precompiler_unavailable_target: the first argument is the triplet of the unavailable target, and the second argument is a list that contains all available targets provided by the precompiler.

One use case is the dll_loader_helper, which only needs to be compiled on windows and ignores the compilation process for all other targets.

For instance (cocoa-xu/dll_loader_helper #8):

  def project do
    [
      ...,
      make_precompiler_unavailable_target: &unavailable_target/2,
      cc_precompiler: [
        only_listed_compilers: true,
        compilers: %{
          {:win32, :nt} => %{
            "x86_64-windows-msvc" => {"cl", "cl"}
          }
        }
      ]
    ]
  end

  defp unavailable_target("x86_64-windows-msvc", _), do: :compile
  defp unavailable_target(_, _), do: :ignore

[josevalim]

What if we make it a callback in the Precompiler that returns either :compile or :ignore. If no callback is specified, we use :compile?

[cocoa-xu]

I agree! I'll update the code soon. :)

[josevalim]

💚 💙 💜 💛 ❤️

[josevalim]

@cocoa-xu please let me know if it works in DLL helpers and, if it does, I will ship a new version!

[cocoa-xu]

Thanks @josevalim! I tested it with the dll_loader_helper on an x86_64 Windows machine, and it works as expected. :)