Better syntax for private constants

Constants are a bit of an odd ball. They don't respect the usual public/protected/private regions like methods do:

class C
  private

  SECRET = 123 # Not actually private!
end

This would need a separate private_constant :SECRET call.

Method def expressions evaluate to the name of the method that was defined, making this possible:

private def method; end

By comparison, constant definitions evaluate to their right hand side, so you can't just do this:

private_constant SECRET = 123 # TypeError: 123 is not a symbol nor a string

The clunky syntax of repeating the constant name every time you want to make it private, means that people don't actually do it in practice. Perhaps we can provide a better syntax for private constants?

Possible solution

Shopify's internal dev tool has a solution for this, using a scoped block, in which all constant definitions are made public. Example usage:

private_constants do
  SECRET = 123
  ANOTHER_CONST = 456
end

Possible implementation

# Mark all constants defined within the block as private
#: { -> void } -> void
def private_constants(&block)
  before = constants(false)
  yield
  private_constant(*(constants(false) - before))
end

Start by examining Ruby's existing constant visibility behavior and the proposed private_constants entry point in the issue. Compare the scoped-block proposal with the current private_constant API; done means the project has an agreed syntax and behavior for marking constants defined in the scope as private.