Missing settings are not validated

[oeoeaio]

Describe the bug

When the Setting::Loader is unable to determine the value of a particular setting it silently fails (does not raise an InvalidSettingsError). The setting value is only evaluated at runtime when some downstream component attempts to access it, and the exception raise is a Dry::Types::ConstraintError which makes no reference to the name of the offending/missing setting making it very hard to debug.

To Reproduce

Declare a settings provider that inherits from the settings provider source:

App.register_provider(:db_config, from: :dry_system, source: :settings) do
  settings do
    setting(:db_host, constructor: Types::String.constrained(filled: true))
  end
end

Then don't set a DB_HOST var in the environment.

Expected behavior

I get an InvalidSettingsError when the settings are loaded. Something like:

Could not load settings. The following settings were invalid: (Dry::System::ProviderSources::Settings::InvalidSettingsError)
db_host: nil violates constraints (type?(String, nil) AND filled?(nil) failed)

My environment

• Affects my production application: Not yet
• Ruby version: reproduced on 2.7.5 and 3.1.2
• OS: macOS