[Installer] Improve error message when using Swift in static libs.

Now it includes the offending Pods so that the user can take action
to remove the Pods, if they don’t want to move to frameworks yet.

CHANGELOG.md

@@ -22,6 +22,12 @@ To install release candidates run `[sudo] gem install cocoapods --pre`
   is now a default plugin.  
   [Samuel Giddins](https://github.com/segiddins)
 
+* Improve the message shown when trying to use Swift Pods without frameworks.
+  Now it includes the offending Pods so that the user can take action to remove
+  the Pods, if they don’t want to move to frameworks yet.  
+  [#3830](https://github.com/CocoaPods/CocoaPods/pull/3830)
+  [Eloy Durán](https://github.com/alloy)
+
 ##### Bug Fixes
 
 * Ensure that the `prepare_command` is run even when skipping the download

lib/cocoapods/installer.rb

@@ -411,10 +411,13 @@ module Pod
         aggregate_target.user_build_configurations.keys.each do |config|
           pod_targets = aggregate_target.pod_targets_for_build_configuration(config)
 
-          if pod_targets.any?(&:uses_swift?)
+          swift_pods = pod_targets.select(&:uses_swift?)
+          unless swift_pods.empty?
             raise Informative, 'Pods written in Swift can only be integrated as frameworks; this ' \
               'feature is still in beta. Add `use_frameworks!` to your Podfile or target to opt ' \
-              'into using it.'
+              'into using it. ' \
+              "The Swift #{swift_pods.size == 1 ? 'Pod being used is' : 'Pods being used are'}: " +
+              swift_pods.map(&:name).to_sentence
           end
         end
       end