[Resolver] Show a helpful error message if the old resolver incorrectly…

[Resolver] Show a helpful error message if the old resolver incorrectly activated a   pre-release version that now leads to a version conflict

CHANGELOG.md

@@ -4,6 +4,15 @@ To install or update CocoaPods see this [guide](http://docs.cocoapods.org/guides
 
 To install release candidates run `[sudo] gem install cocoapods --pre`
 
+## Master
+
+##### Bug Fixes
+
+* Show a helpful error message if the old resolver incorrectly activated a
+  pre-release version that now leads to a version conflict.  
+  [Samuel Giddins](https://github.com/segiddins)
+
+
 ## 0.35.0.rc2
 
 ##### Enhancements

lib/cocoapods/resolver.rb

@@ -55,7 +55,7 @@ module Pod
       @activated = Molinillo::Resolver.new(self, self).resolve(dependencies, locked_dependencies)
       specs_by_target
     rescue Molinillo::ResolverError => e
-      raise Informative, e.message
+      handle_resolver_error(e)
     end
 
     # @return [Hash{Podfile::TargetDefinition => Array<Specification>}]
@@ -360,6 +360,36 @@ module Pod
       end
     end
 
+    # Handles errors that come out of a {Molinillo::Resolver}.
+    #
+    # @todo   The check for version conflicts coming from the {Lockfile}
+    #         requiring a pre-release version can be deleted for version 1.0,
+    #         as it is a migration step for Lockfiles coming from CocoaPods
+    #         versions before `0.35.0`.
+    #
+    # @return [void]
+    #
+    # @param  [Molinillo::ResolverError] error
+    #
+    def handle_resolver_error(error)
+      case error
+      when Molinillo::VersionConflict
+        error.conflicts.each do |name, conflict|
+          lockfile_reqs = conflict.requirements[name_for_locking_dependency_source]
+          if lockfile_reqs.last && lockfile_reqs.last.prerelease? && !conflict.existing
+            raise Informative, 'Due to the previous naïve CocoaPods resolver, ' \
+              "you were using a pre-release version of `#{name}`, " \
+              'without explicitly asking for a pre-release version, which now leads to a conflict. ' \
+              'Please decide to either use that pre-release version by adding the ' \
+              'version requirement to your Podfile ' \
+              "(e.g. `pod '#{name}', '#{lockfile_reqs.map(&:requirement).join("', '")}'`) " \
+              "or revert to a stable version by running `pod update #{name}`."
+          end
+        end
+      end
+      raise Informative, error.message
+    end
+
     # Returns the target-appropriate nodes that are `successors` of `node`,
     # rejecting those that are scoped by target platform and have incompatible
     # targets.

spec/unit/resolver_spec.rb

@@ -342,6 +342,21 @@ module Pod
         version.to_s.should == '1.4'
       end
 
+      it 'shows a helpful error message if the old resolver incorrectly ' \
+         'activated a pre-release version that now leads to a version ' \
+         'conflict' do
+        podfile = Podfile.new do
+          platform :ios, '8.0'
+          pod 'CocoaLumberjack'
+        end
+        locked_deps = dependency_graph_from_array([Dependency.new('CocoaLumberjack', '= 2.0.0-beta2')])
+        resolver = Resolver.new(config.sandbox, podfile, locked_deps, SourcesManager.all)
+        e = lambda { puts resolver.resolve.values.flatten }.should.raise Informative
+        e.message.should.match(/you were using a pre-release version of `CocoaLumberjack`/)
+        e.message.should.match(/`pod 'CocoaLumberjack', '= 2.0.0-beta2'`/)
+        e.message.should.match(/`pod update CocoaLumberjack`/)
+      end
+
       it 'consults all sources when finding a matching spec' do
         podfile = Podfile.new do
           platform :ios