Auto merge of #5003 - bundler:seg-warn-unused-deps, r=indirect

[Definition] Print a helpful warning when a dependency is unused on a…

…ny platform

Should help address the confusion caused by https://github.com/bundler/bundler/issues/5001

lib/bundler/definition.rb

@@ -808,8 +808,16 @@ def expand_dependencies(dependencies, remote = false)
       deps = []
       dependencies.each do |dep|
         dep = Dependency.new(dep, ">= 0") unless dep.respond_to?(:name)
-        next unless remote || dep.current_platform?
-        dep.gem_platforms(@platforms).each do |p|
+        next if !remote && !dep.current_platform?
+        platforms = dep.gem_platforms(@platforms)
+        if platforms.empty?
+          Bundler.ui.warn \
+            "The dependency #{dep} will be unused by any of the platforms Bundler is installing for. " \
+            "Bundler is installing for #{@platforms.join ", "} but the dependency " \
+            "is only for #{dep.platforms.map {|p| Dependency::PLATFORM_MAP[p] }.join ", "}. " \
+            "To add those platforms to the bundle, run `bundle lock --add-platform #{dep.platforms.join ", "}`."
+        end
+        platforms.each do |p|
           deps << DepProxy.new(dep, p) if remote || p == generic_local_platform
         end
       end

spec/install/gemfile/platform_spec.rb

@@ -184,7 +184,7 @@
 
     gemfile <<-G
       source "file://#{gem_repo1}"
-      gem "some_gem", platform: :rbx
+      gem "some_gem", :platform => :rbx
     G
 
     bundle "install --local"
@@ -204,6 +204,23 @@
     bundle "install --local"
     expect(out).not_to match(/Could not find gem 'some_gem/)
   end
+
+  it "prints a helpful warning when a dependency is unused on any platform" do
+    simulate_platform "ruby"
+    simulate_ruby_engine "ruby"
+
+    gemfile <<-G
+      source "file://#{gem_repo1}"
+
+      gem "rack", :platform => :jruby
+    G
+
+    bundle! "install"
+
+    expect(out).to include <<-O.strip
+The dependency #{Gem::Dependency.new("rack", ">= 0")} will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby but the dependency is only for java. To add those platforms to the bundle, run `bundle lock --add-platform jruby`.
+    O
+  end
 end
 
 describe "when a gem has no architecture" do