Speed up bundler/setup by using the raw Gemfile.lock information without extra processing whenever possible

bundler/lib/bundler/definition.rb

@@ -235,6 +235,14 @@ def locked_dependencies
       @locked_deps.values
     end
 
+    def new_deps
+      @new_deps ||= @dependencies - locked_dependencies
+    end
+
+    def deleted_deps
+      @deleted_deps ||= locked_dependencies - @dependencies
+    end
+
     def specs_for(groups)
       return specs if groups.empty?
       deps = dependencies_for(groups)
@@ -259,8 +267,17 @@ def resolve
         Bundler.ui.debug "Frozen, using resolution from the lockfile"
         @locked_specs
       elsif !unlocking? && nothing_changed?
-        Bundler.ui.debug("Found no changes, using resolution from the lockfile")
-        SpecSet.new(filter_specs(@locked_specs, @dependencies.select {|dep| @locked_specs[dep].any? }))
+        if deleted_deps.any?
+          Bundler.ui.debug("Some dependencies were deleted, using a subset of the resolution from the lockfile")
+          SpecSet.new(filter_specs(@locked_specs, @dependencies - deleted_deps))
+        else
+          Bundler.ui.debug("Found no changes, using resolution from the lockfile")
+          if @locked_gems.may_include_redundant_platform_specific_gems?
+            SpecSet.new(filter_specs(@locked_specs, @dependencies))
+          else
+            @locked_specs
+          end
+        end
       else
         last_resolve = converge_locked_specs
         # Run a resolve against the locally available gems
@@ -359,9 +376,6 @@ def ensure_equivalent_gemfile_and_lockfile(explicit_flag = false)
       added.concat new_platforms.map {|p| "* platform: #{p}" }
       deleted.concat deleted_platforms.map {|p| "* platform: #{p}" }
 
-      new_deps = @dependencies - locked_dependencies
-      deleted_deps = locked_dependencies - @dependencies
-
       added.concat new_deps.map {|d| "* #{pretty_dep(d)}" } if new_deps.any?
       deleted.concat deleted_deps.map {|d| "* #{pretty_dep(d)}" } if deleted_deps.any?
 

bundler/lib/bundler/lockfile_parser.rb

@@ -93,6 +93,10 @@ def initialize(lockfile)
         "and then `bundle install` to generate a new lockfile."
     end
 
+    def may_include_redundant_platform_specific_gems?
+      bundler_version.nil? || bundler_version < Gem::Version.new("1.16.2")
+    end
+
     private
 
     TYPES = {

bundler/spec/install/gemfile/platform_spec.rb

@@ -216,28 +216,28 @@
         pry
 
       BUNDLED WITH
-         #{Bundler::VERSION}
+         1.16.1
     L
 
     aggregate_failures do
       lockfile bad_lockfile
-      bundle :install
+      bundle :install, :env => { "BUNDLER_VERSION" => Bundler::VERSION }
       expect(lockfile).to eq good_lockfile
 
       lockfile bad_lockfile
-      bundle :update, :all => true
+      bundle :update, :all => true, :env => { "BUNDLER_VERSION" => Bundler::VERSION }
       expect(lockfile).to eq good_lockfile
 
       lockfile bad_lockfile
-      bundle "update ffi"
+      bundle "update ffi", :env => { "BUNDLER_VERSION" => Bundler::VERSION }
       expect(lockfile).to eq good_lockfile
 
       lockfile bad_lockfile
-      bundle "update empyrean"
+      bundle "update empyrean", :env => { "BUNDLER_VERSION" => Bundler::VERSION }
       expect(lockfile).to eq good_lockfile
 
       lockfile bad_lockfile
-      bundle :lock
+      bundle :lock, :env => { "BUNDLER_VERSION" => Bundler::VERSION }
       expect(lockfile).to eq good_lockfile
     end
   end

bundler/spec/runtime/setup_spec.rb

@@ -637,6 +637,22 @@ def clean_load_path(lp)
       expect(err).to be_empty
     end
 
+    it "doesn't re-resolve when deleting dependencies" do
+      install_gemfile <<-G
+        source "#{file_uri_for(gem_repo1)}"
+        gem "rack"
+        gem "actionpack"
+      G
+
+      install_gemfile <<-G, :verbose => true
+        source "#{file_uri_for(gem_repo1)}"
+        gem "rack"
+      G
+
+      expect(out).to include("Some dependencies were deleted, using a subset of the resolution from the lockfile")
+      expect(err).to be_empty
+    end
+
     it "remembers --without and does not include groups passed to Bundler.setup" do
       bundle "config set --local without rails"
       install_gemfile <<-G