[rubygems/rubygems] Undeprecate Gemfiles without a global source

After having a second look at this deprecation, the explanation that
we're giving does not make a lot of sense. When working only with local
gems, Bundler will indeed generate a different lockfile depending on
the latest installed version of each gem is at `bundle install` time.

That's the same situation that happens with remote sources: Bundler will
generate a different lockfile depending on the latest version of each
gem available remotely.

So, I don't think "a consistent lockfile not getting generated" is a
good motivation for deprecating this.

Also, this deprecation brings additional challenges, since for example,
it should arguably not get printed when using `bundle install --local`?

The original problem when this deprecation was introduced was an
incorrect message about a missing gem having been yanked.

So, I think a better solution is to, as long as we give proper error
messages when things go wrong, let users do what's best for them and
undo the deprecation.

https://github.com/rubygems/rubygems/commit/17499cb83f

lib/bundler/definition.rb

@@ -608,10 +608,20 @@ module Bundler
         missing_specs.each do |s|
           locked_gem = @locked_specs[s.name].last
           next if locked_gem.nil? || locked_gem.version != s.version || sources.local_mode?
-          raise GemNotFound, "Your bundle is locked to #{locked_gem} from #{locked_gem.source}, but that version can " \
-                             "no longer be found in that source. That means the author of #{locked_gem} has removed it. " \
-                             "You'll need to update your bundle to a version other than #{locked_gem} that hasn't been " \
-                             "removed in order to install."
+
+          message = if sources.implicit_global_source?
+            "Because your Gemfile specifies no global remote source, your bundle is locked to " \
+            "#{locked_gem} from #{locked_gem.source}. However, #{locked_gem} is not installed. You'll " \
+            "need to either add a global remote source to your Gemfile or make sure #{locked_gem} is " \
+            "available locally before rerunning Bundler."
+          else
+            "Your bundle is locked to #{locked_gem} from #{locked_gem.source}, but that version can " \
+            "no longer be found in that source. That means the author of #{locked_gem} has removed it. " \
+            "You'll need to update your bundle to a version other than #{locked_gem} that hasn't been " \
+            "removed in order to install."
+          end
+
+          raise GemNotFound, message
         end
 
         missing_specs_list = missing_specs.group_by(&:source).map do |source, missing_specs_for_source|

lib/bundler/dsl.rb

@@ -503,18 +503,7 @@ module Bundler
     end
 
     def check_rubygems_source_safety
-      if @sources.implicit_global_source?
-        implicit_global_source_warning
-      elsif @sources.aggregate_global_source?
-        multiple_global_source_warning
-      end
-    end
-
-    def implicit_global_source_warning
-      Bundler::SharedHelpers.major_deprecation 2, "This Gemfile does not include an explicit global source. " \
-        "Not using an explicit global source may result in a different lockfile being generated depending on " \
-        "the gems you have installed locally before bundler is run. " \
-        "Instead, define a global source in your Gemfile like this: source \"https://rubygems.org\"."
+      multiple_global_source_warning if @sources.aggregate_global_source?
     end
 
     def multiple_global_source_warning

spec/bundler/bundler/dsl_spec.rb

@@ -350,21 +350,4 @@ RSpec.describe Bundler::Dsl do
       end
     end
   end
-
-  describe "#check_primary_source_safety" do
-    context "when a global source is not defined implicitly" do
-      it "will raise a major deprecation warning" do
-        not_a_global_source = double("not-a-global-source", no_remotes?: true)
-        allow(Bundler::Source::Rubygems).to receive(:new).and_return(not_a_global_source)
-
-        warning = "This Gemfile does not include an explicit global source. " \
-          "Not using an explicit global source may result in a different lockfile being generated depending on " \
-          "the gems you have installed locally before bundler is run. " \
-          "Instead, define a global source in your Gemfile like this: source \"https://rubygems.org\"."
-        expect(Bundler::SharedHelpers).to receive(:major_deprecation).with(2, warning)
-
-        subject.check_primary_source_safety
-      end
-    end
-  end
 end

spec/bundler/commands/install_spec.rb

@@ -340,15 +340,36 @@ RSpec.describe "bundle install with gem sources" do
       expect(the_bundle).to include_gems "myrack 1.2", "activesupport 1.2.3"
     end
 
-    it "gives a useful error if no sources are set" do
+    it "gives useful errors if no global sources are set, and gems not installed locally, with and without a lockfile" do
       install_gemfile <<-G, raise_on_error: false
         gem "myrack"
       G
 
-      expect(err).to include("This Gemfile does not include an explicit global source. " \
-        "Not using an explicit global source may result in a different lockfile being generated depending on " \
-        "the gems you have installed locally before bundler is run. " \
-        "Instead, define a global source in your Gemfile like this: source \"https://rubygems.org\".")
+      expect(err).to eq("Could not find gem 'myrack' in locally installed gems.")
+
+      lockfile <<~L
+        GEM
+          specs:
+            myrack (1.0.0)
+
+        PLATFORMS
+          #{lockfile_platforms}
+
+        DEPENDENCIES
+          myrack
+
+        BUNDLED WITH
+           #{Bundler::VERSION}
+      L
+
+      bundle "install", raise_on_error: false
+
+      expect(err).to include(
+        "Because your Gemfile specifies no global remote source, your bundle is locked to " \
+        "myrack (1.0.0) from locally installed gems. However, myrack (1.0.0) is not installed. " \
+        "You'll need to either add a global remote source to your Gemfile or make sure myrack (1.0.0) " \
+        "is available locally before rerunning Bundler."
+      )
     end
 
     it "creates a Gemfile.lock on a blank Gemfile" do