Warn users that bundle now display the help:

- In 31d67ec
  we enforced the new behaviour where running `bundle` no longer
  installs gems but displays the help.
  Users now have a way to configure their preferred default command using
  the `BUNDLE_DEFAULT_CLI_COMMAND` flag.

  With the preview of Ruby 4.0 now being released, some people will
  start to see this new change.

  The problem is that the previous behaviour had existed for like an
  eternity and we didn't warn users about this change in advance.
  I'd like to provide a deprecation/warning cycle because this is
  confusing users already and this breaks various CI setup that now
  needs to be changed immediately.

Author: Edouard-chin

bundler/lib/bundler/cli.rb

@@ -107,7 +107,22 @@ def cli_help
       shell.say
       self.class.send(:class_options_help, shell)
     end
-    default_task(Bundler.settings[:default_cli_command])
+
+    def self.default_command(meth = nil)
+      return super if meth
+
+      default_cli_command = Bundler.settings[:default_cli_command]
+      return default_cli_command if default_cli_command
+
+      Bundler.ui.warn(<<~MSG)
+        In the next version of Bundler, running `bundle` without argument will no longer run `bundle install`.
+        Instead, the `help` command will be displayed.
+
+        If you'd like to keep the previous behaviour please run `bundle config set default_cli_command install --global`.
+      MSG
+
+      "install"
+    end
 
     class_option "no-color", type: :boolean, desc: "Disable colorization in output"
     class_option "retry", type: :numeric, aliases: "-r", banner: "NUM",

bundler/lib/bundler/settings.rb

@@ -83,7 +83,6 @@ class Settings
       "BUNDLE_VERSION" => "lockfile",
       "BUNDLE_LOCKFILE_CHECKSUMS" => true,
       "BUNDLE_CACHE_ALL" => true,
-      "BUNDLE_DEFAULT_CLI_COMMAND" => "cli_help",
       "BUNDLE_PLUGINS" => true,
       "BUNDLE_GLOBAL_GEM_CACHE" => false,
       "BUNDLE_UPDATE_REQUIRES_ALL_FLAG" => false,

bundler/spec/bundler/cli_spec.rb

@@ -87,14 +87,10 @@ def out_with_macos_man_workaround
   end
 
   context "with no arguments" do
-    it "prints a concise help message by default" do
-      bundle ""
-      expect(err).to be_empty
-      expect(out).to include("Bundler version #{Bundler::VERSION}").
-        and include("\n\nBundler commands:\n\n").
-        and include("\n\n  Primary commands:\n").
-        and include("\n\n  Utilities:\n").
-        and include("\n\nOptions:\n")
+    it "installs and log a warning by default" do
+      bundle "", raise_on_error: false
+      expect(err).to include("running `bundle` without argument will no longer run `bundle install`.")
+      expect(err).to include("Could not locate Gemfile")
     end
 
     it "prints a concise help message when default_cli_command set to cli_help" do