LSP mode configurable from external source

The original LSP mode determination relied on the `--lsp` command-line argument,
making it hard to set the mode from 3rd party LSP(-like) tools.
This PR refines the API to resolve this issue. Additionally, as a side effect,
it improves the mocking part of spec.

However, the APIs are still experimental and not yet mature.

lib/rubocop/cop/base.rb

@@ -84,6 +84,24 @@ def self.support_autocorrect?
         false
       end
 
+      ### LSP
+
+      # This experimental feature has been under consideration for a while.
+      # @api private
+      def self.lsp_mode?
+        !!@lsp_mode
+      end
+
+      # This experimental feature has been under consideration for a while.
+      def self.enable_lsp_mode
+        @lsp_mode = true
+      end
+
+      # This experimental feature has been under consideration for a while.
+      def self.disable_lsp_mode
+        @lsp_mode = false
+      end
+
       ### Naming
 
       def self.badge
@@ -481,12 +499,6 @@ def range_for_original(range)
           range.end_pos + @current_offset
         )
       end
-
-      # This experimental feature has been under consideration for a while.
-      # @api private
-      def lsp_mode?
-        ARGV.include?('--lsp')
-      end
     end
   end
 end

lib/rubocop/cop/lint/syntax.rb

@@ -17,7 +17,7 @@ def on_other_file
         private
 
         def add_offense_from_diagnostic(diagnostic, ruby_version)
-          message = if lsp_mode?
+          message = if Base.lsp_mode?
                       diagnostic.message
                     else
                       "#{diagnostic.message}\n(Using Ruby #{ruby_version} parser; " \

lib/rubocop/lsp/server.rb

@@ -20,6 +20,8 @@ module Lsp
     # @api private
     class Server
       def initialize(config_store)
+        RuboCop::Cop::Base.enable_lsp_mode
+
         @reader = LanguageServer::Protocol::Transport::Io::Reader.new($stdin)
         @writer = LanguageServer::Protocol::Transport::Io::Writer.new($stdout)
         @runtime = RuboCop::Lsp::Runtime.new(config_store)

lib/rubocop/rspec/shared_contexts.rb

@@ -130,7 +130,11 @@ def source_range(range, buffer: source_buffer)
 
 RSpec.shared_context 'lsp mode' do
   before do
-    allow(cop).to receive(:lsp_mode?).and_return(true)
+    RuboCop::Cop::Base.enable_lsp_mode
+  end
+
+  after do
+    RuboCop::Cop::Base.disable_lsp_mode
   end
 end
 

spec/rubocop/lsp/server_spec.rb

@@ -5,6 +5,10 @@
 
   subject(:result) { run_server_on_requests(*requests) }
 
+  after do
+    RuboCop::Cop::Base.disable_lsp_mode
+  end
+
   let(:messages) { result[0] }
   let(:stderr) { result[1].string }