🔇 Add silence_thread_safety_deprecation_warnings

This is provided as a temporary workaround, until dependant projects can
update their usage.

A future release will remove this backwards compatibility, but no sooner
than one year after the release that contains this deprecation warning.

lib/net/imap.rb

@@ -759,8 +759,18 @@ class << self
       alias default_imap_port default_port
       alias default_imaps_port default_tls_port
       alias default_ssl_port default_tls_port
+
+      # Set to true to silence deprecation warnings, e.g. from #responses.
+      # Defaults to false.
+      #
+      # These warnings are concerning thread-safety issues, so it is recommended
+      # to update other code and leave this value.  Deprecated usage will
+      # become errors regardless of this setting, so use this only temporarily.
+      attr_accessor :silence_thread_safety_deprecation_warnings
     end
 
+    self.silence_thread_safety_deprecation_warnings = false
+
     # Returns the initial greeting the server, an UntaggedResponse.
     attr_reader :greeting
 
@@ -2501,7 +2511,9 @@ def responses(type = nil)
       elsif type
         raise ArgumentError, "Pass a block or use #clear_responses"
       else
-        warn("DEPRECATED: pass a block or use #clear_responses", uplevel: 1)
+        unless IMAP.silence_thread_safety_deprecation_warnings
+          warn("DEPRECATED: pass a block or use #clear_responses", uplevel: 1)
+        end
         @responses
       end
     end

test/net/imap/test_imap.rb

@@ -1091,6 +1091,8 @@ def test_enable
 
   def test_responses
     with_fake_server do |server, imap|
+      original_silence = Net::IMAP.silence_thread_safety_deprecation_warnings
+      Net::IMAP.silence_thread_safety_deprecation_warnings = false
       # responses available before SELECT/EXAMINE
       assert_equal(%w[IMAP4REV1 NAMESPACE MOVE IDLE UTF8=ACCEPT],
                    imap.responses("CAPABILITY", &:last))
@@ -1108,6 +1110,15 @@ def test_responses
         assert_equal(%i[Answered Flagged Deleted Seen Draft],
                      imap.responses["FLAGS"]&.last)
       end
+      Net::IMAP.silence_thread_safety_deprecation_warnings = true
+      # TODO: assert_no_warn?
+      stderr = EnvUtil.verbose_warning {
+        assert_equal(%i[Answered Flagged Deleted Seen Draft],
+                     imap.responses["FLAGS"]&.last)
+      }
+      assert_empty stderr
+    ensure
+      Net::IMAP.silence_thread_safety_deprecation_warnings = original_silence
     end
   end