Share logic for write transcoding

Co-authored-by: Mike Dalessio <mike.dalessio@gmail.com>

lib/truffle/stringio.rb

@@ -280,12 +280,7 @@ def write(str)
     str = String(str)
     return 0 if str.empty?
 
-    if external_encoding &&
-       external_encoding != str.encoding &&
-       external_encoding != Encoding::BINARY &&
-       str.encoding != Encoding::BINARY
-      str = str.encode(external_encoding)
-    end
+    str = Truffle::IOOperations.write_transcoding(str, external_encoding)
 
     d = @__data__
     TruffleRuby.synchronized(d) do

src/main/ruby/truffleruby/core/io.rb

@@ -2324,11 +2324,7 @@ def write(*objects)
 
       ensure_open_and_writable
 
-      if external_encoding && external_encoding != string.encoding && external_encoding != Encoding::BINARY
-        unless string.ascii_only? && external_encoding.ascii_compatible?
-          string = string.encode(external_encoding)
-        end
-      end
+      string = Truffle::IOOperations.write_transcoding(string, external_encoding)
 
       count = Truffle::POSIX.write_string self, string, true
       bytes_written += count

src/main/ruby/truffleruby/core/truffle/io_operations.rb

@@ -72,6 +72,16 @@ def self.puts(io, *args)
       nil
     end
 
+    def self.write_transcoding(string, external_encoding)
+      if external_encoding && external_encoding != string.encoding &&
+         external_encoding != Encoding::BINARY && string.encoding != Encoding::BINARY &&
+         !(string.ascii_only? && external_encoding.ascii_compatible?)
+        string.encode(external_encoding)
+      else
+        string
+      end
+    end
+
     def self.dup2_with_cloexec(old_fd, new_fd)
       if new_fd < 3
         # STDIO should not be made close-on-exec. `dup2` clears the close-on-exec bit for the destination FD.