Raise FrozenError when a frozen String passed as a destination buffer to Encoding::Convertor#primitive_convert method

Author: andrykonchin

CHANGELOG.md

@@ -29,6 +29,7 @@ Compatibility:
 * Add `Module#refinements` (#3039, @itarato).
 * Add `Refinement#refined_class` (#3039, @itarato).
 * Add `rb_hash_new_capa` function (#3039, @itarato).
+* Fix `Encoding::Converter#primitive_convert` and raise `FrozenError` when a destination buffer argument is frozen (@andrykonchin).
 
 Performance:
 

spec/ruby/core/encoding/converter/primitive_convert_spec.rb

@@ -14,6 +14,10 @@
     -> { @ec.primitive_convert("","") }.should_not raise_error
   end
 
+  it "raises FrozenError when the destination buffer is a frozen String" do
+    -> { @ec.primitive_convert("", "".freeze) }.should raise_error(FrozenError)
+  end
+
   it "accepts nil for the destination byte offset" do
     -> { @ec.primitive_convert("","", nil) }.should_not raise_error
   end

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

@@ -185,6 +185,8 @@ def primitive_convert(source, target, offset = nil, size = nil, options = 0)
       source = source ? StringValue(source) : +''
       target = StringValue(target)
 
+      Primitive.check_mutable_string target
+
       if Primitive.nil? offset
         offset = target.bytesize
       else