Fix File.extname to return unfrozen strings

Author: bjfish

spec/ruby/core/file/extname_spec.rb

@@ -12,6 +12,16 @@
     File.extname(".app.conf").should == ".conf"
   end
 
+  it "returns unfrozen strings" do
+    File.extname("foo.rb").frozen?.should == false
+    File.extname("/foo/bar.rb").frozen?.should == false
+    File.extname("/foo.rb/bar.c").frozen?.should == false
+    File.extname("bar").frozen?.should == false
+    File.extname(".bashrc").frozen?.should == false
+    File.extname("/foo.bar/baz").frozen?.should == false
+    File.extname(".app.conf").frozen?.should == false
+  end
+
   it "returns the extension for edge cases" do
     File.extname("").should ==  ""
     File.extname(".").should ==  ""

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

@@ -484,21 +484,21 @@ def self.extname(path)
     dot_idx = Primitive.find_string_reverse(path, '.', path_size)
 
     # No dots at all
-    return '' unless dot_idx
+    return +'' unless dot_idx
 
     slash_idx = Primitive.find_string_reverse(path, '/', path_size)
 
     # pretend there is / just to the left of the start of the string
     slash_idx ||= -1
 
     # no . in the last component of the path
-    return '' if dot_idx < slash_idx
+    return +'' if dot_idx < slash_idx
 
     # last component starts with a .
-    return '' if dot_idx == slash_idx + 1
+    return +'' if dot_idx == slash_idx + 1
 
     # last component ends with a .
-    return '' if dot_idx == path_size - 1
+    return +'' if dot_idx == path_size - 1
 
     path.byteslice(dot_idx, path_size - dot_idx)
   end