Support null encoding for rb_enc_interned_str_cstr

This commit adds support for a null pointer to be passed as the encoding
argument to rb_enc_interned_str_cstr.

When we implemented this function originally, we noticed that the CRuby
behavior did not match the header documentation, so we opted not to
support it. However, this has now been fixed in CRuby [1], so we can fix
it here as well.

[1]: ruby/ruby#10169

CHANGELOG.md

@@ -9,6 +9,8 @@ Bug fixes:
 
 Compatibility:
 
+* Allow null encoding pointer in `rb_enc_interned_str_cstr` (@thomasmarshall).
+
 Performance:
 
 Changes:

lib/cext/ABI_check.txt

@@ -1 +1 @@
-13
+14

spec/ruby/optional/capi/ext/string_spec.c

@@ -573,7 +573,7 @@ static VALUE string_spec_rb_str_unlocktmp(VALUE self, VALUE str) {
 }
 
 static VALUE string_spec_rb_enc_interned_str_cstr(VALUE self, VALUE str, VALUE enc) {
-  rb_encoding *e = rb_to_encoding(enc);
+  rb_encoding *e = NIL_P(enc) ? 0 : rb_to_encoding(enc);
   return rb_enc_interned_str_cstr(RSTRING_PTR(str), e);
 }
 

spec/ruby/optional/capi/string_spec.rb

@@ -1236,6 +1236,14 @@ def inspect
     it "returns the same string as String#-@" do
       @s.rb_enc_interned_str_cstr("hello", Encoding::UTF_8).should.equal?(-"hello")
     end
+
+    ruby_bug "#20322", ""..."3.4" do
+      it "uses the default encoding if encoding is null" do
+        str = "hello"
+        val = @s.rb_enc_interned_str_cstr(str, nil)
+        val.encoding.should == Encoding::ASCII_8BIT
+      end
+    end
   end
 
   describe "rb_str_to_interned_str" do

src/main/c/cext/string.c

@@ -439,7 +439,7 @@ long rb_str_coderange_scan_restartable(const char *s, const char *e, rb_encoding
 }
 
 VALUE rb_enc_interned_str_cstr(const char *ptr, rb_encoding *enc) {
-  VALUE str = rb_enc_str_new_cstr(ptr, enc);
+  VALUE str = rb_enc_str_new_cstr(ptr, enc ? enc : rb_ascii8bit_encoding());
   return rb_str_to_interned_str(str);
 }