Replace Ruby-based implementation of get_actual_encoding with a primitive.

There's no value in having this logic implemented in both Java and Ruby. `String#inspect` was also altered a bit to make its logic match MRI more closely.

Author: nirvdrum

src/main/java/org/truffleruby/core/encoding/EncodingNodes.java

@@ -474,6 +474,21 @@ public boolean isUnicode(DynamicObject encoding) {
 
     }
 
+    @Primitive(name = "get_actual_encoding", needsSelf = false)
+    public abstract static class GetActualEncodingPrimitiveNode extends PrimitiveArrayArgumentsNode {
+
+        @Specialization
+        public DynamicObject getActualEncoding(DynamicObject string,
+                @Cached("create()") GetActualEncodingNode getActualEncodingNode,
+                @Cached("create()") GetRubyEncodingNode getRubyEncodingNode) {
+            final Rope rope = StringOperations.rope(string);
+            final Encoding actualEncoding = getActualEncodingNode.execute(rope);
+
+            return getRubyEncodingNode.executeGetRubyEncoding(actualEncoding);
+        }
+
+    }
+
     // Port of MRI's `get_actual_encoding`.
     public abstract static class GetActualEncodingNode extends RubyBaseNode {
 

src/main/ruby/core/string.rb

@@ -550,23 +550,11 @@ def inspect
     ascii = enc.ascii_compatible?
     unicode = Truffle.invoke_primitive :encoding_is_unicode, enc
 
-    if unicode
-      if enc.equal? Encoding::UTF_16
-        a = getbyte 0
-        b = getbyte 1
-
-        unless (a == 0xfe and b == 0xff) or (a == 0xff and b == 0xfe)
-          unicode = false
-        end
-      elsif enc.equal? Encoding::UTF_32
-        a = getbyte 0
-        b = getbyte 1
-        c = getbyte 2
-        d = getbyte 3
-
-        unless (a == 0 and b == 0 and c == 0xfe and d == 0xfe) or (a == 0xff and b == 0xfe and c == 0 and d == 0)
-          unicode = false
-        end
+    actual_encoding = enc.dummy? ? Truffle.invoke_primitive(:get_actual_encoding, self) : enc
+    if actual_encoding != enc
+      enc = actual_encoding
+      if unicode
+        unicode = Truffle.invoke_primitive :encoding_is_unicode, enc
       end
     end