Adopt StringFlags

Author: andrykonchin

spec/truffle/parsing/fixtures/strings/encoding/when_non_ascii_characters.yaml

@@ -0,0 +1,14 @@
+subject: "String"
+description: "encoding / when there are ASCII and non-ASCII characters in a literal"
+notes: >
+  String may be forced to the BINARY (ASCII-8BIT) encoding sometimes
+focused_on_node: "org.truffleruby.language.literal.StringLiteralNode"
+ruby: |
+  # encoding: us-ascii
+  "\xFF"
+ast: |
+  StringLiteralNode
+      attributes:
+          encoding = ASCII-8BIT
+          flags = 1
+          tstring = \xFF

spec/truffle/parsing/fixtures/strings/encoding/when_utf8_characters.yaml

@@ -0,0 +1,14 @@
+subject: "String"
+description: "encoding / when there are UTF-8 characters in a literal"
+notes: >
+  String may be forced to the UTF-8 encoding sometimes
+focused_on_node: "org.truffleruby.language.literal.StringLiteralNode"
+ruby: |
+  # encoding: us-ascii
+  "\u{A3}"
+ast: |
+  StringLiteralNode
+      attributes:
+          encoding = UTF-8
+          flags = 1
+          tstring = £

src/main/java/org/truffleruby/parser/YARPTranslator.java

@@ -2711,13 +2711,23 @@ public RubyNode visitStatementsNode(Nodes.StatementsNode node) {
     @Override
     public RubyNode visitStringNode(Nodes.StringNode node) {
         final RubyNode rubyNode;
-        final TruffleString tstring = TStringUtils.fromByteArray(node.unescaped, sourceEncoding);
+        final RubyEncoding encoding;
+
+        if (node.isForcedUtf8Encoding()) {
+            encoding = Encodings.UTF_8;
+        } else if (node.isForcedBinaryEncoding()) {
+            encoding = Encodings.BINARY;
+        } else {
+            encoding = sourceEncoding;
+        }
+
+        final TruffleString tstring = TStringUtils.fromByteArray(node.unescaped, encoding);
 
         if (!node.isFrozen()) {
-            final TruffleString cachedTString = language.tstringCache.getTString(tstring, sourceEncoding);
-            rubyNode = new StringLiteralNode(cachedTString, sourceEncoding);
+            final TruffleString cachedTString = language.tstringCache.getTString(tstring, encoding);
+            rubyNode = new StringLiteralNode(cachedTString, encoding);
         } else {
-            final ImmutableRubyString frozenString = language.getFrozenStringLiteral(tstring, sourceEncoding);
+            final ImmutableRubyString frozenString = language.getFrozenStringLiteral(tstring, encoding);
             rubyNode = new FrozenStringLiteralNode(frozenString, FrozenStrings.EXPRESSION);
         }