[PRISM] Support US-ASCII symbols

prism_compile.c

@@ -216,6 +216,9 @@ parse_string_symbol(const pm_symbol_node_t *symbol, const pm_parser_t *parser)
     else if (symbol->base.flags & PM_SYMBOL_FLAGS_FORCED_BINARY_ENCODING) {
         encoding = "ASCII-8BIT";
     }
+    else if (symbol->base.flags & PM_SYMBOL_FLAGS_FORCED_US_ASCII_ENCODING) {
+        encoding = "US-ASCII";
+    }
     const uint8_t *start = pm_string_source(&symbol->unescaped);
     return parse_symbol(start, start + pm_string_length(&symbol->unescaped), encoding);
 }

test/ruby/test_compile_prism.rb

@@ -791,6 +791,12 @@ module Prism
         # -*- coding: us-ascii -*-
         :"\xff"
       RUBY
+
+      # Test US-ASCII symbol in a ASCII-8BIT file
+      assert_prism_eval(<<~'RUBY', raw: true)
+        # -*- coding: ascii-8bit -*-
+        :a
+      RUBY
     end
 
     def test_XStringNode