Use char_is_identifier_start() to check for valid method name and fix serialization

src/prism.c

@@ -11021,7 +11021,7 @@ parse_write(pm_parser_t *parser, pm_node_t *target, pm_token_t *operator, pm_nod
                     return target;
                 }
 
-                if (*call->message_loc.start == '_' || parser->encoding->alnum_char(call->message_loc.start, call->message_loc.end - call->message_loc.start)) {
+                if (char_is_identifier_start(parser, call->message_loc.start)) {
                     // When we get here, we have a method call, because it was
                     // previously marked as a method call but now we have an =. This
                     // looks like:

templates/lib/prism/serialize.rb.erb

@@ -211,9 +211,9 @@ module Prism
 
           constant =
             if start.nobits?(1 << 31)
-              input.byteslice(start, length).to_sym
+              input.byteslice(start, length).force_encoding(@encoding).to_sym
             else
-              serialized.byteslice(start & ((1 << 31) - 1), length).to_sym
+              serialized.byteslice(start & ((1 << 31) - 1), length).force_encoding(@encoding).to_sym
             end
 
           constant_pool[index] = constant

test/prism/fixtures/emoji_method_calls.txt

@@ -0,0 +1 @@
+foo.🌊 = 1

test/prism/parse_test.rb

@@ -87,7 +87,7 @@ def test_parse_lex_file
 
     relatives.each do |relative|
       # These fail on TruffleRuby due to a difference in Symbol#inspect: :测试 vs :"测试"
-      next if RUBY_ENGINE == "truffleruby" and %w[seattlerb/bug202.txt seattlerb/magic_encoding_comment.txt].include?(relative)
+      next if RUBY_ENGINE == "truffleruby" and %w[emoji_method_calls.txt seattlerb/bug202.txt seattlerb/magic_encoding_comment.txt].include?(relative)
 
       filepath = File.join(base, relative)
       snapshot = File.expand_path(File.join("snapshots", relative), __dir__)