RJIT: Workaround USE_RVARGC=0 CI

lib/ruby_vm/rjit/insn_compiler.rb

@@ -2734,7 +2734,8 @@ module RubyVM::RJIT
     # @param asm [RubyVM::RJIT::Assembler]
     def jit_rb_str_empty_p(jit, ctx, asm, argc, known_recv_class)
       # Assume same offset to len embedded or not so we can use one code path to read the length
-      assert_equal(C.RString.offsetof(:as, :heap, :len), C.RString.offsetof(:as, :embed, :len))
+      #assert_equal(C.RString.offsetof(:as, :heap, :len), C.RString.offsetof(:as, :embed, :len))
+      # `C.RString.offsetof(:as, :embed, :len)` doesn't work because of USE_RVARGC=0 CI
 
       recv_opnd = ctx.stack_pop(1)
       out_opnd = ctx.stack_push

rjit_c.rb

@@ -768,11 +768,6 @@ module RubyVM::RJIT # :nodoc: all
             shared: self.VALUE,
           ), Primitive.cexpr!("OFFSETOF(((struct RString *)NULL)->as.heap, aux)")],
         ),
-        embed: CType::Struct.new(
-          "", Primitive.cexpr!("SIZEOF(((struct RString *)NULL)->as.embed)"),
-          len: [CType::Immediate.parse("long"), Primitive.cexpr!("OFFSETOF(((struct RString *)NULL)->as.embed, len)")],
-          ary: [CType::Pointer.new { CType::Immediate.parse("char") }, Primitive.cexpr!("OFFSETOF(((struct RString *)NULL)->as.embed, ary)")],
-        ),
       ), Primitive.cexpr!("OFFSETOF((*((struct RString *)NULL)), as)")],
     )
   end

tool/rjit/bindgen.rb

@@ -606,6 +606,7 @@ generator = BindingGenerator.new(
   ],
   skip_fields: {
     'rb_execution_context_struct.machine': %w[regs], # differs between macOS and Linux
+    'RString.as': %w[embed], # doesn't compile on USE_RVARGC=0 CI
     rb_execution_context_struct: %w[method_missing_reason], # non-leading bit fields not supported
     rb_iseq_constant_body: %w[yjit_payload], # conditionally defined
     rb_thread_struct: %w[status locking_native_thread to_kill abort_on_exception report_on_exception pending_interrupt_queue_checked],