diff --git a/kernel.rb b/kernel.rb
index e72e7f3f58..24a51310f5 100644
--- a/kernel.rb
+++ b/kernel.rb
@@ -16,7 +16,7 @@ module Kernel
   #++
   #
   def class
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_obj_class(self)'
   end
 
@@ -65,7 +65,7 @@ module Kernel
   #++
   #
   def frozen?
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_obj_frozen_p(self)'
   end
 
diff --git a/numeric.rb b/numeric.rb
index 95e33a9403..23063aefad 100644
--- a/numeric.rb
+++ b/numeric.rb
@@ -86,7 +86,7 @@ class Integer
   #
   # Returns +int+, negated.
   def -@
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_int_uminus(self)'
   end
 
@@ -102,7 +102,7 @@ class Integer
   #
   #   sprintf("%X", ~0x1122334455)    #=> "..FEEDDCCBBAA"
   def ~
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_int_comp(self)'
   end
 
@@ -117,7 +117,7 @@ class Integer
   #    12345.abs      #=> 12345
   #
   def abs
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_int_abs(self)'
   end
 
@@ -163,7 +163,7 @@ class Integer
   #      raise "overflow"
   #    end
   def bit_length
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_int_bit_length(self)'
   end
 
@@ -172,7 +172,7 @@ class Integer
   #
   #  Returns +true+ if +int+ is an even number.
   def even?
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_int_even_p(self)'
   end
 
@@ -191,7 +191,7 @@ class Integer
   #
   #  Returns +true+ if +int+ is an odd number.
   def odd?
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_int_odd_p(self)'
   end
 
@@ -226,7 +226,7 @@ class Integer
   #     (256**40 - 1).size   #=> 40
   #
   def size
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_int_size(self)'
   end
 
@@ -251,7 +251,7 @@ class Integer
   #
   # Returns +true+ if +int+ has a zero value.
   def zero?
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_int_zero_p(self)'
   end
 
@@ -316,12 +316,12 @@ class Float
   #     34.56.abs      #=> 34.56
   #
   def abs
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_float_abs(self)'
   end
 
   def magnitude
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_float_abs(self)'
   end
 
@@ -332,7 +332,7 @@ class Float
   # Returns +float+, negated.
   #
   def -@
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'rb_float_uminus(self)'
   end
 
@@ -343,7 +343,7 @@ class Float
   #  Returns +true+ if +float+ is 0.0.
   #
   def zero?
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'RBOOL(FLOAT_ZERO_P(self))'
   end
 
@@ -354,7 +354,7 @@ class Float
   #  Returns +true+ if +float+ is greater than 0.
   #
   def positive?
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'RBOOL(RFLOAT_VALUE(self) > 0.0)'
   end
 
@@ -365,7 +365,7 @@ class Float
   #  Returns +true+ if +float+ is less than 0.
   #
   def negative?
-    Primitive.attr! 'inline'
+    Primitive.attr! :inline
     Primitive.cexpr! 'RBOOL(RFLOAT_VALUE(self) < 0.0)'
   end
 
diff --git a/tool/mk_builtin_loader.rb b/tool/mk_builtin_loader.rb
index 112c82c8f4..dce76d1c5f 100644
--- a/tool/mk_builtin_loader.rb
+++ b/tool/mk_builtin_loader.rb
@@ -6,6 +6,7 @@ require_relative 'ruby_vm/helpers/c_escape'
 
 SUBLIBS = {}
 REQUIRED = {}
+BUILTIN_ATTRS = %w[inline]
 
 def string_literal(lit, str = [])
   while lit
@@ -25,6 +26,17 @@ def string_literal(lit, str = [])
   end
 end
 
+# e.g. [:symbol_literal, [:symbol, [:@ident, "inline", [19, 21]]]]
+def symbol_literal(lit)
+  symbol_literal, symbol_lit = lit
+  raise "#{lit.inspect} was not :symbol_literal" if symbol_literal != :symbol_literal
+  symbol, ident_lit = symbol_lit
+  raise "#{symbol_lit.inspect} was not :symbol" if symbol != :symbol
+  ident, symbol_name, = ident_lit
+  raise "#{ident.inspect} was not :@ident" if ident != :@ident
+  symbol_name
+end
+
 def inline_text argc, arg1
   raise "argc (#{argc}) of inline! should be 1" unless argc == 1
   arg1 = string_literal(arg1)
@@ -32,6 +44,15 @@ def inline_text argc, arg1
   arg1.join("").rstrip
 end
 
+def inline_attr(argc, arg1)
+  raise "argc (#{argc}) of attr! should be 1" unless argc == 1
+  attr = symbol_literal(arg1)
+  unless BUILTIN_ATTRS.include?(attr)
+    raise "attr (#{attr}) was not in: #{BUILTIN_ATTRS.join(', ')}"
+  end
+  attr
+end
+
 def make_cfunc_name inlines, name, lineno
   case name
   when /\[\]/
@@ -138,10 +159,7 @@ def collect_builtin base, tree, name, bs, inlines, locals = nil
         if /(.+)[\!\?]\z/ =~ func_name
           case $1
           when 'attr'
-            text = inline_text(argc, args.first)
-            if text != 'inline'
-              raise "Only 'inline' is allowed to be annotated (but got: '#{text}')"
-            end
+            text = inline_attr(argc, args.first)
             break
           when 'cstmt'
             text = inline_text argc, args.first