YJIT: Abort expandarray optimization if method_missing is defined

Fixes: [Bug #21707]
[AW: rewrote comments]
Co-authored-by: Alan Wu <[email protected]>

Author: rwstauner

bootstraptest/test_yjit.rb

@@ -2680,6 +2680,22 @@ def expandarray_redefined_nilclass
   expandarray_redefined_nilclass
 }
 
+assert_equal 'not_array', %q{
+  def expandarray_not_array(obj)
+    a, = obj
+    a
+  end
+
+  obj = Object.new
+  def obj.method_missing(m, *args, &block)
+    return [:not_array] if m == :to_ary
+    super
+  end
+
+  expandarray_not_array(obj)
+  expandarray_not_array(obj)
+}
+
 assert_equal '[1, 2, nil]', %q{
   def expandarray_rhs_too_small
     a, b, c = [1, 2]

yjit/src/codegen.rs

@@ -2258,7 +2258,8 @@ fn gen_expandarray(
 
     let comptime_recv = jit.peek_at_stack(&asm.ctx, 0);
 
-    // If the comptime receiver is not an array
+    // If the comptime receiver is not an array, speculate for when the `rb_check_array_type()`
+    // conversion returns nil and without side-effects (e.g. arbitrary method calls).
     if !unsafe { RB_TYPE_P(comptime_recv, RUBY_T_ARRAY) } {
         // at compile time, ensure to_ary is not defined
         let target_cme = unsafe { rb_callable_method_entry_or_negative(comptime_recv.class_of(), ID!(to_ary)) };
@@ -2270,6 +2271,13 @@ fn gen_expandarray(
             return None;
         }
 
+        // Bail when method_missing is defined to avoid generating code to call it.
+        // Also, for simplicity, bail when BasicObject#method_missing has been removed.
+        if !assume_method_basic_definition(jit, asm, comptime_recv.class_of(), ID!(method_missing)) {
+            gen_counter_incr(jit, asm, Counter::expandarray_method_missing);
+            return None;
+        }
+
         // invalidate compile block if to_ary is later defined
         jit.assume_method_lookup_stable(asm, target_cme);
 

yjit/src/cruby.rs

@@ -816,6 +816,7 @@ pub(crate) mod ids {
     def_ids! {
         name: NULL               content: b""
         name: respond_to_missing content: b"respond_to_missing?"
+        name: method_missing     content: b"method_missing"
         name: to_ary             content: b"to_ary"
         name: to_s               content: b"to_s"
         name: eq                 content: b"=="

yjit/src/stats.rs

@@ -496,6 +496,7 @@ make_counters! {
     expandarray_postarg,
     expandarray_not_array,
     expandarray_to_ary,
+    expandarray_method_missing,
     expandarray_chain_max_depth,
 
     // getblockparam