Proc made by Hash#to_proc should be a lambda [Bug #12671]

Like `Symbol#to_proc` (f0b815dc670b61eba1daaa67a8613ac431d32b16)

proc.c

@@ -667,7 +667,7 @@ bind_location(VALUE bindval)
 }
 
 static VALUE
-cfunc_proc_new(VALUE klass, VALUE ifunc, int8_t is_lambda)
+cfunc_proc_new(VALUE klass, VALUE ifunc)
 {
     rb_proc_t *proc;
     cfunc_proc_t *sproc;
@@ -685,7 +685,7 @@ cfunc_proc_new(VALUE klass, VALUE ifunc, int8_t is_lambda)
 
     /* self? */
     RB_OBJ_WRITE(procval, &proc->block.as.captured.code.ifunc, ifunc);
-    proc->is_lambda = is_lambda;
+    proc->is_lambda = TRUE;
     return procval;
 }
 
@@ -736,14 +736,14 @@ MJIT_FUNC_EXPORTED VALUE
 rb_func_proc_new(rb_block_call_func_t func, VALUE val)
 {
     struct vm_ifunc *ifunc = rb_vm_ifunc_proc_new(func, (void *)val);
-    return cfunc_proc_new(rb_cProc, (VALUE)ifunc, 0);
+    return cfunc_proc_new(rb_cProc, (VALUE)ifunc);
 }
 
 MJIT_FUNC_EXPORTED VALUE
 rb_func_lambda_new(rb_block_call_func_t func, VALUE val, int min_argc, int max_argc)
 {
     struct vm_ifunc *ifunc = rb_vm_ifunc_new(func, (void *)val, min_argc, max_argc);
-    return cfunc_proc_new(rb_cProc, (VALUE)ifunc, 1);
+    return cfunc_proc_new(rb_cProc, (VALUE)ifunc);
 }
 
 static const char proc_without_block[] = "tried to create Proc object without a block";

spec/ruby/core/hash/to_proc_spec.rb

@@ -19,8 +19,16 @@ describe "Hash#to_proc" do
       @proc = @hash.to_proc
     end
 
-    it "is not a lambda" do
-      @proc.lambda?.should == false
+    ruby_version_is ""..."2.8" do
+      it "is not a lambda" do
+        @proc.lambda?.should == false
+      end
+    end
+
+    ruby_version_is "2.8" do
+      it "is a lambda" do
+        @proc.lambda?.should == true
+      end
     end
 
     it "raises ArgumentError if not passed exactly one argument" do

test/ruby/test_hash.rb

@@ -1627,6 +1627,8 @@ class TestHash < Test::Unit::TestCase
     }
 
     assert_equal([10, 20, 30], [1, 2, 3].map(&h))
+
+    assert_equal(true, h.to_proc.lambda?)
   end
 
   def test_transform_keys