Avoid array allocation for *nil, by not calling nil.to_a

The following method call:

```ruby
a(*nil)
```

A method call such as `a(*nil)` previously allocated an array, because
it calls `nil.to_a`, but I have determined this array allocation is
unnecessary.  The instructions in this case are:

```
0000 putself                                                          (   1)[Li]
0001 putnil
0002 splatarray                             false
0004 opt_send_without_block                 <calldata!mid:a, argc:1, ARGS_SPLAT|FCALL>
0006 leave
```

The method call uses `ARGS_SPLAT` without `ARGS_SPLAT_MUT`, so the
returned array doesn't need to be mutable.  I believe all cases where
`splatarray false` are used allow the returned object to be frozen,
since the `false` means to not duplicate the array.  The optimization
in this case is to have `splatarray false` push a shared empty frozen
array, instead of calling `nil.to_a` to return a newly allocated array.

There is a slightly backwards incompatibility with this optimization,
in that `nil.to_a` is not called.  However, I believe the new behavior
of `*nil` not calling `nil.to_a` is more consistent with how `**nil`
does not call `nil.to_hash`.  Also, so much Ruby code would break if
`nil.to_a` returned something different from the empty hash, that it's
difficult to imagine anyone actually doing that in real code, though
we have a few tests/specs for that.

I think it would be bad for consistency if `*nil` called `nil.to_a`
in some cases and not others, so this changes other cases to not
call `nil.to_a`:

For `[*nil]`, this uses `splatarray true`, which now allocates a
new array for a `nil` argument without calling `nil.to_a`.

For `[1, *nil]`, this uses `concattoarray`, which now returns
the first array if the second array is `nil`.

This updates the allocation tests to check that the array allocations
are avoided where possible.

Implements [Feature #21047]

1 files changed, 15 insertions, 4 deletions

diff --git a/spec/ruby/language/variables_spec.rb b/spec/ruby/language/variables_spec.rb
index bd993acb0f..eb080eea55 100644
--- a/spec/ruby/language/variables_spec.rb
+++ b/spec/ruby/language/variables_spec.rb
@@ -363,11 +363,22 @@ describe "Multiple assignment" do
       a.should == []
     end
 
-    it "calls #to_a to convert nil to an empty Array" do
-      nil.should_receive(:to_a).and_return([])
+    ruby_version_is "3.5" do
+      it "converts nil to empty array without calling a method" do
+        nil.should_not_receive(:to_a)
 
-      (*a = *nil).should == []
-      a.should == []
+        (*a = *nil).should == []
+        a.should == []
+      end
+    end
+
+    ruby_version_is ""..."3.5"  do
+      it "calls #to_a to convert nil to an empty Array" do
+        nil.should_receive(:to_a).and_return([])
+
+        (*a = *nil).should == []
+        a.should == []
+      end
     end
 
     it "does not call #to_a on an Array" do