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, 2 insertions, 2 deletions

diff --git a/bootstraptest/test_literal.rb b/bootstraptest/test_literal.rb
index 7295f7a148..39e6527027 100644
--- a/bootstraptest/test_literal.rb
+++ b/bootstraptest/test_literal.rb
@@ -117,8 +117,8 @@ assert_equal '1',               'a = [obj = Object.new]; a.size'
 assert_equal 'true',            'a = [obj = Object.new]; a[0] == obj'
 assert_equal '5',               'a = [1,2,3]; a[1] = 5; a[1]'
 assert_equal 'bar',             '[*:foo];:bar'
-assert_equal '[1, 2]',          'def nil.to_a; [2]; end; [1, *nil]'
-assert_equal '[1, 2]',          'def nil.to_a; [1, 2]; end; [*nil]'
+assert_equal '[]',              'def nil.to_a; [1, 2]; end; [*nil]'
+assert_equal '[1]',             'def nil.to_a; [2]; end; [1, *nil]'
 assert_equal '[0, 1, {2 => 3}]', '[0, *[1], 2=>3]', "[ruby-dev:31592]"