[tannevaled]

Closes #805.

What it was

$ echo 'END { puts "E" }' > p.rb && rbgo p.rb
panic: runtime error: slice bounds out of range [:1] with capacity 0
$ echo $?
2

END { } has been in Ruby since 1.0. For a program this is a crash instead of
an answer; for an embedder it is worse — ruby.Run does not return an
error, it takes the host process down, and nothing the host writes around the
call prevents it.

A Go zero value that means something

END { } desugars to at_exit { }, and the Block the parser built left
SplatIndex at Go's zero value. Zero is a perfectly good index, and that
field's "no splat" value is -1:

SplatIndex int // index of the top-level *splat param in Params, or -1

So the block announced a *splat parameter at position 0 with no parameters to
hold it, and this compiler's rest-binding believed it and read past an empty
environment.

One line upstream: go-ruby-parser/parser#52, released as v0.13.1 — a
patch, as gorelease -base v0.13.0 reports, since no exported API moved.

	rbgo before	rbgo after	MRI 4.0.7
END { puts "E" }	panic, exit 2	E	E
puts "main" + END	panic	main E	main E
BEGIN + main + END	panic	B main E	B main E

Why the regression test lives here too

Not belt and braces — the defect was invisible in the AST. The parser's own
suite was green throughout, because a parser does not run what it builds: the
entire cost landed on the consumer that lowers a Block. A downgrade of this
dependency brings the crash back, and this is what notices.

It asserts the ordering as well as the absence of a crash: END and
at_exit handlers run LIFO, measured on both engines — MRI 4.0.7 and rbgo agree
on second registered then first registered. A test that only checked "did it
crash" would pass on a handler that never ran. BEGIN and at_exit sit in the
same test as controls, so a regression in any of the three is attributable to
the right half.

The README row goes in the same change

The row added this morning says END { } crashes the process. It did, and
this is the commit that stops it — so the row goes now. A document that is wrong
on the day it ships is precisely what #801 was about.

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