[tannevaled]

One line, and a Go zero value that means something

END { } desugars to at_exit { }, and the Block it built left SplatIndex
at Go's zero value. Zero is a perfectly good index, and this 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.

Nothing here crashed, which is why it lasted

A parser does not run what it builds, so the cost landed entirely in a consumer.
go-embedded-ruby's compiler believed the claim and emitted a rest-binding that
read past an empty environment:

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

An uncontained Go panic from ordinary Ruby — END has been in the language
since 1.0 — which for an embedder takes the host process down rather than
returning an error. go-embedded-ruby/ruby#805.

Verified end to end with a replace directive. With this one line:

	rbgo	MRI 4.0.7
END { puts "E" }	E	E
puts "main" + END	main E	main E
BEGIN + main + END	B main E	B main E

The test that matters is the invariant, not the case

Two of the three &ast.Block literals in parser.go already set SplatIndex —
the two that build their value from parsed parameters. The one that forgot
is the one with no parameters to parse, and that is the shape any future
desugaring will have.

So TestEveryBlockHasAConsistentSplatIndex parses a corpus of seventeen sources
and asserts, over every Block in every tree, that SplatIndex is -1 or a
valid index into Params.

• It walks by reflection rather than a type switch over the AST, so a node
  added later is covered without anyone remembering to extend the walker — not
  remembering is precisely the failure this guard exists to catch.
• It fails when the walk finds no Block at all, because a walk that read
  nothing is indistinguishable from a tree with no defects.

Both tests were checked in the reverse direction: reverting the one-line fix
turns both red, with the guard naming the block and its bad index.

🤖 Generated with Claude Code