tco_test — tail call optimization

Two functions that would blow the stack in any non-TCO interpreter. count_down recurses 1,000,000 deep; sum_acc does the same with an accumulator. Rail compiles these to tight loops with a subs and a b.gt — no stack growth.

Source (examples/tco_test.rail):

-- Tail call optimization test
-- These would stack overflow without TCO

count_down : i32 -> i32
count_down n =
  if n == 0 then 0 else count_down (n - 1)

sum_acc : i32 -> i32 -> i32
sum_acc n acc =
  if n == 0 then acc else sum_acc (n - 1) (acc + n)

main =
  let _ = print (count_down 1000000)
  let _ = print (sum_acc 1000000 0)
  0

Run:

./rail_native run examples/tco_test.rail

Output:

0
500000500000

count_down 1000000 reaches 0 and returns it. sum_acc 1000000 0 computes 1 + 2 + ... + 1,000,000 = 500,000,500,000. Both run in constant stack space.

The : i32 -> i32 line is a type-annotation comment hint — Rail's parser tolerates the syntax but the compiler infers types itself.