-- `ys = +=xs item` must NOT mutate `xs`, even when `xs` is RC=1.
--
-- The RC=1 in-place fast path on `OP_LISTAPPEND` (VM) and `jit_listappend`
-- (Cranelift) only fires for the rebind shape `name = += name item` — the
-- pattern the compiler peephole emits for accumulator loops. When the source
-- and destination registers differ (`ys = += xs item`), we always allocate a
-- fresh list and copy. Otherwise we'd silently mutate the caller's source.
--
-- Mirror of the `OP_MSET` / `jit_mset_inplace` split landed in PR #249.
-- xs is a fresh list literal (RC=1 at the append). After the non-rebind
-- append, xs must still be the original three elements. Pre-fix, both VM and
-- Cranelift returned [1, 2, 3, 99].
preserve-source>L n;xs=[1,2,3];ys=+=xs 99;xs
-- The rebind shape is the accumulator pattern and must still grow in place
-- (amortised O(1) per push). This stresses the fast path stays available.
rebind-accumulator>n;xs=[];@i 0..50{xs=+=xs i};len xs
-- Both source and destination visible in one return. Length pair pins the
-- aliasing contract: xs must be untouched.
both-visible>t;xs=[1,2,3];ys=+=xs 99;fmt "{};{}" (len xs) (len ys)
-- run: preserve-source
-- out: [1, 2, 3]
-- run: rebind-accumulator
-- out: 50
-- run: both-visible
-- out: 3;4