-- `b = +a suffix` must NOT mutate `a`, even when `a` is RC=1.
--
-- The RC=1 in-place fast path on `OP_ADD` / `OP_ADD_SS` (VM) and
-- `jit_add` / `jit_concat` (Cranelift) only fires for the rebind shape
-- `name = +name suffix` — the pattern the compiler peephole emits for
-- string-building accumulators. When the source and destination registers
-- differ (`b = +a suffix`), we always allocate a fresh string and copy.
-- Otherwise we'd silently mutate the caller's source.
--
-- Mirror of the `OP_LISTAPPEND` split landed in PR #250 and the
-- `OP_MSET` / `jit_mset_inplace` split landed in PR #249.
-- `a` is built by fmt (RC=1 at the concat). After the non-rebind concat,
-- `a` must still be the original "k1". Pre-fix, the VM returned nil (slot
-- nullified) and Cranelift returned "k1_x" (in-place mutation).
preserve-source>t;a=fmt "k{}" 1;b=+a "_x";a
-- The rebind shape is the accumulator pattern and must still grow in place
-- (amortised O(n), not O(n²)). This stresses the fast path stays available.
rebind-accumulator n:n>n;s="";@i 0..n{s=+s "x"};len s
-- Both source and dest visible in one return. Pins the aliasing contract:
-- `a` must be untouched.
both-visible>t;a=fmt "k{}" 1;b=+a "_x";fmt "{}|{}" a b
-- run: preserve-source
-- out: k1
-- run: rebind-accumulator 50
-- out: 50
-- run: both-visible
-- out: k1|k1_x