A mutated capture first assigned inside a loop body miscompiles: the loop hoist declares it as a plain value, the assignment writes through it as a cell

[nicolas-maman]

Repro

run(cb: fn) { cb() }

main() {
    seen = 0
    j = 0
    while j < 20 {
        local = j
        run() callback { local = local * 2 }
        seen = seen + local
        j = j + 1
    }
    println("${seen}")
}

error: invalid type argument of unary '*' (have 'int')
  local = j
error: passing argument 1 of '_aether_make_closure_0' makes pointer from integer without a cast

Verified on main (0.670.0) and on the #2019 branch — the two schemes for the cell differ, the miscompile is the same.

What happens

local is assigned inside the closure, so it is a promoted capture and must live in a heap cell (int* local). But its first assignment is inside a while body, and hoist_loop_vars (compiler/codegen/codegen_stmt.c) pre-declares every variable first assigned in a loop body at the top of the body as a plain value:

        int local;          // the hoist
*local = j;                 // the first assignment, now seen as a reassignment through the cell

The hoist marks the name declared, so the first-assignment path — the one that would have emitted the cell declaration — takes the "already declared, write through the pointer" branch instead, against an int.

Fix shape

hoist_loop_vars (and hoist_if_branch_vars, which has the same job for if arms) must not pre-declare a promoted capture as a value. Either skip promoted names there — the in-body first assignment then declares the cell where it is assigned, with its scope-exit release — or hoist the cell itself, which needs the per-iteration allocate/release to still land inside the body. Skipping is the smaller and the correct one: a cell is per-iteration state, and the body already knows how to declare one.

Needs a regression test that runs the loop above and checks 380, and the same for an if arm.

Found while writing tests/regression/test_capture_cell_lifetime.ae for #2019; that test leaves this shape out so the two do not block each other.