A deeper move does not narrow the enclosing value's row
Gap
Row narrowing (RFC-0117 for anonymous records, RFC-0137 for structs) narrows a
binding's type when a field is moved directly out of it: after let x := o.a;,
o : Outer.{ d }. Only a depth-1 projection narrows. The type checker reads
moved_shallow_projections, which keeps just moves whose place has exactly one
projection, and record_move_of_place returns early for anything deeper. The
Language Spec states the boundary: "A record-typed field is moved as a
unit — a residual's row never carries a narrower type for a field it still
holds; narrowing a field of a field in place is RFC-0150's."
Reproduced against current develop:
struct Inner { b: String, c: String }
struct Outer { a: Inner, d: String }
fun take(o: Outer) -> i64 { 1 }
fun main() {
let o := Outer { a = Inner { b = "b", c = "c" }, d = "d" };
let x := o.a.b; // deeper move
take(o); // whole use after the partial move
}
Without flags this type-checks (exit 0). With --move-check it is rejected
(T0019 use of partially moved value o). For contrast, a depth-1 move
(let x := o.a; then take(o)) is always a type error: "a partially-moved
Outer (now Outer.{ d }) cannot be used where the whole Outer is required".
Impact
After a deeper partial move the value is still typed as whole, so passing it (or
its enclosing field) where the whole type is required is not caught by ordinary
type checking; only the opt-in move checker reports it. That mirrors the rest of
the ownership model, which is enforced only under --move-check, but it means
the always-on narrowing does not cover nested paths.
Affects
Resolution
Planned: RFC-0150 (Nested Row Narrowing, 1-under-review) is tracked as
metel-core#900, milestone v0.18.1. When it is accepted and implemented, a move
of root.a.b narrows the row of a inside root's residual, and this gap
closes against that RFC.