A field cannot be moved out of a Drop type, even where its destructor would not need it

GAP-OWNERSHIP-003

Moving a field out of a Drop type is always rejected; row-bounded Drop dispatch would allow it where the destructor does not need that field.

Lifecycle

planned

Owner

language

Scope

reference/spec/ownership.md#spec.ownership.partial-moves.legality-2

Origin

metel-core#1235 conversion of the Planned for notes in reference/spec/ownership.md (RFC-0137 §5)

Affects · linked records

spec.ownership.partial-moves.legality-2language spec rule

spec.ownership.drop-dispatch-against-a-narrowed-residual.legality-1language spec rule

RFC-0137rfc

gapKnown gaps ↗

Gap

The Language Spec bans moving a field out of a Drop type unconditionally (legality-2), so a destructor never runs against a value whose fields have been narrowed away. RFC-0137 §5 replaces the ban with row-bounded Drop dispatch: a Drop impl's required field set is the residual row its drop receiver declares, and a move is allowed when the remaining row still covers it. That needs the narrowed drop receiver (RFC-0109). The section "Drop dispatch against a narrowed residual" specifies the mechanism; until it is built the ban is enforced exactly as stated.

Impact

Code that wants to take one field of a Drop type and let the rest be dropped later must restructure, for example by moving the whole value.

Affects

• spec.ownership.partial-moves.legality-2
• spec.ownership.drop-dispatch-against-a-narrowed-residual.legality-1
• RFC-0137

Resolution

Planned for v0.16.0, tracked as metel-core#949 (RFC-0137 §5).