A DEBUG-only structural check, the analog of ILInstruction.CheckInvariant, run
after every AST transform in RunTransforms. It recursively asserts that each
required (non-optional) single slot holds a child, every child's Parent points
back, the stored flattened index matches the slot position, and the runtime type
fits the slot, so a transform that corrupts the tree fails at that transform
instead of as a downstream output diff. CSharpSlotInfo gains an IsOptional flag
(emitted by the generator) so the check can tell required from optional slots.
Assisted-by: Claude:claude-opus-4-8:Claude Code
Nodes use nullable reference types now, so the generated null-object
node and its hasNullNode / NeedsNullNode / NullNodeBaseCtorParamCount
plumbing are dead. Drop them and reword the affected comments.
Introduce the successor to node.Role for child-slot identity: the generator
emits a CSharpSlotInfo per [Slot], exposed as node.Slot, plus a shared SlotKind
enum for the polymorphic "is this node in an embedded-statement / condition /
base-type slot?" comparisons a per-node identity cannot express. Migrate the
printer and transform position checks from node.Role to node.Slot and
node.Slot.Kind, and read identifier children and role-keyed writes through the
typed properties. Role is still present and is removed later; output is
unchanged.
Assisted-by: Claude:claude-opus-4-8:Claude Code
Siegfried Pammer