Enum SpanSource 

pub enum SpanSource {
    Slot(u32),
    MetaRef(u32),
    Known(Pos),
    SelfBase,
}

One candidate in a pending endpoint walk, in lark’s _pp_get_meta order.

Variants

Slot(u32)

An unfilled batching-callback slot: classify the drained host value at resolution (positioned/positionless Token or stamped Tree contribute; anything else skips).

MetaRef(u32)

A tree child whose own meta is unresolved — an index into the same meta arena the owning Meta::Pending lives in.

Known(Pos)

Engine-resolved contributor; terminates the walk (later raw children cannot be reached by lark’s scan).

SelfBase

The owning node’s OWN prior meta — an expand1 (?rule) collapse whose scan walked over the very node it is about to stamp (lark wraps PropagatePositions outside ExpandSingleChild, so res_meta is first_meta). Resolves against the pending meta’s base, NOT the meta arena: the arena slot is mid-overwrite, and reading it would look like a dependency cycle.

Keeping it as a source — rather than deleting it — is what makes the WALK faithful: a non-empty base CONTRIBUTES and terminates the walk (lark stops at that child and self-assigns), while an empty base SKIPS and the walk continues to later raw children. Deleting it always continued, which silently took the endpoint from a later child lark can never reach.

Scope of “faithful”: this models lark’s _pp_get_meta WALK, not the mutation ORDER of PropagatePositions.__call__, which applies the start update before scanning the reversed children — so lark can observe (and, on an empty base, crash on) a half-written res_meta. Both plans here resolve against the same base. The only observable difference is that lark raises AttributeError where we return the value its own algorithm prescribes; deliberately not emulated (see dev/post-milestone-1.md).

Trait Implementations

impl Clone for SpanSource

fn clone(&self) -> SpanSource

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for SpanSource

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Eq for SpanSource

impl PartialEq for SpanSource

fn eq(&self, other: &SpanSource) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for SpanSource