Function insert_cast_fixups 

pub fn insert_cast_fixups(
    graph: &mut Graph,
    casts: &[(NodeId, NodeId, DType)],
) -> usize

Insert Op::Cast fixups that CHANGE a consumer’s input dtype — the sibling of insert_layout_fixups, for dtype rather than layout. Given precomputed (consumer, input, target_dtype) edges (an input whose dtype should be promoted so the consumer’s (op, per-operand-dtypes) key becomes servable), insert one Op::Cast(target) per distinct (input, target) — CSE-shared across consumers — and rewire each consumer’s edge to it.

Unlike Graph::rewrite_input (whose contract is a same-dtype redirect), this DELIBERATELY changes the consumer’s input dtype — that is the point: it is the graph-side half of the optimizer’s dtype-reconciliation pass (fuel_dispatch::optimize::insert_dtype_fixups), which decides WHICH edges to promote from backend kernel coverage. Edges apply in the given order (deterministic when the caller passes topo order). Returns the rewires applied.