Function rustc_const_eval::util::aggregate::expand_aggregate

pub fn expand_aggregate<'tcx>(
    orig_lhs: Place<'tcx>,
    operands: impl Iterator<Item = (Operand<'tcx>, Ty<'tcx>)> + TrustedLen,
    kind: AggregateKind<'tcx>,
    source_info: SourceInfo,
    tcx: TyCtxt<'tcx>
) -> impl Iterator<Item = Statement<'tcx>> + TrustedLen

Expand lhs = Rvalue::Aggregate(kind, operands) into assignments to the fields.

Produces something like

(lhs as Variant).field0 = arg0;     // We only have a downcast if this is an enum
(lhs as Variant).field1 = arg1;
discriminant(lhs) = variant_index;  // If lhs is an enum or generator.