internal/core/adt: optimize slice

Dynamic structs are discarded after use, so if we are
taking a slice, we might as well repurpose the arcs.

This was discovered when fixing a bug when that was
exposed by this test.
   b: strings.Join(["t", "e", "s", "t"][1:], "")
in combination with an upcoming change that changes
to use for iteration from CompleteArcs to
CompleteArcsOnly.

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I9f57c403d703678cbf4d2a79d0b96baae923f425
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1202210
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>

Author: mpvl

internal/core/adt/expr.go

@@ -1125,6 +1125,13 @@ func (x *SliceExpr) evaluate(c *OpContext, state combinedFlags) Value {
 				})
 				return nil
 			}
+			if v.IsDynamic {
+				// If the list is dynamic, there is no need to recompute the
+				// arcs.
+				a.Label = label
+				n.Arcs = append(n.Arcs, a)
+				continue
+			}
 			arc := *a
 			arc.Parent = n
 			arc.Label = label