internal/core/adt: detect closed lists as fatal errors

Tightening of error handling. This may eliminate some
disjunctions earlier, causing better error handling.

Issue #2244

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: I0b059969b2b272c903ed4e4eea368822da93d822
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/549658
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

cue/testdata/references/index.txtar

@@ -30,15 +30,15 @@ varNegativeIndex: {
 	a: [1, 2, 3][n]
 }
 -- out/eval/stats --
-Leaks:  15
+Leaks:  12
 Freed:  36
 Reused: 33
-Allocs: 18
-Retain: 15
+Allocs: 15
+Retain: 12
 
-Unifications: 43
-Conjuncts:    56
-Disjuncts:    48
+Unifications: 40
+Conjuncts:    50
+Disjuncts:    45
 -- out/eval --
 Errors:
 outOfBoundsDisjunction: invalid list index 1 (out of bounds):

cue/testdata/resolve/016_index.txtar

@@ -92,15 +92,15 @@ e7: def["b"]
   e7: 〈0;def〉["b"]
 }
 -- out/eval/stats --
-Leaks:  16
+Leaks:  10
 Freed:  32
 Reused: 26
-Allocs: 22
-Retain: 23
+Allocs: 16
+Retain: 16
 
-Unifications: 40
-Conjuncts:    61
-Disjuncts:    53
+Unifications: 34
+Conjuncts:    48
+Disjuncts:    46
 -- out/eval --
 Errors:
 c: invalid list index "3" (type string):

internal/core/adt/context.go

@@ -876,6 +876,9 @@ func (c *OpContext) lookup(x *Vertex, pos token.Pos, l Feature, state VertexStat
 		// inability to look up a field as "final", as it may resolve down the
 		// line.
 		permanent := x.status > Conjuncts
+		if m, _ := x.BaseValue.(*ListMarker); m != nil && !m.IsOpen {
+			permanent = true
+		}
 		if (state > Partial || permanent) && !x.Accept(c, l) {
 			code = 0
 		} else if hasCycle {