internal/core/adt: hacky alternative for cyclicReferences

We did not yet implement cyclicReferences for the new
evaluator. This mechanism is an important aspect for performance.
It is hard to implement for the new evaluator, at least for now,
as we want to retain the flexibility of not having to evaluate
a referenced node first. Maybe later that is okay.
We implement an approximate alternative. This may give some
spurious structural cycles.

The idea is also that once we have structure sharing implemented,
we could probably implement a neater alternative to using
cyclicReferences.

Also, this change is blocking the implementation of structure
sharing. Altogether we think it is a good tradeoff to introduce
the new errors in favor of fixing the others.

Issue #2854
Issue #2884

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: If5f9b0d570cd8d122bd535e1fbc9b3ceafa848ba
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1191589
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cue/testdata/benchmarks/issue1684.txtar

@@ -33,15 +33,15 @@ out: #secrets & {
 	ONE: TWO: THREE: $secret: _id: "123"
 }
 -- out/evalalpha/stats --
-Leaks:  80025
-Freed:  18309
-Reused: 18309
-Allocs: 80025
+Leaks:  2115
+Freed:  539
+Reused: 539
+Allocs: 2115
 Retain: 0
 
-Unifications: 19610
-Conjuncts:    161517
-Disjuncts:    32694
+Unifications: 614
+Conjuncts:    5842
+Disjuncts:    846
 -- out/eval --
 (struct){
   #Secret: (#struct){
@@ -85,18 +85,18 @@ diff old new
 -Freed:  1064043
 -Reused: 1063992
 -Allocs: 51
-+Leaks:  80025
-+Freed:  18309
-+Reused: 18309
-+Allocs: 80025
++Leaks:  2115
++Freed:  539
++Reused: 539
++Allocs: 2115
  Retain: 0
 
 -Unifications: 791999
 -Conjuncts:    2479541
 -Disjuncts:    1064043
-+Unifications: 19610
-+Conjuncts:    161517
-+Disjuncts:    32694
++Unifications: 614
++Conjuncts:    5842
++Disjuncts:    846
 -- out/eval/stats --
 Leaks:  0
 Freed:  1064043

cue/testdata/benchmarks/listdedup.txtar

@@ -33,15 +33,15 @@ C: #steps: #Script & {mount: [B]}
 
 #ref: {a: 1} | {b: 2}
 -- out/evalalpha/stats --
-Leaks:  261590
-Freed:  35214
-Reused: 35213
-Allocs: 261591
+Leaks:  12448
+Freed:  1388
+Reused: 1387
+Allocs: 12449
 Retain: 0
 
-Unifications: 23412
-Conjuncts:    326798
-Disjuncts:    87912
+Unifications: 2716
+Conjuncts:    48270
+Disjuncts:    3272
 -- out/eval --
 (struct){
   A: (#struct){ |((#struct){
@@ -216,18 +216,18 @@ diff old new
 -Reused: 24051
 -Allocs: 45
 -Retain: 1
-+Leaks:  261590
-+Freed:  35214
-+Reused: 35213
-+Allocs: 261591
++Leaks:  12448
++Freed:  1388
++Reused: 1387
++Allocs: 12449
 +Retain: 0
 
 -Unifications: 18724
 -Conjuncts:    100730
 -Disjuncts:    24097
-+Unifications: 23412
-+Conjuncts:    326798
-+Disjuncts:    87912
++Unifications: 2716
++Conjuncts:    48270
++Disjuncts:    3272
 -- out/eval/stats --
 Leaks:  0
 Freed:  24096

cue/testdata/cycle/023_reentrance.txtar

@@ -76,10 +76,6 @@ Disjuncts:    0
 Errors:
 structural cycle:
     ./in.cue:3:25
-structural cycle:
-    ./in.cue:8:9
-structural cycle:
-    ./in.cue:8:38
 
 Result:
 (_|_){
@@ -100,28 +96,21 @@ Result:
     n: (int){ int }
   }
   fib1: (int){ 1 }
-  fib2: (int){ 1 }
+  fib2: (_|_){
+    // [structural cycle] structural cycle:
+    //     ./in.cue:3:25
+  }
   fib3: (_|_){
     // [structural cycle] structural cycle:
     //     ./in.cue:3:25
-    // structural cycle:
-    //     ./in.cue:8:38
   }
   fib7: (_|_){
     // [structural cycle] structural cycle:
     //     ./in.cue:3:25
-    // structural cycle:
-    //     ./in.cue:8:9
-    // structural cycle:
-    //     ./in.cue:8:38
   }
   fib12: (_|_){
     // [structural cycle] structural cycle:
     //     ./in.cue:3:25
-    // structural cycle:
-    //     ./in.cue:8:9
-    // structural cycle:
-    //     ./in.cue:8:38
   }
 }
 -- diff/-out/evalalpha/stats<==>+out/eval/stats --
@@ -139,7 +128,7 @@ diff old new
 +Reused: 0
 +Allocs: 70
 +Retain: 0
-
+ 
 -Unifications: 196
 -Conjuncts:    464
 -Disjuncts:    268
@@ -150,7 +139,18 @@ diff old new
 diff old new
 --- old
 +++ new
-@@ -16,10 +16,6 @@
+@@ -1,10 +1,6 @@
+ Errors:
+ structural cycle:
+     ./in.cue:3:25
+-structural cycle:
+-    ./in.cue:8:9
+-structural cycle:
+-    ./in.cue:8:38
+
+ Result:
+ (_|_){
+@@ -16,10 +12,6 @@
        //     ./in.cue:7:5
        //     ./in.cue:3:35
        //     ./in.cue:5:5
@@ -161,7 +161,7 @@ diff old new
      }
    }
    fib: (_|_){
-@@ -26,9 +22,6 @@
+@@ -26,33 +18,23 @@
      // [incomplete] fib: non-concrete value int in operand to >=:
      //     ./in.cue:7:5
      //     ./in.cue:5:5
@@ -171,6 +171,36 @@ diff old new
      n: (int){ int }
    }
    fib1: (int){ 1 }
+-  fib2: (int){ 1 }
++  fib2: (_|_){
++    // [structural cycle] structural cycle:
++    //     ./in.cue:3:25
++  }
+   fib3: (_|_){
+     // [structural cycle] structural cycle:
+     //     ./in.cue:3:25
+-    // structural cycle:
+-    //     ./in.cue:8:38
+   }
+   fib7: (_|_){
+     // [structural cycle] structural cycle:
+     //     ./in.cue:3:25
+-    // structural cycle:
+-    //     ./in.cue:8:9
+-    // structural cycle:
+-    //     ./in.cue:8:38
+   }
+   fib12: (_|_){
+     // [structural cycle] structural cycle:
+     //     ./in.cue:3:25
+-    // structural cycle:
+-    //     ./in.cue:8:9
+-    // structural cycle:
+-    //     ./in.cue:8:38
+   }
+ }
+-- diff/todo/p3 --
+Note that it is okay for fib2 to fail.
 -- out/eval/stats --
 Leaks:  16
 Freed:  180
@@ -181,8 +211,6 @@ Retain: 148
 Unifications: 196
 Conjuncts:    464
 Disjuncts:    268
--- diff/todo/p3 --
-Note that it is okay for fib2 to fail.
 -- out/eval --
 Errors:
 structural cycle: