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.

Issue #2854
Issue #2884

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: If5f9b0d570cd8d122bd535e1fbc9b3ceafa848ba

cue/testdata/cycle/023_reentrance.txtar

@@ -76,10 +76,6 @@ Disjuncts:    268
 Errors:
 structural cycle:
     ./in.cue:3:25
-structural cycle:
-    ./in.cue:8:9
-structural cycle:
-    ./in.cue:8:38
 
 Result:
 (_|_){
@@ -100,35 +96,39 @@ 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<==>+out/eval --
 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
@@ -139,7 +139,7 @@ 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
@@ -149,6 +149,36 @@ 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 --
 Errors:
 structural cycle: