internal/core/adt: new cycle algorithm

V3 only partially implemented the V2 cycle algorithm.
Rather than implementing the rest, we came up with a
new algorithm that fits better with the depth-first nature
of V3. It is overall considerably simpler than the V2
algorithm.

This CL reflects a "mostly done" state. There are still
improvements to be made (e.g. inline and let fields), but
it is already in better shape that V3 was before. Further
fixes will be done in subsequent CLs.

Note that this algorith is much simpler. It no longer
needs:
- track individual reference points (although it
  still does so for debugging purposes, but it
  does not use it to influence outcome)
- track nodeContext.cyclicReferences
- have a special mechanism to detect and disable
  mutual references
- track depth of optional markers
- track depth of references
- additional finding of evidence of non-cyclic nodes
  in some cases

CHANGES IN TESTS

benchmarks/issue1684:
    more efficient at trimming disjunctions
comprehensions/errors:
    more efficient at trimming disjunctions
cycle/comprehenion:
    changes are strict improvements, even though they
    do not fix the underlying bugs
cycle/constraints:
    - both shorter and deeper cutoffs
cycle/evaluate:
    - reordering, mostly
cycle/inline_non_recursive:
    - FIXES
cycle/inline:
    - Minor improvements
    - Does not yet fix underlying bug
cycle/structural:
    - Some processing is now deeper
    - Some less deep
	- Some P0-level bugs now FIXED that were
	  somehow not listed in diff/todo/*.
eval/v0.7:
	- NEW BUG: notification mechanism bypassed,
	  it seems.
export/030:
    - performance improvements

Issue #2850

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: I280fcf9cb333735fcc1774f980bf34e7ed72c9a9
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201897
Reviewed-by: Matthew Sackman <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>

cue/testdata/benchmarks/issue1684.txtar

@@ -62,15 +62,15 @@ nestedClosed: passing: {
 	D: {id: {}} | {[string]: D}
 }
 -- out/evalalpha/stats --
-Leaks:  2533
-Freed:  612
-Reused: 612
-Allocs: 2533
+Leaks:  2135
+Freed:  658
+Reused: 658
+Allocs: 2135
 Retain: 0
 
-Unifications: 707
-Conjuncts:    5373
-Disjuncts:    1102
+Unifications: 475
+Conjuncts:    4781
+Disjuncts:    918
 -- out/evalalpha --
 (struct){
   #Secret: (#struct){
@@ -127,12 +127,6 @@ Disjuncts:    1102
           id: (struct){ |((struct){
               id: (struct){
               }
-            }, (struct){
-              id: (struct){ |((struct){
-                  id: (struct){
-                  }
-                }, (struct){
-                }) }
             }, (struct){
             }) }
         }, (struct){
@@ -149,28 +143,26 @@ diff old new
 -Freed:  1064333
 -Reused: 1064282
 -Allocs: 51
-+Leaks:  2533
-+Freed:  612
-+Reused: 612
-+Allocs: 2533
++Leaks:  2135
++Freed:  658
++Reused: 658
++Allocs: 2135
  Retain: 0
 
 -Unifications: 792123
 -Conjuncts:    2480117
 -Disjuncts:    1064333
-+Unifications: 707
-+Conjuncts:    5373
-+Disjuncts:    1102
++Unifications: 475
++Conjuncts:    4781
++Disjuncts:    918
 -- diff/-out/evalalpha<==>+out/eval --
 diff old new
 --- old
 +++ new
-@@ -54,35 +54,11 @@
-               id: (struct){
-               }
+@@ -56,36 +56,6 @@
              }, (struct){
--            }) }
--        }, (struct){
+             }) }
+         }, (struct){
 -          id: (struct){ |((struct){
 -              id: (struct){
 -              }
@@ -198,14 +190,14 @@ diff old new
 -          id: (struct){ |((struct){
 -              id: (struct){
 -              }
-+              id: (struct){ |((struct){
-+                  id: (struct){
-+                  }
-+                }, (struct){
-+                }) }
-             }, (struct){
-             }) }
-         }, (struct){
+-            }, (struct){
+-            }) }
+-        }, (struct){
+         }) }
+     }
+   }
+-- diff/explanation --
+New algorithm is better at trimming recursive disjunctions.
 -- out/eval/stats --
 Leaks:  0
 Freed:  1064333

cue/testdata/benchmarks/listdisj.txtar

@@ -64,17 +64,6 @@ Retain: 0
 Unifications: 15
 Conjuncts:    43
 Disjuncts:    4
--- out/evalalpha --
-(struct){
-  #T: (list){
-    0: (string){ "d" }
-  }
-  x: ~(#T)
-  y: ~(#T)
-  z: ~(#T)
-  v: ~(#T)
-  #X: ~(#T)
-}
 -- diff/-out/evalalpha/stats<==>+out/eval/stats --
 diff old new
 --- old
@@ -96,6 +85,17 @@ diff old new
 +Unifications: 15
 +Conjuncts:    43
 +Disjuncts:    4
+-- out/evalalpha --
+(struct){
+  #T: (list){
+    0: (string){ "d" }
+  }
+  x: ~(#T)
+  y: ~(#T)
+  z: ~(#T)
+  v: ~(#T)
+  #X: ~(#T)
+}
 -- diff/-out/evalalpha<==>+out/eval --
 diff old new
 --- old

cue/testdata/comprehensions/errors.txtar

@@ -68,9 +68,7 @@ Result:
   // [eval]
   circularIf: (struct){
     #list: (#struct){
-      tail: ((null|struct)){ |(*(null){ null }, (#struct){
-          tail: (null){ null }
-        }) }
+      tail: (null){ null }
     }
   }
   circularFor: (_|_){
@@ -116,7 +114,18 @@ diff old new
  circularFor.#list: cannot range over tail != null (found bool, want list or struct):
      ./in.cue:12:12
  intField: integer fields not supported:
-@@ -35,10 +34,6 @@
+@@ -13,9 +12,7 @@
+   // [eval]
+   circularIf: (struct){
+     #list: (#struct){
+-      tail: ((null|struct)){ |(*(null){ null }, (#struct){
+-          tail: (null){ null }
+-        }) }
++      tail: (null){ null }
+     }
+   }
+   circularFor: (_|_){
+@@ -35,10 +32,6 @@
    intField: (_|_){
      // [eval] intField: integer fields not supported:
      //     ./in.cue:27:4
@@ -127,7 +136,7 @@ diff old new
    }
    conflictRangingOverSelf: (_|_){
      // [eval]
-@@ -49,7 +44,6 @@
+@@ -49,7 +42,6 @@
          // [eval] conflictRangingOverSelf.x.age: conflicting values int and "age" (mismatched types int and string):
          //     ./in.cue:36:9
          //     ./in.cue:40:3