internal/core/adt: avoid loops in debug printing

Printing adt values can cause infinite loops with structure
sharing enabled. This is especially the case for partially
evaluated Vertices, if a shared Vertex is printed for which
the structural cycle is not yet detected.

We use the recently added cycle detection mechanism to
whether a node is cyclic and/or shared in a Vertex.
This is a rather conservative method and may mark false
positives. This is okay as it serves the purpose of
avoiding printing cycles only and is not otherwise
involved in operations.

We could enabled printing with structure sharing in more
tests, but this would produce rather large diffs. We could
still consider this later, especially once the old
evaluator has been removed and the output from the new
evaluator become the new golden files.

Issue #2884

Signed-off-by: Marcel van Lohuizen <[email protected]>
Change-Id: Icc5641446c8b5b9a4d4f67b584ccaa7114939890
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193845
TryBot-Result: CUEcueckoo <[email protected]>
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>

Author: mpvl

cue/testdata/benchmarks/listdisj.txtar

@@ -64,26 +64,16 @@ Retain: 0
 Unifications: 15
 Conjuncts:    43
 Disjuncts:    4
--- out/eval --
+-- out/evalalpha --
 (struct){
   #T: (list){
     0: (string){ "d" }
   }
-  x: (list){
-    0: (string){ "d" }
-  }
-  y: (list){
-    0: (string){ "d" }
-  }
-  z: (list){
-    0: (string){ "d" }
-  }
-  v: (list){
-    0: (string){ "d" }
-  }
-  #X: (list){
-    0: (string){ "d" }
-  }
+  x: ~(#T)
+  y: ~(#T)
+  z: ~(#T)
+  v: ~(#T)
+  #X: ~(#T)
 }
 -- diff/-out/evalalpha/stats<==>+out/eval/stats --
 diff old new
@@ -106,6 +96,58 @@ diff old new
 +Unifications: 15
 +Conjuncts:    43
 +Disjuncts:    4
+-- diff/-out/evalalpha<==>+out/eval --
+diff old new
+--- old
++++ new
+@@ -2,19 +2,9 @@
+   #T: (list){
+     0: (string){ "d" }
+   }
+-  x: (list){
+-    0: (string){ "d" }
+-  }
+-  y: (list){
+-    0: (string){ "d" }
+-  }
+-  z: (list){
+-    0: (string){ "d" }
+-  }
+-  v: (list){
+-    0: (string){ "d" }
+-  }
+-  #X: (list){
+-    0: (string){ "d" }
+-  }
++  x: ~(#T)
++  y: ~(#T)
++  z: ~(#T)
++  v: ~(#T)
++  #X: ~(#T)
+ }
+-- out/eval --
+(struct){
+  #T: (list){
+    0: (string){ "d" }
+  }
+  x: (list){
+    0: (string){ "d" }
+  }
+  y: (list){
+    0: (string){ "d" }
+  }
+  z: (list){
+    0: (string){ "d" }
+  }
+  v: (list){
+    0: (string){ "d" }
+  }
+  #X: (list){
+    0: (string){ "d" }
+  }
+}
+-- diff/explanation --
+Differences fully due to rendered structure sharing.
 -- out/eval/stats --
 Leaks:  0
 Freed:  447

cue/testdata/cycle/constraints.txtar

@@ -664,15 +664,7 @@ Result:
     // [structural cycle]
     t1: (_|_){
       // [structural cycle]
-      a: (_|_){
-        // [structural cycle]
-        b: (_|_){
-          // [structural cycle]
-          b: (_|_){
-            // [structural cycle] selfTriggerCycle.t1.#T.b.b: structural cycle
-          }
-        }
-      }
+      a: ~(selfTriggerCycle.t1.#T)
       #T: (_|_){
         // [structural cycle]
         b: (_|_){
@@ -735,49 +727,16 @@ Result:
             link: (#struct){
               a: (#struct){
                 two: (#struct){
-                  link: (_|_){
-                    // [structural cycle]
-                    a: (_|_){
-                      // [structural cycle]
-                      b: (_|_){
-                        // [structural cycle]
-                        link: (_|_){
-                          // [structural cycle] selfTriggerCycle.issue1503.#T.a.b.link: structural cycle
-                        }
-                      }
-                    }
-                  }
+                  link: ~(selfTriggerCycle.issue1503.#T)
                 }
                 b: (#struct){
-                  link: (_|_){
-                    // [structural cycle]
-                    a: (_|_){
-                      // [structural cycle]
-                      b: (_|_){
-                        // [structural cycle]
-                        link: (_|_){
-                          // [structural cycle] selfTriggerCycle.issue1503.#T.a.b.link: structural cycle
-                        }
-                      }
-                    }
-                  }
+                  link: ~(selfTriggerCycle.issue1503.#T)
                 }
               }
             }
           }
           b: (#struct){
-            link: (_|_){
-              // [structural cycle]
-              a: (_|_){
-                // [structural cycle]
-                b: (_|_){
-                  // [structural cycle]
-                  link: (_|_){
-                    // [structural cycle] selfTriggerCycle.issue1503.#T.a.b.link: structural cycle
-                  }
-                }
-              }
-            }
+            link: ~(selfTriggerCycle.issue1503.#T)
           }
         }
       }
@@ -854,17 +813,21 @@ diff old new
          }
        }
      }
-@@ -222,6 +222,9 @@
+@@ -218,12 +218,7 @@
+     // [structural cycle]
+     t1: (_|_){
+       // [structural cycle]
+-      a: (_|_){
+-        // [structural cycle]
+-        b: (_|_){
+-          // [structural cycle]
+-        }
+-      }
++      a: ~(selfTriggerCycle.t1.#T)
+       #T: (_|_){
          // [structural cycle]
          b: (_|_){
-           // [structural cycle]
-+          b: (_|_){
-+            // [structural cycle] selfTriggerCycle.t1.#T.b.b: structural cycle
-+          }
-         }
-       }
-       #T: (_|_){
-@@ -280,22 +283,52 @@
+@@ -280,25 +275,22 @@
      }
      issue1503: (_|_){
        // [structural cycle]
@@ -884,55 +847,28 @@ diff old new
 -              // [structural cycle]
 -              a: (struct){
 -                two: (struct){
+-                }
+-              }
+-            }
 +      a: (#struct){
 +        a: (#struct){
 +          one: (#struct){
 +            link: (#struct){
 +              a: (#struct){
 +                two: (#struct){
-+                  link: (_|_){
-+                    // [structural cycle]
-+                    a: (_|_){
-+                      // [structural cycle]
-+                      b: (_|_){
-+                        // [structural cycle]
-+                        link: (_|_){
-+                          // [structural cycle] selfTriggerCycle.issue1503.#T.a.b.link: structural cycle
-+                        }
-+                      }
-+                    }
-+                  }
++                  link: ~(selfTriggerCycle.issue1503.#T)
 +                }
 +                b: (#struct){
-+                  link: (_|_){
-+                    // [structural cycle]
-+                    a: (_|_){
-+                      // [structural cycle]
-+                      b: (_|_){
-+                        // [structural cycle]
-+                        link: (_|_){
-+                          // [structural cycle] selfTriggerCycle.issue1503.#T.a.b.link: structural cycle
-+                        }
-+                      }
-+                    }
-+                  }
++                  link: ~(selfTriggerCycle.issue1503.#T)
 +                }
 +              }
 +            }
 +          }
 +          b: (#struct){
-+            link: (_|_){
-+              // [structural cycle]
-+              a: (_|_){
-+                // [structural cycle]
-+                b: (_|_){
-+                  // [structural cycle]
-+                  link: (_|_){
-+                    // [structural cycle] selfTriggerCycle.issue1503.#T.a.b.link: structural cycle
-+                  }
-                 }
-               }
-             }
++            link: ~(selfTriggerCycle.issue1503.#T)
+           }
+         }
+       }
 -- diff/todo/p2 --
 issue1503: cycle detected too late (but not super late)
 selfTriggerCycle.t1.a.b.b: cycle detected slightly too late