internal/core/adt: unshare once Pending arc is known

If an arc for a comprehension is added, that arc would
invalidate sharing if it would be certain that the comprehension
will yield a result. We do not generally know this, though.
Unset sharing as soon as an arc loses Pending status.

This also means that at the time we need to indirect
disjunctions, we cannot yet know whether arcs added by
comprehensions are pending or not. So we need to indirect
these two different types of indirections at different points
in the unification algorithm.

Note that issue990.txtar introduces a spurious structural cycle.
The structural cycle algorithm is known to be low-fidelity at
the moment, and as this fixes other issues, we can leave this
to be addressed in another CL. In fact, this issue is fixed
in an upcoming CL that is still WIP.

Issue #2884
Issue #2854

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: Ia7f7a31f861217ccf018f6d17c6d8fe30956a32f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1193674
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cue/testdata/comprehensions/pushdown.txtar

@@ -784,6 +784,9 @@ embed.fail4.p: field not allowed:
 fieldMismatch.a: cannot combine regular field "x" with 2:
     ./in.cue:139:7
     ./in.cue:137:3
+structShare.err1.x.d.e: field not allowed:
+    ./in.cue:578:12
+    ./in.cue:578:9
 
 Result:
 (_|_){
@@ -1280,17 +1283,31 @@ Result:
       }
     }
   }
-  structShare: (struct){
+  structShare: (_|_){
+    // [eval]
     ok1: (struct){
       a: (struct){
+        d: (struct){
+          e: (bool){ true }
+        }
       }
       E: (struct){
       }
     }
-    err1: (struct){
-      x: (struct){
+    err1: (_|_){
+      // [eval]
+      x: (_|_){
+        // [eval]
         #E: (#struct){
         }
+        d: (_|_){
+          // [eval]
+          e: (_|_){
+            // [eval] structShare.err1.x.d.e: field not allowed:
+            //     ./in.cue:578:12
+            //     ./in.cue:578:9
+          }
+        }
       }
     }
   }
@@ -1417,7 +1434,7 @@ Result:
 diff old new
 --- old
 +++ new
-@@ -1,29 +1,13 @@
+@@ -1,32 +1,19 @@
  Errors:
 +noStackOverflowStructCycle.#list.tail: structural cycle
 +noStackOverflowStructCycle.list.tail: structural cycle
@@ -1451,7 +1468,13 @@ diff old new
  fieldMismatch.a: cannot combine regular field "x" with 2:
      ./in.cue:139:7
      ./in.cue:137:3
-@@ -68,8 +52,8 @@
++structShare.err1.x.d.e: field not allowed:
++    ./in.cue:578:12
++    ./in.cue:578:9
+
+ Result:
+ (_|_){
+@@ -68,8 +55,8 @@
    }
    fail: (struct){
      a: (_|_){
@@ -1462,7 +1485,7 @@ diff old new
      }
    }
    embed: (_|_){
-@@ -78,10 +62,7 @@
+@@ -78,10 +65,7 @@
        // [eval]
        p: (_|_){
          // [eval] embed.fail1.p: field not allowed:
@@ -1474,7 +1497,7 @@ diff old new
          //     ./in.cue:46:4
          //     ./in.cue:49:9
        }
-@@ -102,10 +83,7 @@
+@@ -102,10 +86,7 @@
        // [eval]
        p: (_|_){
          // [eval] embed.fail4.p: field not allowed:
@@ -1486,15 +1509,15 @@ diff old new
          //     ./in.cue:74:4
          q: (int){ 1 }
        }
-@@ -130,7 +108,6 @@
+@@ -130,7 +111,6 @@
          // [incomplete] embed.incomplete7.p: non-concrete value int in operand to +:
          //     ./in.cue:93:6
          //     ./in.cue:94:6
 -        //     ./in.cue:95:3
        }
        q: (int){ int }
      }
-@@ -185,8 +162,7 @@
+@@ -185,8 +165,7 @@
        }
      }
    }
@@ -1504,7 +1527,7 @@ diff old new
      t1: (struct){
        #a: (_|_){
          // [incomplete] provideIncompleteSuccess.t1.#a: incomplete bool: bool:
-@@ -194,16 +170,15 @@
+@@ -194,16 +173,15 @@
          b: (bool){ bool }
        }
        x: (#struct){
@@ -1528,7 +1551,7 @@ diff old new
        #a: (#struct){
          c: (int){ 4 }
          b: (bool){ true }
-@@ -210,17 +185,8 @@
+@@ -210,17 +188,8 @@
        }
        #c: (#struct){
        }
@@ -1548,7 +1571,7 @@ diff old new
          }
          b: (bool){ true }
        }
-@@ -250,17 +216,22 @@
+@@ -250,17 +219,22 @@
    }
    cyclicError: (struct){
      a: (_|_){
@@ -1576,7 +1599,7 @@ diff old new
      }
    }
    midwayReferences: (struct){
-@@ -397,7 +368,7 @@
+@@ -397,7 +371,7 @@
        E: (#struct){
          e: (bool){ bool }
          f: (_|_){
@@ -1585,7 +1608,7 @@ diff old new
            //     ./in.cue:389:7
          }
        }
-@@ -417,17 +388,10 @@
+@@ -417,17 +391,10 @@
          }
        }
        E: (_|_){
@@ -1604,7 +1627,7 @@ diff old new
        }
      }
      derefDisj2: (struct){
-@@ -438,17 +402,10 @@
+@@ -438,17 +405,10 @@
          }
        }
        E: (_|_){
@@ -1623,48 +1646,18 @@ diff old new
        }
      }
      bulk1: (struct){
-@@ -550,33 +507,17 @@
-       }
-     }
-   }
--  structShare: (_|_){
--    // [eval]
-+  structShare: (struct){
-     ok1: (struct){
-       a: (struct){
--        d: (struct){
--          e: (bool){ true }
--        }
--      }
--      E: (struct){
--      }
--    }
--    err1: (_|_){
--      // [eval]
--      x: (_|_){
--        // [eval]
-+      }
-+      E: (struct){
-+      }
-+    }
-+    err1: (struct){
-+      x: (struct){
-         #E: (#struct){
-         }
--        d: (_|_){
--          // [eval]
--          e: (_|_){
--            // [eval] structShare.err1.x.d.e: field not allowed:
+@@ -571,9 +531,7 @@
+           // [eval]
+           e: (_|_){
+             // [eval] structShare.err1.x.d.e: field not allowed:
 -            //     ./in.cue:575:15
 -            //     ./in.cue:576:9
 -            //     ./in.cue:577:2
--            //     ./in.cue:578:9
--          }
--        }
-       }
-     }
-   }
-@@ -597,13 +538,13 @@
++            //     ./in.cue:578:12
+             //     ./in.cue:578:9
+           }
+         }
+@@ -597,13 +555,13 @@
        }
        envs: (struct){
          e1: (#struct){
@@ -1682,7 +1675,7 @@ diff old new
          }
        }
      }
-@@ -625,7 +566,7 @@
+@@ -625,7 +583,7 @@
        y: (int){ 1 }
      }
      a: (struct){
@@ -1691,7 +1684,7 @@ diff old new
      }
    }
    nestedWithEmbeddingOK: (struct){
-@@ -640,9 +581,7 @@
+@@ -640,9 +598,7 @@
      _c: (struct){
        y: (int){ 1 }
      }