internal/core/adt: add test for dropped field issue

Currently, inline struct were reapproriated
by linking them into the destination struct
by setting the parent and label. However,
labels are used to look up fields. This resulted
in fields being dropped.

This test exposes that.

Issue #3601

Signed-off-by: Marcel van Lohuizen <mpvl@gmail.com>
Change-Id: Icdfd3e9edac458ffb08a3759a2fd350e3cfdf363
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1205231
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Matthew Sackman <matthew@cue.works>

cue/testdata/eval/sharing.txtar

@@ -10,6 +10,12 @@ issue3062: ok1: {
 	A: "a"
 }
 
+issue3601: ok1: {
+	Y: (X & __no_sharing).a
+	X: a: b: a.b
+	out: Y & __no_sharing
+}
+
 // Test debug facilities to turn of sharing.
 debug: {
 	sharingOn: {
@@ -32,15 +38,15 @@ debug: {
 	}
 }
 -- out/eval/stats --
-Leaks:  0
-Freed:  33
-Reused: 26
-Allocs: 7
-Retain: 7
+Leaks:  2
+Freed:  40
+Reused: 33
+Allocs: 9
+Retain: 10
 
-Unifications: 33
-Conjuncts:    49
-Disjuncts:    38
+Unifications: 42
+Conjuncts:    64
+Disjuncts:    48
 -- out/evalalpha --
 (struct){
   issue3062: (struct){
@@ -58,6 +64,24 @@ Disjuncts:    38
       A: (string){ "a" }
     }
   }
+  issue3601: (struct){
+    ok1: (struct){
+      Y: (struct){
+        b: (_){ _ }
+      }
+      X: (struct){
+        a: (struct){
+          b: (_){ _ }
+        }
+      }
+      out: (struct){
+        b: (_|_){
+          // [incomplete] issue3601.ok1.out.b: undefined field: a:
+          //     ./in.cue:12:11
+        }
+      }
+    }
+  }
   debug: (struct){
     sharingOn: (struct){
       a: ~(debug.sharingOn.b)
@@ -108,12 +132,42 @@ diff old new
    issue3062: (struct){
      ok1: (struct){
        #S: (string){ "a" }
-@@ -19,42 +14,32 @@
+@@ -19,12 +14,10 @@
        A: (string){ "a" }
      }
    }
+-  issue3601: (_|_){
+-    // [eval]
+-    ok1: (_|_){
+-      // [eval]
+-      Y: (_|_){
+-        // [eval] no sharing
++  issue3601: (struct){
++    ok1: (struct){
++      Y: (struct){
++        b: (_){ _ }
+       }
+       X: (struct){
+         a: (struct){
+@@ -31,47 +24,40 @@
+           b: (_){ _ }
+         }
+       }
+-      out: (_|_){
+-        // [eval] no sharing
+-      }
+-    }
+-  }
 -  debug: (_|_){
 -    // [eval]
++      out: (struct){
++        b: (_|_){
++          // [incomplete] issue3601.ok1.out.b: undefined field: a:
++          //     ./in.cue:12:11
++        }
++      }
++    }
++  }
 +  debug: (struct){
      sharingOn: (struct){
 -      a: (struct){
@@ -194,6 +248,23 @@ Result:
       A: (string){ "a" }
     }
   }
+  issue3601: (_|_){
+    // [eval]
+    ok1: (_|_){
+      // [eval]
+      Y: (_|_){
+        // [eval] no sharing
+      }
+      X: (struct){
+        a: (struct){
+          b: (_){ _ }
+        }
+      }
+      out: (_|_){
+        // [eval] no sharing
+      }
+    }
+  }
   debug: (_|_){
     // [eval]
     sharingOn: (struct){
@@ -256,6 +327,17 @@ Result:
       A: "a"
     }
   }
+  issue3601: {
+    ok1: {
+      Y: (〈0;X〉 & _|_(no sharing)).a
+      X: {
+        a: {
+          b: 〈1;a〉.b
+        }
+      }
+      out: (〈0;Y〉 & _|_(no sharing))
+    }
+  }
   debug: {
     sharingOn: {
       a: 〈0;b〉