internal/core/eval: handle structural cycles

Issue #29
Issue #42

Change-Id: I86191d7bf4fdcf8705d740171aedd81e04819dd5
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6522
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/testdata/cycle/015_reference_across_tuples_and_back.txtar

@@ -54,6 +54,6 @@ b:
   }
   b: (struct){
     e: (int){ 3 }
-    f: (_){ _ }
+    f: (int){ 3 }
   }
 }

cue/testdata/cycle/023_reentrance.txtar

@@ -79,7 +79,8 @@ fib12: 144
   }).out
 }
 -- out/eval --
-(struct){
+(_|_){
+  // [structural cycle]
   fibRec: (struct){
     nn: (int){ int }
     out: (_|_){
@@ -93,6 +94,10 @@ fib12: 144
     n: (int){ int }
   }
   fib2: (int){ 1 }
-  fib7: (int){ 13 }
-  fib12: (int){ 144 }
+  fib7: (_|_){
+    // [structural cycle]
+  }
+  fib12: (_|_){
+    // [structural cycle]
+  }
 }

cue/testdata/cycle/025_cannot_resolve_references_that_would_be_ambiguous.txtar

@@ -56,22 +56,26 @@ c2: (*{
 -- out/eval --
 (struct){
   a1: (_|_){
-    // [incomplete]
+    // [incomplete] incomplete cause disjunction
   }
   a2: (_|_){
-    // [incomplete]
+    // [incomplete] incomplete cause disjunction
   }
   a3: (int){ 1 }
-  b1: (_|_){
-    // [incomplete] ambiguous disjunction
-  }
-  b2: (_|_){
-    // [incomplete] ambiguous disjunction
-  }
-  c1: (_|_){
-    // [incomplete] ambiguous disjunction
-  }
-  c2: (_|_){
-    // [incomplete] ambiguous disjunction
-  }
+  b1: (int){ |((int){ 0 }, (int){ 1 }) }
+  b2: (int){ |((int){ 1 }, (int){ 0 }) }
+  c1: (struct){ |((struct){
+      a: (int){ 1 }
+      b: (int){ 2 }
+    }, (struct){
+      b: (int){ 1 }
+      a: (int){ 2 }
+    }) }
+  c2: (struct){ |((struct){
+      a: (int){ 2 }
+      b: (int){ 1 }
+    }, (struct){
+      b: (int){ 2 }
+      a: (int){ 1 }
+    }) }
 }

cue/testdata/cycle/051_resolved_self-reference_cycles_with_disjunction.txtar

@@ -169,10 +169,14 @@ Result:
   xa2: (int){ 8 }
   xa3: (int){ 6 }
   xa4: (int){ 10 }
-  xb1: (int){ 8 }
-  xb2: (int){ 8 }
-  xb3: (int){ 6 }
-  xb4: (int){ 10 }
+  xb1: (int){ |((int){ 8 }, (int){ 9 }) }
+  xb2: (_|_){
+    // [incomplete]
+  }
+  xb3: (int){ |((int){ 6 }, (int){ 9 }) }
+  xb4: (_|_){
+    // [cycle] cycle error
+  }
   xc1: (int){ |((int){ 8 }, (int){ 9 }) }
   xc2: (int){ 8 }
   xc3: (int){ 6 }
@@ -202,10 +206,14 @@ Result:
     // [eval] incompatible values 7 and 6:
     //     ./in.cue:44:6
   }
-  xf1: (int){ 8 }
-  xf2: (int){ 8 }
-  xf3: (int){ 6 }
-  xf4: (int){ 10 }
+  xf1: (int){ |((int){ 8 }, (int){ 9 }) }
+  xf2: (_|_){
+    // [incomplete]
+  }
+  xf3: (int){ |((int){ 6 }, (int){ 9 }) }
+  xf4: (_|_){
+    // [cycle] cycle error
+  }
   z1: (int){ |((int){ 11 }, (int){ 13 }) }
   z2: (int){ 10 }
   z3: (int){ 8 }

cue/testdata/cycle/052_resolved_self-reference_cycles_with_disjunction_with_defaults.txtar

@@ -139,10 +139,14 @@ Result:
   xa2: (int){ 8 }
   xa3: (int){ 6 }
   xa4: (int){ 10 }
-  xb1: (int){ 8 }
-  xb2: (int){ 8 }
+  xb1: (int){ |(*(int){ 8 }, (int){ 9 }) }
+  xb2: (_|_){
+    // [incomplete]
+  }
   xb3: (int){ 6 }
-  xb4: (int){ 10 }
+  xb4: (_|_){
+    // [cycle] cycle error
+  }
   xc1: (int){ |(*(int){ 8 }, (int){ 9 }) }
   xc2: (int){ 8 }
   xc3: (int){ 6 }

cue/testdata/cycle/disjunction.txtar

@@ -0,0 +1,150 @@
+-- in.cue --
+
+// cycle is a structural cycle
+cycle: a: cycle
+
+// reference to outside structural cycle
+r1a: cycle | int
+r1b: int | cycle
+
+r2a: cycle | 1
+r2b: 1 | cycle
+
+r3a: cycle | null
+r3b: null | cycle
+
+r4a: cycle | {}
+r4b: {} | cycle
+
+r5a: cycle | []
+r5b: [] | cycle
+
+// reference to ancestor node
+s1a: x: s1a | int
+s1b: x: int | s1b
+
+s2a: x: s2a | 1
+s2b: x: 1 | s2b
+
+s3a: x: s3a | null
+s3b: x: null | s3b
+
+s4a: x: s4a | {}
+s4b: x: {} | s4b
+
+s5a: x: s5a | []
+s5b: x: [] | s5b
+
+
+
+-- out/eval --
+Errors:
+structural cycle
+
+Result:
+(_|_){
+  // [structural cycle]
+  cycle: (_|_){
+    // [structural cycle]
+    a: (_|_){
+      // [structural cycle] structural cycle
+      a: (_|_){// 〈1;cycle〉
+      }
+    }
+  }
+  r1a: (int){ int }
+  r1b: (int){ int }
+  r2a: (int){ 1 }
+  r2b: (int){ 1 }
+  r3a: (null){ null }
+  r3b: (null){ null }
+  r4a: (struct){
+  }
+  r4b: (struct){
+  }
+  r5a: (#list){
+  }
+  r5b: (#list){
+  }
+  s1a: (struct){
+    x: (int){ int }
+  }
+  s1b: (struct){
+    x: (int){ int }
+  }
+  s2a: (struct){
+    x: (int){ 1 }
+  }
+  s2b: (struct){
+    x: (int){ 1 }
+  }
+  s3a: (struct){
+    x: (null){ null }
+  }
+  s3b: (struct){
+    x: (null){ null }
+  }
+  s4a: (struct){
+    x: (struct){
+    }
+  }
+  s4b: (struct){
+    x: (struct){
+    }
+  }
+  s5a: (struct){
+    x: (#list){
+    }
+  }
+  s5b: (struct){
+    x: (#list){
+    }
+  }
+}
+-- out/compile --
+--- in.cue
+{
+  cycle: {
+    a: 〈1;cycle〉
+  }
+  r1a: (〈0;cycle〉|int)
+  r1b: (int|〈0;cycle〉)
+  r2a: (〈0;cycle〉|1)
+  r2b: (1|〈0;cycle〉)
+  r3a: (〈0;cycle〉|null)
+  r3b: (null|〈0;cycle〉)
+  r4a: (〈0;cycle〉|{})
+  r4b: ({}|〈0;cycle〉)
+  r5a: (〈0;cycle〉|[])
+  r5b: ([]|〈0;cycle〉)
+  s1a: {
+    x: (〈1;s1a〉|int)
+  }
+  s1b: {
+    x: (int|〈1;s1b〉)
+  }
+  s2a: {
+    x: (〈1;s2a〉|1)
+  }
+  s2b: {
+    x: (1|〈1;s2b〉)
+  }
+  s3a: {
+    x: (〈1;s3a〉|null)
+  }
+  s3b: {
+    x: (null|〈1;s3b〉)
+  }
+  s4a: {
+    x: (〈1;s4a〉|{})
+  }
+  s4b: {
+    x: ({}|〈1;s4b〉)
+  }
+  s5a: {
+    x: (〈1;s5a〉|[])
+  }
+  s5b: {
+    x: ([]|〈1;s5b〉)
+  }
+}