internal/core/eval: nested comprehension fix

While iterating over comprehension to execute, new
ones may be added.

The iterator terminated at the end of the start of the
iteration, however, and the added elements were
promptly deleted.

Change-Id: I22b420e5d9fbb0a7c8b783ae1620aee3775f0bca
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6761
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

cue/testdata/comprehensions/nested2.txtar

@@ -0,0 +1,92 @@
+-- in.cue --
+given: {
+	INC: USD: 2.0
+	USD: GBP: 3.0
+}
+hydrated: {
+	for k,v in given {
+		for k1,r in v {
+			"\(k)": "\(k)":  1.0
+			"\(k1)": "\(k1)": 1.0
+			"\(k)": "\(k1)": r
+			"\(k1)": "\(k)": number
+		}
+	}
+}
+
+foo: {
+    a: 10
+    if a < 20 {
+        if a < 50 {
+            b: 20
+        }
+    }
+}
+-- out/eval --
+(struct){
+  given: (struct){
+    INC: (struct){
+      USD: (float){ 2.0 }
+    }
+    USD: (struct){
+      GBP: (float){ 3.0 }
+    }
+  }
+  hydrated: (struct){
+    INC: (struct){
+      INC: (float){ 1.0 }
+      USD: (float){ 2.0 }
+    }
+    USD: (struct){
+      USD: (float){ 1.0 }
+      INC: (number){ number }
+      GBP: (float){ 3.0 }
+    }
+    GBP: (struct){
+      GBP: (float){ 1.0 }
+      USD: (number){ number }
+    }
+  }
+  foo: (struct){
+    a: (int){ 10 }
+    b: (int){ 20 }
+  }
+}
+-- out/compile --
+--- in.cue
+{
+  given: {
+    INC: {
+      USD: 2.0
+    }
+    USD: {
+      GBP: 3.0
+    }
+  }
+  hydrated: {
+    for k, v in 〈1;given〉 {
+      for k1, r in 〈1;v〉 {
+        "\(〈3;k〉)": {
+          "\(〈4;k〉)": 1.0
+        }
+        "\(〈1;k1〉)": {
+          "\(〈2;k1〉)": 1.0
+        }
+        "\(〈3;k〉)": {
+          "\(〈2;k1〉)": 〈2;r〉
+        }
+        "\(〈1;k1〉)": {
+          "\(〈4;k〉)": number
+        }
+      }
+    }
+  }
+  foo: {
+    a: 10
+    if (〈0;a〉 < 20) {
+      if (〈1;a〉 < 50) {
+        b: 20
+      }
+    }
+  }
+}

internal/core/eval/eval.go

@@ -1482,11 +1482,11 @@ func (n *nodeContext) expandOne() (done bool) {
 		return true
 	}
 
-	if n.ifClauses, progress = n.injectEmbedded(n.ifClauses); progress {
+	if progress = n.injectEmbedded(&(n.ifClauses)); progress {
 		return true
 	}
 
-	if n.forClauses, progress = n.injectEmbedded(n.forClauses); progress {
+	if progress = n.injectEmbedded(&(n.forClauses)); progress {
 		return true
 	}
 
@@ -1542,7 +1542,7 @@ func (n *nodeContext) injectDynamic() (progress bool) {
 // injectEmbedded evaluates and inserts embeddings. It first evaluates all
 // embeddings before inserting the results to ensure that the order of
 // evaluation does not matter.
-func (n *nodeContext) injectEmbedded(all []envYield) (a []envYield, progress bool) {
+func (n *nodeContext) injectEmbedded(all *[]envYield) (progress bool) {
 	ctx := n.ctx
 	type envStruct struct {
 		env *adt.Environment
@@ -1554,12 +1554,13 @@ func (n *nodeContext) injectEmbedded(all []envYield) (a []envYield, progress boo
 	}
 
 	k := 0
-	for _, d := range all {
+	for i := 0; i < len(*all); i++ {
+		d := (*all)[i]
 		sa = sa[:0]
 
 		if err := ctx.Yield(d.env, d.yield, f); err != nil {
 			if err.IsIncomplete() {
-				all[k] = d
+				(*all)[k] = d
 				k++
 			} else {
 				// continue to collect other errors.
@@ -1573,7 +1574,8 @@ func (n *nodeContext) injectEmbedded(all []envYield) (a []envYield, progress boo
 		}
 	}
 
-	return all[:k], k < len(all)
+	*all = (*all)[:k]
+	return k < len(*all)
 }
 
 // addLists