explode: move "as" check to semantic pass

Move the "as" check on explode proc from kernel compilation phase to
semantic phase where this belongs.

compiler/ast/dag/op.go

@@ -41,7 +41,7 @@ type (
 		Kind string `json:"kind" unpack:""`
 		Args []Expr `json:"args"`
 		Type string `json:"type"`
-		As   Expr   `json:"as"`
+		As   string `json:"as"`
 	}
 	Filter struct {
 		Kind string `json:"kind" unpack:""`

compiler/kernel/op.go

@@ -224,14 +224,7 @@ func (b *Builder) compileLeaf(o dag.Op, parent zbuf.Puller) (zbuf.Puller, error)
 		if err != nil {
 			return nil, err
 		}
-		as, err := compileLval(v.As)
-		if err != nil {
-			return nil, err
-		}
-		if len(as) != 1 {
-			return nil, errors.New("explode field must be a top-level field")
-		}
-		return explode.New(b.octx.Zctx, parent, args, typ, as.Leaf())
+		return explode.New(b.octx.Zctx, parent, args, typ, v.As)
 	case *dag.Over:
 		return b.compileOver(parent, v)
 	case *dag.Yield:

compiler/semantic/op.go

@@ -692,17 +692,19 @@ func (a *analyzer) semOp(o ast.Op, seq dag.Seq) (dag.Seq, error) {
 		if err != nil {
 			return nil, err
 		}
-		var as dag.Expr
+		var as string
 		if o.As == nil {
-			as = &dag.This{
-				Kind: "This",
-				Path: field.Path{"value"},
-			}
+			as = "value"
 		} else {
-			as, err = a.semExpr(o.As)
+			e, err := a.semExpr(o.As)
 			if err != nil {
 				return nil, err
 			}
+			if this, ok := e.(*dag.This); !ok {
+				return nil, errors.New("explode: as clause must be a field reference")
+			} else if len(this.Path) != 1 {
+				return nil, errors.New("explode: field must be a top-level field")
+			}
 		}
 		return append(seq, &dag.Explode{
 			Kind: "Explode",