hcldec: Variables must visit deeply-nested specifications

Previously this implementation was doing only one level of recursion in
its walk, which gave the appearance of working until the
transform/container-type specs (DefaultSpec, TransformSpec, ...) were
introduced, creating the possibility of "same body children" being more
than one level away from the initial spec.

It's still correct to only process the schema and content once, because
ImpliedSchema is already collecting all of the requirements from the
"same body children", and so our content object will include everything
that the nested specs should need to analyze needed variables.

hcldec/variables.go

@@ -15,20 +15,22 @@ import (
 // be incomplete, but that's assumed to be okay because the eventual call
 // to Decode will produce error diagnostics anyway.
 func Variables(body hcl.Body, spec Spec) []hcl.Traversal {
-	schema := ImpliedSchema(spec)
-
-	content, _, _ := body.PartialContent(schema)
-
 	var vars []hcl.Traversal
+	schema := ImpliedSchema(spec)
+	content, _, _ := body.PartialContent(schema)
 
 	if vs, ok := spec.(specNeedingVariables); ok {
 		vars = append(vars, vs.variablesNeeded(content)...)
 	}
-	spec.visitSameBodyChildren(func(s Spec) {
+
+	var visitFn visitFunc
+	visitFn = func(s Spec) {
 		if vs, ok := s.(specNeedingVariables); ok {
 			vars = append(vars, vs.variablesNeeded(content)...)
 		}
-	})
+		s.visitSameBodyChildren(visitFn)
+	}
+	spec.visitSameBodyChildren(visitFn)
 
 	return vars
 }

hcldec/variables_test.go

@@ -5,8 +5,8 @@ import (
 	"reflect"
 	"testing"
 
-	"github.com/hashicorp/hcl2/hcl/hclsyntax"
 	"github.com/hashicorp/hcl2/hcl"
+	"github.com/hashicorp/hcl2/hcl/hclsyntax"
 )
 
 func TestVariables(t *testing.T) {
@@ -42,6 +42,37 @@ func TestVariables(t *testing.T) {
 				},
 			},
 		},
+		{
+			"a = foo\nb = bar\n",
+			&DefaultSpec{
+				Primary: &AttrSpec{
+					Name: "a",
+				},
+				Default: &AttrSpec{
+					Name: "b",
+				},
+			},
+			[]hcl.Traversal{
+				{
+					hcl.TraverseRoot{
+						Name: "foo",
+						SrcRange: hcl.Range{
+							Start: hcl.Pos{Line: 1, Column: 5, Byte: 4},
+							End:   hcl.Pos{Line: 1, Column: 8, Byte: 7},
+						},
+					},
+				},
+				{
+					hcl.TraverseRoot{
+						Name: "bar",
+						SrcRange: hcl.Range{
+							Start: hcl.Pos{Line: 2, Column: 5, Byte: 12},
+							End:   hcl.Pos{Line: 2, Column: 8, Byte: 15},
+						},
+					},
+				},
+			},
+		},
 		{
 			"a = foo\n",
 			&ObjectSpec{