allow json output to marshal ConfigModeAttr blocks

In order to marshal config blocks using ConfigModeAttr, we need to
insert the fixup body to map hcl blocks to the attribute in the schema.

internal/command/jsonconfig/expression.go

@@ -10,6 +10,7 @@ import (
 	"github.com/hashicorp/terraform/internal/addrs"
 	"github.com/hashicorp/terraform/internal/configs/configschema"
 	"github.com/hashicorp/terraform/internal/lang"
+	"github.com/hashicorp/terraform/internal/lang/blocktoattr"
 	"github.com/zclconf/go-cty/cty"
 	ctyjson "github.com/zclconf/go-cty/cty/json"
 )
@@ -96,6 +97,9 @@ func marshalExpressions(body hcl.Body, schema *configschema.Block) expressions {
 	// (lowSchema is an hcl.BodySchema:
 	// https://godoc.org/github.com/hashicorp/hcl/v2/hcl#BodySchema )
 
+	// fix any ConfigModeAttr blocks present from legacy providers
+	body = blocktoattr.FixUpBlockAttrs(body, schema)
+
 	// Use the low-level schema with the body to decode one level We'll just
 	// ignore any additional content that's not covered by the schema, which
 	// will effectively ignore "dynamic" blocks, and may also ignore other

internal/command/jsonconfig/expression_test.go

@@ -80,6 +80,29 @@ func TestMarshalExpressions(t *testing.T) {
 				},
 			},
 		},
+		{
+			hcltest.MockBody(&hcl.BodyContent{
+				Blocks: hcl.Blocks{
+					{
+						Type: "block_to_attr",
+						Body: hcltest.MockBody(&hcl.BodyContent{
+
+							Attributes: hcl.Attributes{
+								"foo": {
+									Name: "foo",
+									Expr: hcltest.MockExprTraversalSrc(`module.foo.bar`),
+								},
+							},
+						}),
+					},
+				},
+			}),
+			expressions{
+				"block_to_attr": expression{
+					References: []string{"module.foo.bar", "module.foo"},
+				},
+			},
+		},
 	}
 
 	for _, test := range tests {
@@ -89,6 +112,11 @@ func TestMarshalExpressions(t *testing.T) {
 					Type:     cty.String,
 					Optional: true,
 				},
+				"block_to_attr": {
+					Type: cty.List(cty.Object(map[string]cty.Type{
+						"foo": cty.String,
+					})),
+				},
 			},
 		}