fix: disambiguate variable defaults with "empty" values from undefined

- the default is wrapped in a struct, so that the wrapper being nil
  can indicate that `default` was undefined. if a default was defined,
  the Value field is serialized

tfconfig/load_hcl.go

@@ -148,7 +148,7 @@ func loadModule(dir string) (*Module, Diagnostics) {
 							// guaranteed valid by ctyjson.Marshal.
 							panic(fmt.Errorf("failed to re-parse default value from JSON: %s", err))
 						}
-						v.Default = def
+						v.Default = &Default{Value: def}
 					}
 				}
 

tfconfig/load_legacy.go

@@ -101,7 +101,7 @@ func loadModuleLegacyHCL(dir string) (*Module, Diagnostics) {
 					Name:        name,
 					Type:        block.Type,
 					Description: block.Description,
-					Default:     block.Default,
+					Default:     &Default{Value: block.Default},
 					Pos:         sourcePosLegacyHCL(item.Pos(), filename),
 				}
 				if _, exists := mod.Variables[name]; exists {

tfconfig/test-fixtures/variable-types/variable-types.out.json

@@ -34,6 +34,60 @@
                 "filename": "test-fixtures/variable-types/variable-types.tf",
                 "line": 8
             }
+        },
+        "string_default_empty": {
+            "name": "string_default_empty",
+            "type": "string",
+            "default": "",
+            "pos": {
+                "filename": "test-fixtures/variable-types/variable-types.tf",
+                "line": 14
+            }
+        },
+        "string_default_null": {
+            "name": "string_default_null",
+            "type": "string",
+            "default": null,
+            "pos": {
+                "filename": "test-fixtures/variable-types/variable-types.tf",
+                "line": 19
+            }
+        },
+        "list_default_empty": {
+            "name": "list_default_empty",
+            "type": "list(string)",
+            "default": [],
+            "pos": {
+                "filename": "test-fixtures/variable-types/variable-types.tf",
+                "line": 24
+            }
+        },
+        "object_default_empty": {
+            "name": "object_default_empty",
+            "type": "object({})",
+            "default": {},
+            "pos": {
+                "filename": "test-fixtures/variable-types/variable-types.tf",
+                "line": 29
+            }
+        },
+        "number_default_zero": {
+            "name": "number_default_zero",
+            "type": "number",
+            "default": 0,
+            "pos": {
+                "filename": "test-fixtures/variable-types/variable-types.tf",
+                "line": 34
+            }
+        },
+        "bool_default_false": {
+            "name": "bool_default_false",
+            "type": "bool",
+            "default": false,
+            "pos": {
+                "filename": "test-fixtures/variable-types/variable-types.tf",
+                "line": 39
+            }
         }
     },
     "outputs": {},

tfconfig/test-fixtures/variable-types/variable-types.tf

@@ -10,3 +10,33 @@ variable "map" {
   # with older configurations.
   type = "map"
 }
+
+variable "string_default_empty" {
+  type    = string
+  default = ""
+}
+
+variable "string_default_null" {
+  type    = string
+  default = null
+}
+
+variable "list_default_empty" {
+  type    = list(string)
+  default = []
+}
+
+variable "object_default_empty" {
+  type    = object({})
+  default = {}
+}
+
+variable "number_default_zero" {
+  type    = number
+  default = 0
+}
+
+variable "bool_default_false" {
+  type    = bool
+  default = false
+}

tfconfig/variable.go

@@ -1,5 +1,13 @@
 package tfconfig
 
+import (
+	"encoding/json"
+)
+
+type Default struct {
+	Value interface{}
+}
+
 // Variable represents a single variable from a Terraform module.
 type Variable struct {
 	Name        string `json:"name"`
@@ -10,7 +18,11 @@ type Variable struct {
 	// the native Go type system. The conversion from the value given in
 	// configuration may be slightly lossy. Only values that can be
 	// serialized by json.Marshal will be included here.
-	Default interface{} `json:"default,omitempty"`
+	Default *Default `json:"default,omitempty"`
 
 	Pos SourcePos `json:"pos"`
 }
+
+func (d *Default) MarshalJSON() ([]byte, error) {
+	return json.Marshal(&d.Value)
+}