helper/schema: Add validation to prevent write-only attributes in set nested blocks

.changes/unreleased/BUG FIXES-20250218-172440.yaml

@@ -0,0 +1,6 @@
+kind: BUG FIXES
+body: 'helper/schema: Fixed bug that allowed write-only attributes within set nested blocks. 
+Any attribute within a set nested block with `WriteOnly` set to `true` will now trigger an error message.'
+time: 2025-02-18T17:24:40.023079-05:00
+custom:
+    Issue: "1427"

.changes/unreleased/NOTES-20250218-172625.yaml

@@ -0,0 +1,5 @@
+kind: NOTES
+body: Write-only attribute support is in technical preview and offered without compatibility promises until Terraform 1.11 is generally available.
+time: 2025-02-18T17:26:25.941391-05:00
+custom:
+    Issue: "1375"

helper/schema/schema.go

@@ -975,7 +975,13 @@ func (m schemaMap) internalValidate(topSchemaMap schemaMap, attrsOnly bool) erro
 			case *Resource:
 				attrsOnly := attrsOnly || v.ConfigMode == SchemaConfigModeAttr
 
-				if v.Computed && schemaMap(t.SchemaMap()).hasWriteOnly() {
+				blockHasWriteOnly := schemaMap(t.SchemaMap()).hasWriteOnly()
+
+				if v.Type == TypeSet && blockHasWriteOnly {
+					return fmt.Errorf("%s: Set Block type cannot contain WriteOnly attributes", k)
+				}
+
+				if v.Computed && blockHasWriteOnly {
 					return fmt.Errorf("%s: Block types with Computed set to true cannot contain WriteOnly attributes", k)
 				}
 

helper/schema/schema_test.go

@@ -5303,7 +5303,7 @@ func TestSchemaMap_InternalValidate(t *testing.T) {
 			},
 			true,
 		},
-		"Set configuration block nested attribute with WriteOnly set returns no errors": {
+		"Set configuration block nested attribute with WriteOnly set returns error": {
 			map[string]*Schema{
 				"config_block_attr": {
 					Type:     TypeSet,
@@ -5319,7 +5319,7 @@ func TestSchemaMap_InternalValidate(t *testing.T) {
 					},
 				},
 			},
-			false,
+			true,
 		},
 		"List configuration block with ConfigModeAttr set, sub block nested attribute with WriteOnly set returns no errors": {
 			map[string]*Schema{
@@ -5350,7 +5350,7 @@ func TestSchemaMap_InternalValidate(t *testing.T) {
 			false,
 		},
 
-		"Set configuration block with ConfigModeAttr set, sub block nested attribute with WriteOnly set returns no errors": {
+		"Set configuration block with ConfigModeAttr set, sub block nested attribute with WriteOnly set returns error": {
 			map[string]*Schema{
 				"block": {
 					Type:       TypeSet,
@@ -5376,7 +5376,7 @@ func TestSchemaMap_InternalValidate(t *testing.T) {
 					},
 				},
 			},
-			false,
+			true,
 		},
 		"List computed block nested attribute with WriteOnly set returns error": {
 			map[string]*Schema{

website/docs/plugin/sdkv2/resources/write-only-arguments.mdx

@@ -16,7 +16,9 @@ which should either use the value by making the appropriate change to the API or
 
 The following are high level differences between `Required`/`Optional` arguments and write-only arguments:
 
-- Write-only arguments can accept ephemeral and non-ephemeral values
+- Write-only arguments can accept ephemeral and non-ephemeral values.
+
+- Write-only arguments cannot be used with set attributes and set nested blocks.
 
 - Write-only argument values are only available in the configuration. The prior state, planned state, and final state values for
 write-only arguments should always be `null`.
@@ -51,6 +53,7 @@ write-only arguments should always be `null`.
 - Cannot be used when `Default` is `specified`
 - Cannot be used with `DefaultFunc`
 - Cannot be used with aggregate schema types (e.g. `typeMap`, `typeList`, `typeSet`), but non-computed nested block types can contain write-only arguments.
+- Cannot be used within a set nested block type.
 
 ## Retrieving Write-only Values
 

website/docs/plugin/sdkv2/schemas/schema-behaviors.mdx

@@ -213,7 +213,8 @@ resource "example_instance" "ex" {
 - Cannot be used when `ForceNew` is `true`
 - Cannot be used when `Default` is `specified`
 - Cannot be used with `DefaultFunc`
-- Cannot be used with aggregate schema types (e.g. `typeMap`, `typeList`, `typeSet`), but non-computed nested block types can contain write-only arguments.
+- Cannot be used with aggregate schema types (e.g. `typeMap`, `typeList`, `typeSet`), but non-computed list nested block types can contain write-only arguments.
+- Cannot be used within a set nested block type
 
 `WriteOnly` should be used for arguments that handle secret values that do not need to be persisted in Terraform plan or state,
 such as passwords, API keys, etc. Write-only argument values are not sent to Terraform and do not