Add a rule for removing diff suppress functions

docs/content/develop/breaking-changes/breaking-changes.md

@@ -79,7 +79,7 @@ For more information, see
     and modifying examples and modules may achieve the intended effect with a smaller blast radius.
 * <a name="field-changing-data-format"></a> Modifying how field data is stored in state
   * For example, changing the case of a value returned by the API in a flattener or decorder
-* Removing diff suppression from a field.
+* <a name="field-removing-diff-suppress"></a> Removing diff suppression from a field.
   * For MMv1 resources, removing `diff_suppress_func` from a field.
   * For handwritten resources, removing `DiffSuppressFunc` from a field.
 * Removing update support from a field.

tools/diff-processor/rules/rules_field.go

@@ -27,6 +27,7 @@ var FieldRules = []FieldRule{
 	fieldRule_DefaultModification,
 	fieldRule_GrowingMin,
 	fieldRule_ShrinkingMax,
+	fieldRule_RemovingDiffSuppress,
 	fieldRule_ChangingFieldDataFormat,
 }
 
@@ -211,6 +212,25 @@ func fieldRule_ShrinkingMax_func(old, new *schema.Schema, mc MessageContext) str
 	return ""
 }
 
+var fieldRule_RemovingDiffSuppress = FieldRule{
+	name:        "Removing Diff Suppress Function",
+	definition:  "Diff suppress functions cannot be removed. Otherwise terraform configurations that previously had no diffs would show diffs.",
+	message:     "Field {{field}} lost its diff suppress function",
+	identifier:  "field-removing-diff-suppress",
+	isRuleBreak: fieldRule_RemovingDiffSuppress_func,
+}
+
+func fieldRule_RemovingDiffSuppress_func(old, new *schema.Schema, mc MessageContext) string {
+	// ignore for added / removed fields
+	if old == nil || new == nil {
+		return ""
+	}
+	if old.DiffSuppressFunc != nil && new.DiffSuppressFunc == nil {
+		return populateMessageContext(mc.message, mc)
+	}
+	return ""
+}
+
 func fieldRulesToRuleArray(frs []FieldRule) []Rule {
 	var rules []Rule
 	for _, fr := range frs {