First stab at attr.TypeWithModifyPlan.

.changelog/162.txt

@@ -0,0 +1,3 @@
+```release-note:feature
+Added support for a ModifyPlan method on the `attr.Type` type, allowing custom types to include plan modification logic that will be applied to every attribute or element using that type.
+```

attr/type.go

@@ -86,6 +86,17 @@ type TypeWithValidate interface {
 	Validate(context.Context, tftypes.Value, *tftypes.AttributePath) diag.Diagnostics
 }
 
+// TypeWithModifyPlan extends the Type interface to include a ModifyPlan method,
+// used to bundle consistent plan modification logic with the Type.
+type TypeWithModifyPlan interface {
+	Type
+
+	// ModifyPlan returns the Value that should be used in the plan. It is
+	// generally used to suppress diffs that do not correspond to semantic
+	// differences. In these cases, the `state` Value should be returned.
+	ModifyPlan(ctx context.Context, state, plan Value, path *tftypes.AttributePath) (Value, diag.Diagnostics)
+}
+
 // TypeWithPlaintextDescription extends the Type interface to include a
 // Description method, used to bundle extra information to include in attribute
 // descriptions with the Type. It expects the description to be written as

tfsdk/serve.go

@@ -698,6 +698,21 @@ func (s *server) planResourceChange(ctx context.Context, req *tfprotov6.PlanReso
 		return
 	}
 
+	// Execute any attr.TypeWithModifyPlans
+	//
+	// We do this first because type plan modifications should be
+	// overridable by resource and attribute level plan modifications. As a
+	// rule of thumb, more specific plan modifiers should happen after more
+	// generic plan modifiers.
+	//
+	// We only do this if there's a plan to modify; otherwise, it
+	// represents a resource being deleted and there's no point.
+	newPlan, ok := runTypePlanModifiers(ctx, state, plan, resourceSchema, resp)
+	if !ok {
+		return
+	}
+	plan = newPlan
+
 	// Execute any AttributePlanModifiers.
 	//
 	// This pass is before any Computed-only attributes are marked as unknown
@@ -764,6 +779,10 @@ func (s *server) planResourceChange(ctx context.Context, req *tfprotov6.PlanReso
 		plan = modifiedPlan
 	}
 
+	// TODO: execute any type plan modifiers again to allow overwriting
+	// unknown values. Do we even want to do that? What could the use case
+	// possibly be?
+
 	// Execute any AttributePlanModifiers again. This allows overwriting
 	// any unknown values.
 	//