helper/schema: Simplify setDiff, remove exported SetDiff mentions

setDiff does not make use of its new parameter anymore, so it has been
removed. Also, as there is no more SetDiff (exported) function, mentions
of that have been removed from comments too.

helper/schema/resource_diff.go

@@ -131,10 +131,9 @@ type ResourceDiff struct {
 	// A writer that writes overridden new fields.
 	newWriter *newValueWriter
 
-	// Tracks which keys have been updated by SetNew, SetNewComputed, and SetDiff
-	// to ensure that the diff does not get re-run on keys that were not touched,
-	// or diffs that were just removed (re-running on the latter would just roll
-	// back the removal).
+	// Tracks which keys have been updated by ResourceDiff to ensure that the
+	// diff does not get re-run on keys that were not touched, or diffs that were
+	// just removed (re-running on the latter would just roll back the removal).
 	updatedKeys map[string]bool
 }
 
@@ -198,8 +197,8 @@ func newResourceDiff(schema map[string]*Schema, config *terraform.ResourceConfig
 	return d
 }
 
-// UpdatedKeys returns the keys that were updated by SetNew, SetNewComputed, or
-// SetDiff. These are the only keys that a diff should be re-calculated for.
+// UpdatedKeys returns the keys that were updated by this ResourceDiff run.
+// These are the only keys that a diff should be re-calculated for.
 func (d *ResourceDiff) UpdatedKeys() []string {
 	var s []string
 	for k := range d.updatedKeys {
@@ -208,9 +207,9 @@ func (d *ResourceDiff) UpdatedKeys() []string {
 	return s
 }
 
-// Clear wipes the diff for a particular key. It is called by SetDiff to remove
-// any possibility of conflicts, but can be called on its own to just remove a
-// specific key from the diff completely.
+// Clear wipes the diff for a particular key. It is called by ResourceDiff's
+// functionality to remove any possibility of conflicts, but can be called on
+// its own to just remove a specific key from the diff completely.
 //
 // Note that this does not wipe an override. This function is only allowed on
 // computed keys.
@@ -260,7 +259,7 @@ func (d *ResourceDiff) SetNew(key string, value interface{}) error {
 		return err
 	}
 
-	return d.setDiff(key, d.get(strings.Split(key, "."), "state").Value, value, false)
+	return d.setDiff(key, value, false)
 }
 
 // SetNewComputed functions like SetNew, except that it blanks out a new value
@@ -272,11 +271,11 @@ func (d *ResourceDiff) SetNewComputed(key string) error {
 		return err
 	}
 
-	return d.setDiff(key, d.get(strings.Split(key, "."), "state").Value, nil, true)
+	return d.setDiff(key, nil, true)
 }
 
 // setDiff performs common diff setting behaviour.
-func (d *ResourceDiff) setDiff(key string, old, new interface{}, computed bool) error {
+func (d *ResourceDiff) setDiff(key string, new interface{}, computed bool) error {
 	if err := d.clear(key); err != nil {
 		return err
 	}
@@ -300,9 +299,9 @@ func (d *ResourceDiff) ForceNew(key string) error {
 		return fmt.Errorf("ForceNew: No changes for %s", key)
 	}
 
-	old, new := d.GetChange(key)
+	_, new := d.GetChange(key)
 	d.schema[key].ForceNew = true
-	return d.setDiff(key, old, new, false)
+	return d.setDiff(key, new, false)
 }
 
 // Get hands off to ResourceData.Get.