command: UiHook to report when it gets incorrect values

For PreApply hook purposes we only actually use the Delete, Create, and
Update actions, because other actions are handled in different ways than
a direct call to ApplyResourceChange.

However, if there's a bug in core that causes it to pass a different
action, it's better for us to mark it as being explicitly unknown in the
UI rather than simply defaulting to "Modifying...", which can thus obscure
the problem and make for a confusing result.

command/hook_ui.go

@@ -64,6 +64,11 @@ const (
 func (h *UiHook) PreApply(addr addrs.AbsResourceInstance, gen states.Generation, action plans.Action, priorState, plannedNewState cty.Value) (terraform.HookAction, error) {
 	h.once.Do(h.init)
 
+	dispAddr := addr.String()
+	if gen != states.CurrentGen {
+		dispAddr = fmt.Sprintf("%s (%s)", dispAddr, gen)
+	}
+
 	var operation string
 	var op uiResourceOp
 	switch action {
@@ -73,9 +78,14 @@ func (h *UiHook) PreApply(addr addrs.AbsResourceInstance, gen states.Generation,
 	case plans.Create:
 		operation = "Creating..."
 		op = uiResourceCreate
-	default:
+	case plans.Update:
 		operation = "Modifying..."
 		op = uiResourceModify
+	default:
+		// We don't expect any other actions in here, so anything else is a
+		// bug in the caller but we'll ignore it in order to be robust.
+		h.ui.Output(fmt.Sprintf("(Unknown action %s for %s)", action, dispAddr))
+		return terraform.HookActionContinue, nil
 	}
 
 	attrBuf := new(bytes.Buffer)
@@ -135,7 +145,7 @@ func (h *UiHook) PreApply(addr addrs.AbsResourceInstance, gen states.Generation,
 
 	h.ui.Output(h.Colorize.Color(fmt.Sprintf(
 		"[reset][bold]%s: %s%s[reset]%s",
-		addr,
+		dispAddr,
 		operation,
 		stateIdSuffix,
 		attrString,