Create SafeMarkup::escapeIfUnsafe()

Something like this?

SafeMarkup::checkPlain() will be deprecated no?

+++ b/core/lib/Drupal/Component/Utility/PlaceholderTrait.php
@@ -34,17 +34,13 @@ protected static function placeholderFormat($string, array $args, &$safe = TRUE)
+          $value = SafeMarkup::escapeIfUnsafe($value);
           $args[$key] = '<em class="placeholder">' . $value . '</em>';

+++ b/core/modules/views/views.module
@@ -666,9 +666,7 @@ function views_pre_render_views_form_views_form($element) {
+    $substitution = SafeMarkup::escapeIfUnsafe($substitution);
     $replace[] = $substitution;

There doesn't appear to be any need for these intermediate variables anymore.

Added test to cover also the safe string that needs to stay unchanged. Hope that's good :)

1. +++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
   @@ -237,4 +237,22 @@ public static function format($string, array $args) {
   +   * @param string $string
   +   *   The text string to be escaped if unsafe.
   

   $string can also be a SafeString object, no? That's exactly what were are checking here. So we should document the type as string|SafeStringInterface.

2. +++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
   @@ -237,4 +237,22 @@ public static function format($string, array $args) {
   +   * @return string
   +   *   The escaped string if it was unsafe.
   

   empty line between @param and @return (but not between multiple @param's).

   The description is also not clear what happens if the string was safe, it sounds like it doesn't return anything. Maybe just "An escaped or safe string." ?

1. +++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
   @@ -237,4 +237,22 @@ public static function format($string, array $args) {
   +   * @param string $string
   

   This can be also SafeString.

2. +++ b/core/lib/Drupal/Component/Utility/SafeMarkup.php
   @@ -237,4 +237,22 @@ public static function format($string, array $args) {
   +   * @return string
   +   *   The escaped string if it was unsafe.
   

   This is not a string that its returning but instead a SafeStringInterface.

Here we go.

#20.2
If a SafeStringInterface is passed, then by definition it is safe and simply returned immediately. If a regular string is passed, then it is checked against the static list and either returned (if it was already marked as safe) or run through Html::escape(), which also results in a plain old string being returned.
In my opinion, I don't think the current documentation does a good job of explaining this.

Also, #9 was never addressed.

@MattA,

Sorry, I overlooked #9. Please see the interdiff for the changes you mentioned.
As for the documentation, I think I see what you mean, I tried to adjust it.

I discussed about this issue with @alexpott and @stefan.r and we agreed that we don't want to do this now. The reason why its not a good idea to add a helper for this is that in most of the cases where strings has to be manually escaped, there is better alternative than using this helper method. By adding this helper we would encourage people use this method on top of other solutions.

I also looked the use cases in core and I didn't find good use cases so far in the core where there would get much benefit out of this. I also think that this will get better by the time we get rid of SafeMarkup::isSafe().