Convert function_exists() to is_callable()

@quietone Should we replace all existences of function_exists() with is_callable() ?

@annmarysruthy, I think each usage needs to be examined. For example, the usages in tests should probably not change, In may turn out that there are no places where making this change is an improvement. I have not looked closely at all the instances.

Checked all occurrences of function_exists() and changed 6 files. In my opinion, Rest of the occurrences are better kept as function_exists() itself. Kindly review MR

This looks great! I have to review more closely, but this already covers the three I was reviewing.

curious to compare performance of both methods

I'd take another look at EntityResolverManager.

Found some more hooks to exclude.

For 11.

Ok I am going through this and here are the criteria

I don't think we should be changing the string ones here. It's pointless and I think should be address in the issue that does the change.

I agree, that is the first bullet point, sorry if that wasn't clear.

If it is called on a string we know it's a function and we can address it when we convert the function itself

Ok I did the evaluation, it could use confirmation, I've attached it.

I ran this command to get a list of function_exists calls
grep -rn "function_exists" core > functionExistsCallsLine.txt

I then removed comments and broke it into three categories:

• Actionable (here or in a follow up)
• Procedural hooks, update, install, legacy invoke or theme related
• strings or library functions like is_file

Here is the list of actionable ones, I wasn't sure about the css test.
Category 1 Actionable
core/modules/views/src/Plugin/views/HandlerBase.php:572: if (isset($this->definition['access callback']) && function_exists($this->definition['access callback'])) {
core/includes/form.inc:955: if (($function = $batch['redirect_callback']) && function_exists($function)) {
core/modules/comment/src/CommentManager.php:210: if (function_exists($function)) {
core/modules/views_ui/src/ViewUI.php:341: $cancel_submit = function_exists($form_id . '_cancel') ? $form_id . '_cancel' : [$this, 'standardCancel'];
core/tests/Drupal/Tests/Core/Asset/CssOptimizerUnitTest.php:298:if (!function_exists('Drupal\Core\Asset\file_uri_scheme')) {
core/lib/Drupal/Core/Entity/EntityResolverManager.php:93: if (function_exists($controller)) {