Problem/Motivation
On #3041924: [META] Convert hook_help() module overview text to topics we are converting our hook_help() text into Help Topics (and making improvements). As part of that effort, on #3072312: Review/fix/delete existing help topics, we are going through the initial topics we added as proof of concept when the original Help Topics patch went in. We came across a problem that is sort of a regression from things we could do in hook_help() that we can't currently do in a help topic.
In hook_help(), we could have code like this, to make a link only if another module exists:
$url = (\Drupal::moduleHandler()->moduleExists('block')) ? Url::fromRoute('help.page', ['name' => 'block'])->toString() : '#';
But we don't have a way to do that in a Twig template. (At least, not that I know of?)
As a note, we actually don't even have a good way to make URLs at all in Twig -- see this related issue: #2996305: Add support for the url() function to twig {%trans%}
Proposed resolution
Let twig's url function recover from route not found and missing parameter exceptions.
Remaining tasks
Figure out how to do this, and make a patch.
User interface changes
None.
API changes
New Twig functionality will be added. TBD.
Data model changes
None.
Release notes snippet
TBD.