\Drupal\Core\Routing\Router::generate() is deprecated but it is on an interface

I think we would want to end up with an exception, but if we do that, should we redeprecate for Drupal 10? We'd need to update the message too to say it'll throw an exception as opposed to being removed.

Also there's 4 other implementations of interface except \Drupal\Core\Routing\Router::generate()
- \Drupal\Core\Routing\AccessAwareRouter::generate()
- \Drupal\Core\Routing\UrlGenerator::generate()
- \Drupal\Core\Render\MetadataBubblingUrlGenerator::generate()
- testing code \Drupal\help_test\SupernovaGenerator::generate() (throws exception)

In discussion @catch proposed this:

I think we would be redeprecating for Drupal 10, but instead of saying it's removed, say it will start throwing an exception instead.

We cannot remove this as it is part of a Symfony interface. That said, it is not entirely fulfilling the interface contract if we are throwing an exception form it either, but that we can resolve in Drupal 9 to 10.

It was introduced deprecated outright in #2810303: Reunite the router: One router to rule them all before 8.3.0, that much I digged up. I don't think that is a good URL to include in the trigger_error() so I did not add that.

How does this look like?

+++ b/core/lib/Drupal/Core/Routing/Router.php
@@ -331,7 +331,7 @@ public function getRouteCollection() {
+    @trigger_error(__NAMESPACE__ . '\Router::generate() is deprecated in drupal:8.3.0 and will throw an exception from drupal:10.0.0. Use the \Drupal\Core\Url object instead.', E_USER_DEPRECATED);

It could use __METHOD__ instead of __NAMESPACE__

And needs a test to catch this error

There's todo for this class #2915772: Deprecate non_lazy_route_enhancer service tag in favor of route_enhancer, same for non_lazy_route_filter and route_filter
Also the CR needs update https://www.drupal.org/node/2820197

Here's a patch with test, no interdiff cos it bigger then previous patch

Looks good to me. The change record at https://www.drupal.org/node/2820197 does not mention anything that this patch changes. I don't think it needs to explain what this specific method does.