Drupal 9 compatibility

Ran drupal-check and found no issues:

➜  drupal-8.9 git:(8.9.x) ✗ ./vendor/bin/drupal-check   modules/contrib/simplify_menu
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 [OK] No errors

Rector O/P

➜  drupal-8.9 git:(8.9.x) ✗ ./vendor/bin/rector process  modules/contrib/simplify_menu --dry-run
Rector 0.8.x-dev@8896f0a
Config file: rector.yml

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 [OK] Rector is done!

Patch #3. LGTM. Updating the status to RTBC

Per a Slack discussion with Gábor Hojtsy last year regarding usage of D9 tags (Drupal 9, Drupal 9 compatibility, Drupal 9 readiness, etc.), "Drupal 9 compatibility" should be used for contributed projects that need updating and "Drupal 9" was the old tag for D8 issues before the D9 branch was ready. Doing title and tag cleanup here based on that discussion. Please use "Drupal 9 compatibility" for titles and tags on other issues. Thanks.

I tried #3 on Drupal 9.0.0 and it didn't work for me.
I created a new patch.

#6 worked for me

I saw two more problems.

/docroot (release/8.5.x-dev ✘)✹✭ ᐅ ~/.composer/vendor/mglaman/drupal-check/drupal-check  -d modules/contrib/simplify_menu
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 ------ ----------------------------------------------------------------------
  Line   src/Tests/SimplifyMenuTest.php
 ------ ----------------------------------------------------------------------
  12     Class Drupal\simplify_menu\Tests\SimplifyMenuTest extends deprecated
         class Drupal\simpletest\WebTestBase:
         in drupal:8.8.0 and is removed from drupal:9.0.0. Instead,
         use \Drupal\Tests\BrowserTestBase. See
         https://www.drupal.org/node/3030340.
  34     Call to method setUp() of deprecated class
         Drupal\simpletest\WebTestBase:
         in drupal:8.8.0 and is removed from drupal:9.0.0. Instead,
         use \Drupal\Tests\BrowserTestBase. See
         https://www.drupal.org/node/3030340.
 ------ ----------------------------------------------------------------------

... and I learned patches can't be tested against D9.

In order to progress on a D9 release I would suggest that a maintainer creates a D9 release branch, so that we can also test against the D9 codebase.

Ran rector tools and scanned upgrade_status on simplify menu module: No errors reported

my_site_name_dir % vendor/bin/rector process web/modules/contrib/simplify_menu --dry-run
Rector 0.8.x-dev@8896f0a
Config file: rector.yml

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 [OK] Rector is done!                                                                                                   
                                                                                                                        

Ran drupal-check, I saw some errors
After applied #8 patch

my_site_name_dir % vendor/bin/drupal-check -ad web/modules/contrib/simplify_menu        
 4/4 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

 ------ ----------------------------------------------------------------------------------------------------------------- 
  Line   src/MenuItems.php                                                                                                
 ------ ----------------------------------------------------------------------------------------------------------------- 
  89     Cannot access offset mixed on Drupal\Core\Menu\MenuLinkTree.                                                     
  89     Result of && is always false.                                                                                    
  134    PHPDoc tag @param has invalid value ($menu_id                                                                    
         The id of the menu to check for active links.): Unexpected token "$menu_id", expected type at offset 91          
  138    Property Drupal\simplify_menu\MenuItems::$activeMenuTree (Drupal\Core\Menu\MenuLinkTree) does not accept array.  
 ------ ----------------------------------------------------------------------------------------------------------------- 

 ------ ----------------------------------------------------------------------------------------------------------------- 
  Line   src/Tests/SimplifyMenuTest.php                                                                                   
 ------ ----------------------------------------------------------------------------------------------------------------- 
  95     PHPDoc tag @param has invalid value ($group                                                                      
         The type of assertion - examples are "Browser", "PHP".): Unexpected token "$group", expected type at offset 347  
  95     PHPDoc tag @param has invalid value ($message                                                                    
         The message to display along with the assertion.): Unexpected token "$message", expected type at offset 270      
  95     PHPDoc tag @param has invalid value ($xpath                                                                      
         An XPath expression.): Unexpected token "$xpath", expected type at offset 71                                     
  95     PHPDoc tag @return has invalid value (): Unexpected token "\n   *", expected type at offset 433                  
  97     Method Drupal\Tests\BrowserTestBase::assertTrue() invoked with 3 parameters, 1-2 required.                       
  97     Result of method Drupal\Tests\BrowserTestBase::assertTrue() (void) is used.                                      
 ------ ----------------------------------------------------------------------------------------------------------------- 

~/htdocs/drupal (9.0.x ✘)✹✭ ᐅ vendor/bin/drupal-check  -ad --drupal-root ~/htdocs/drupal  modules/simplify_menu
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 [OK] No errors

I move the test into the tests folder and some other small improvements.

Improved coding standards.

After applied #20 patch
i saw one error when i run drupal check on simplify menu module.

my_site_name_dir % vendor/bin/drupal-check -ad web/modules/contrib/simplify_menu
 4/4 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

 ------ ------------------------------------------------------------------------------------------------------------------------------------------------ 
  Line   src/MenuItems.php                                                                                                                               
 ------ ------------------------------------------------------------------------------------------------------------------------------------------------ 
  39     Property Drupal\simplify_menu\MenuItems::$menuLinkTree (Drupal\Core\Menu\MenuLinkTree) does not accept Drupal\Core\Menu\MenuLinkTreeInterface.  
 ------ ------------------------------------------------------------------------------------------------------------------------------------------------ 
                                                                                                                        
 [ERROR] Found 1 error                                                                                                  
                                                                                                                        

#23 patch applied cleanly.
Ran drupal check, rector tools and scanned upgrade_status on simplify menu module: No errors reported

my_site_name_dir % vendor/bin/rector process web/modules/contrib/simplify_menu --dry-run
Rector 0.8.x-dev@8896f0a
Config file: rector.yml

 12/12 [▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓] 100%

                                                                                                                        
 [OK] Rector is done!                                                                                                   
                                                                                                                        
my_site_name_dir %  vendor/bin/drupal-check -ad web/modules/contrib/simplify_menu   

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 [OK] No errors                                                                                                         
                                                                                                                     

+1 RTBC
#23 patch works fine for me. Adding a screenshot for ref.

8.x-2.1 has been released with Drupal 8 and 9 compatibility. Closing this issue. Thank you all!