How to use projectile-find-test-file

projectile guesses the test file name given a source file by adding suffix/prefix to your source file. For example, if you have a hello.rb then it tries to find a file called hello_test.rb in your project.

It by default has some rules that maps from project type to test suffix/prefix. Here is the actual code:

(defun projectile-test-suffix (project-type)
  "Find default test files suffix based on PROJECT-TYPE."
  (cond
   ((member project-type '(rails-rspec ruby-rspec)) "_spec")
   ((member project-type '(rails-test ruby-test lein-test go)) "_test")
   ((member project-type '(scons)) "test")
   ((member project-type '(maven symfony)) "Test")
   ((member project-type '(gradle grails)) "Spec")))

If you don't see anything in projectile-find-test-file that means most likely it cannot find anything related to your source file by adding suffix/prefix.

This can be customized via projectile-test-suffix-function. By default the variable points to the function above, but you can override it with your own rules.

This isn't a great answer to your question, but projectile-find-test-file was just hanging for me before I even got a prompt. I didn't have the inclination to figure out what was going on there so I just created:

(defun my-test-prefix (project-type) "test_")

and customized projectile-test-function-prefix to point to that. That got projectile-toggle-between-implementation-and-test working for me which is all I really needed.