Consider task analyzer/fixer advising helper methods to take AbsolutePath

[rainersigwald]

Consider a task implementation that uses a helper method, like this trivial one:

public class MyTask : Task
{
    [Required]
    public string Input {get; set;}

    public bool Execute()
    {
        return Helper(Input);
    }

    private static bool Helper(string input)
    {
        return File.Exists(input);
    }
}

When making this multithread-aware, we'd like to get an MSBuildTask003 at the File.Exists call, because we need to use a thread-aware path instead of relying on CWD. But the straightforward fixer (wrap to File.Exists(TaskEnvironment.GetAbsolutePath(input))) won't work, because the TaskEnvironment property is not available in static context.

But there's another possible fix, which is "make the static method take an AbsolutePath and push creating it elsewhere":

+ [MSBuildMultiThreadableTask]
  public class MyTask : Task
  {
      [Required]
      public string Input {get; set;}

      public bool Execute()
      {
-         return Helper(Input);
+         return Helper(TaskEnvironment.GetAbsolutePath(Input));
      }

-     private static bool Helper(string input)
+     private static bool Helper(AbsolutePath input)
      {
          return File.Exists(input);
      }
  }

This isn't necessarily applicable everywhere but would be a nice suggestion in many cases.

Ideally it could also be used to build a barrier between "MSBuild task" code and "helper code" where we could establish guarantees about the absolute-ness of paths passed to "helper code" and not require that the helpers internally use AbsolutePath, since they may be shared with another non-MSBuild use case.

[ViktorHofer]

Agreed. I think that's also a great example to highlight the value of the new strongly typed task parameter bindings. If you replace the string type of the task property with AbsolutePath, you could just pass that through or make the helper function non-static and access the property directly.

Though, I don't think we should push that recommendation to the end user until #14680 is fixed.