Visual Studio has a feature for C# that displays how many times a given method has been referenced. This information is displayed as tiny greyed out text above the method names. It is not possible to edit this text and it will be ignored by the cursor when you transition over it. (I believe they are called adornments or glyphs, but I'm not entirely sure)
For illustration purpose, example images of the reference counters can be seen in the images at http://www.axtools.com/products-codesmart-vsnet.php and http://www.kunal-chowdhury.com/2013/08/reference-counter-in-visual-studio-2013.html#MYmS5iViRWPY3Lzm.97.
Is it possible to achieve the same in Vim?
I am curious because I dream about having a plugin in Vim that displays type signatures of F# code on top of the functions. And to be useful in Vim, the type signatures must be ignored by the cursor and Vim editing functionality. I have never developed a Vim plugin, so I guess this would be my first journey into Vim-scripting...