I have to be that guy: Using TODOs is considered bad practice by many programmers. You should rather use an issue tracker (if you are using something like bitbucket or github you already have one) or even better write a failing unit test.
To also add something helpful: What you want is super easy to implement as an RStudio addin. You just need to get the current project (rstudioapi for getting the file currently open in the editor, rprojroot for getting the project directory). Search all .R script files (
# TODO, and then present the results however you want (
print(), shiny app).
If you really make it an rstudio addin, you can even assign a hotkey to your function.