Huh? How would a static site generator serve both RSS and the HTML view of the RSS from the same file?
To be extra clear: I want to have <a href="feed.xml">My RSS Feed</a> link on my blog so everyone can find my feed. I also want users who don't know about RSS to see something other than a wall of plain-text XML.
You don't serve them from the same file. You serve them from separate files.
As I mention in my other comment to you, I don't know why you want an RSS file to be viewable. That's not an expected behavior. RSS is for aggregators to consume, not for viewing.
Technically, the web server can do content negotiation based on Accept headers with static files. But… In theory, you shouldn't need a direct link to the RSS feed on your web page. Most feed readers support a link-alternate in the HTML header:
Someone who wants to subscribe can just drop example.com/blog in to the feed reader and it will do the right thing. The "RSS Feed" interactive link then could go to a HTML web page with instructions for subscribing and/or a preview.