Whenever two mitzvos come into conflict with each other, and one is given precedence over the other (such as in the case of מילה on Shabbos, or any עשה דוחה לא תעשה), there is usually discussion over whether it is "הותרא" or "דחויה," whether the mitzvah that gave way effectively ceases to exist (הותרא), or if it's still there, but we're allowed to override it anyway (דחויא).

We know ([Yoma 85a](http://hebrewbooks.org/shas.aspx?mesechta=6&daf=85&format=pdf)-b, and several other sources) that when someone's life is in danger on Shabbos, and the Shabbos laws have to be violated in order to save his life, that we not only may but ***must*** violate Shabbos to save his life. However, it's unclear to me whether this is הותרא or דחויא.  

**Is פיקוח נפש on Shabbos הותרא or דיחויא?**

*To clarify; I'm looking for sources from the Rishonim and Acharonim that discuss this question.*