Add unresolved question of break 'fn ..

text/0000-fn-lifetime.md

@@ -68,4 +68,19 @@ What other designs have been considered? What is the impact of not doing this?
 # Unresolved questions
 [unresolved]: #unresolved-questions
 
+- Should we allow `break 'fn ..` with the same semantics as `return ..`. In
+  [RFC 1624](https://github.com/rust-lang/rfcs/blob/master/text/1624-loop-break-value.md)
+  we got breaking with a value out of loops, so the value part is already
+  established. This provides one canonnical syntax for breaking out of a loops
+  or functions, leaving `return` just a convenience. In a future with full
+  lifetime ascription, this would further generalize to supporting all labeled
+  blocks, not just loops and functions.
+
+  A downside is `continue 'a` will still only make sense when `'a` is bound to a
+  loop, and `break 'a` won't make sense when `'a` is a lifetime parameter. That
+  means users will need to understand their are three tiers of lifetimes: loop,
+  block (including `'fn`), and, parameter, where each is less usable than the
+  last. Today loop lables and lifetimes are disjoint in that neither can be used
+  where the other is expected, though they do share the same syntax.
+
 What parts of the design are still TBD?