Clarify and improve comment, removing a TODO.

src/librustc_typeck/check/mod.rs

@@ -1891,12 +1891,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
 impl<'a, 'tcx> RegionScope for FnCtxt<'a, 'tcx> {
     fn object_lifetime_default(&self, span: Span) -> Option<ty::Region> {
-        // TODO. RFC #599 specifies that object lifetime defaults take
-        // precedence over other defaults. *However,* within a fn
-        // body, we typically use inference to allow users to elide
-        // lifetimes whenever they like, and then just infer it to
-        // whatever it must be. So I interpret that as applying only
-        // to fn sigs.
+        // RFC #599 specifies that object lifetime defaults take
+        // precedence over other defaults. But within a fn body we
+        // don't have a *default* region, rather we use inference to
+        // find the *correct* region, which is strictly more general
+        // (and anyway, within a fn body the right region may not even
+        // be something the user can write explicitly, since it might
+        // be some expression).
         Some(self.infcx().next_region_var(infer::MiscVariable(span)))
     }