Add Place::base_local method and improve doc for Place::local to …

…clarify why we need the former.

src/librustc/mir/mod.rs

@@ -1967,7 +1967,10 @@ impl<'tcx> Place<'tcx> {
         Place::Projection(Box::new(PlaceProjection { base: self, elem }))
     }
 
-    /// Find the innermost `Local` from this `Place`.
+    /// Find the innermost `Local` from this `Place`, *if* it is either a local itself or
+    /// a single deref of a local.
+    ///
+    /// FIXME: can we safely swap the semantics of `fn base_local` below in here instead?
     pub fn local(&self) -> Option<Local> {
         match self {
             Place::Local(local) |
@@ -1978,6 +1981,15 @@ impl<'tcx> Place<'tcx> {
             _ => None,
         }
     }
+
+    /// Find the innermost `Local` from this `Place`.
+    pub fn base_local(&self) -> Option<Local> {
+        match self {
+            Place::Local(local) => Some(*local),
+            Place::Projection(box Projection { base, elem: _ }) => base.base_local(),
+            Place::Promoted(..) | Place::Static(..) => None,
+        }
+    }
 }
 
 impl<'tcx> Debug for Place<'tcx> {