Cleanup fn is_mutable by removing some unnecessary control-flow breaks.

(This makes it a little easier to add instrumentation of the entry and
exit by adding `debug!` at the beginning and end, though note that the
function body *does* use the `?` operator...)

Author: pnkfelix

src/librustc_mir/borrow_check/mod.rs

@@ -1943,7 +1943,7 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
         }
     }
 
-    /// Whether this value be written or borrowed mutably.
+    /// Whether this value can be written or borrowed mutably.
     /// Returns the root place if the place passed in is a projection.
     fn is_mutable<'d>(
         &self,
@@ -2021,14 +2021,14 @@ impl<'cx, 'gcx, 'tcx> MirBorrowckCtxt<'cx, 'gcx, 'tcx> {
                             ty::RawPtr(tnm) => {
                                 match tnm.mutbl {
                                     // `*const` raw pointers are not mutable
-                                    hir::MutImmutable => return Err(place),
+                                    hir::MutImmutable => Err(place),
                                     // `*mut` raw pointers are always mutable, regardless of
                                     // context. The users have to check by themselves.
                                     hir::MutMutable => {
-                                        return Ok(RootPlace {
+                                        Ok(RootPlace {
                                             place,
                                             is_local_mutation_allowed,
-                                        });
+                                        })
                                     }
                                 }
                             }