librustc: Redo the unsafe checker and make unsafe methods not callable from safe code

src/libstd/task/spawn.rs

@@ -159,13 +159,17 @@ struct AncestorList(Option<Exclusive<AncestorNode>>);
 // Accessors for taskgroup arcs and ancestor arcs that wrap the unsafety.
 #[inline(always)]
 fn access_group<U>(x: &TaskGroupArc, blk: &fn(TaskGroupInner) -> U) -> U {
-    x.with(blk)
+    unsafe {
+        x.with(blk)
+    }
 }
 
 #[inline(always)]
 fn access_ancestors<U>(x: &Exclusive<AncestorNode>,
                        blk: &fn(x: &mut AncestorNode) -> U) -> U {
-    x.with(blk)
+    unsafe {
+        x.with(blk)
+    }
 }
 
 // Iterates over an ancestor list.