Update comment and make it truer by actually doing what it says

Author: oli-obk

src/librustc/mir/interpret/value.rs

@@ -13,6 +13,9 @@ pub struct RawConst<'tcx> {
     pub ty: Ty<'tcx>,
     /// the allocation that would be returned by using
     /// `tcx.alloc_map.lock().unwrap_memory(self.alloc_id)`
+    ///
+    /// This is an optimization so we don't actually have to go fetch said allocation from the
+    /// `alloc_map` at most use sites of the `const_eval_raw` query
     pub alloc: &'tcx Allocation,
 }
 

src/librustc_mir/interpret/memory.rs

@@ -348,10 +348,9 @@ impl<'a, 'mir, 'tcx, M: Machine<'a, 'mir, 'tcx>> Memory<'a, 'mir, 'tcx, M> {
                 ErrorHandled::TooGeneric => EvalErrorKind::TooGeneric.into(),
             }
         }).map(|raw_const| {
-            let allocation = tcx.alloc_map.lock().unwrap_memory(raw_const.alloc_id);
             // We got tcx memory. Let the machine figure out whether and how to
             // turn that into memory with the right pointer tag.
-            M::adjust_static_allocation(allocation, memory_extra)
+            M::adjust_static_allocation(raw_const.alloc, memory_extra)
         })
     }