avoid computing precise span for const_eval query

Author: RalfJung

src/librustc_mir/interpret/eval_context.rs

@@ -860,7 +860,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         } else {
             self.param_env
         };
-        let val = self.tcx.const_eval_global_id(param_env, gid, Some(self.cur_span()))?;
+        let val = self.tcx.const_eval_global_id(param_env, gid, Some(self.root_span))?;
 
         // Even though `ecx.const_eval` is called from `eval_const_to_op` we can never have a
         // recursion deeper than one level, because the `tcx.const_eval` above is guaranteed to not

src/test/ui/consts/const-size_of-cycle.stderr

@@ -17,8 +17,8 @@ LL |     bytes: [u8; std::mem::size_of::<Foo>()]
 note: ...which requires const-evaluating `std::mem::size_of`...
   --> $SRC_DIR/libcore/mem/mod.rs:LL:COL
    |
-LL |     intrinsics::size_of::<T>()
-   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL | pub const fn size_of<T>() -> usize {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 note: ...which requires const-evaluating + checking `std::intrinsics::size_of`...
   --> $SRC_DIR/libcore/intrinsics.rs:LL:COL
    |