Fix potential crash on large constant ZST slice

Author: bjorn3

src/constant.rs

@@ -186,8 +186,7 @@ pub(crate) fn codegen_const_value<'tcx>(
         ConstValue::Slice { data, meta } => {
             let alloc_id = fx.tcx.reserve_and_set_memory_alloc(data);
             let ptr = pointer_for_allocation(fx, alloc_id).get_addr(fx);
-            // FIXME: the `try_from` here can actually fail, e.g. for very long ZST slices.
-            let len = fx.bcx.ins().iconst(fx.pointer_type, i64::try_from(meta).unwrap());
+            let len = fx.bcx.ins().iconst(fx.pointer_type, meta as i64);
             CValue::by_val_pair(ptr, len, layout)
         }
     }