diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 1d2222adb9da..751efe0e71ec 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -259,6 +259,10 @@ use crate::vec::Vec;
 #[cfg(test)]
 mod tests;
 
+// This is repr(C) to future-proof against possible field-reordering, which
+// would interfere with otherwise safe [into|from]_raw() of transmutable
+// inner types.
+#[repr(C)]
 struct RcBox<T: ?Sized> {
     strong: Cell<usize>,
     weak: Cell<usize>,
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index f9c8da58c75c..b44d78f31ec9 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -270,6 +270,10 @@ impl<T: ?Sized + fmt::Debug> fmt::Debug for Weak<T> {
     }
 }
 
+// This is repr(C) to future-proof against possible field-reordering, which
+// would interfere with otherwise safe [into|from]_raw() of transmutable
+// inner types.
+#[repr(C)]
 struct ArcInner<T: ?Sized> {
     strong: atomic::AtomicUsize,