From: Mark Rousskov <mark.simulacrum@gmail.com>
Date: Sun, 22 Dec 2019 01:35:41 +0000 (-0500)
Subject: Warn against relying on ?Sized being last
X-Git-Url: https://git.lizzy.rs/?a=commitdiff_plain;h=a34c2677afeee2747d680536f302a8c5665a65f4;p=rust.git

Warn against relying on ?Sized being last
---

diff --git a/src/liballoc/rc.rs b/src/liballoc/rc.rs
index 42a278de98b..00c4b5462be 100644
--- a/src/liballoc/rc.rs
+++ b/src/liballoc/rc.rs
@@ -2106,6 +2106,8 @@ impl<T: ?Sized> Unpin for Rc<T> { }
 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
     // Align the unsized value to the end of the `RcBox`.
     // Because it is ?Sized, it will always be the last field in memory.
+    // Note: This is a detail of the current implementation of the compiler,
+    // and is not a guaranteed language detail. Do not rely on it outside of std.
     data_offset_align(align_of_val(&*ptr))
 }
 
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index a99564c0dac..37995a7ec79 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -2161,6 +2161,8 @@ impl<T: ?Sized> Unpin for Arc<T> { }
 unsafe fn data_offset<T: ?Sized>(ptr: *const T) -> isize {
     // Align the unsized value to the end of the `ArcInner`.
     // Because it is `?Sized`, it will always be the last field in memory.
+    // Note: This is a detail of the current implementation of the compiler,
+    // and is not a guaranteed language detail. Do not rely on it outside of std.
     data_offset_align(align_of_val(&*ptr))
 }