Do not create dangling &T in Weak::drop by CAD97 · Pull Request #80488 · rust-lang/rust (original) (raw)

Since at this point all strong pointers have been dropped, the wrapped T has also been dropped. As such, creating a &T to the dropped place is negligent at best (language UB at worst). Since we have Layout::for_value_raw now, use that instead of Layout::for_value to avoid creating the &T.

This does have implications for custom (potentially thin) DSTs, though much less severe than those discussed in #80407. Specifically, one of two things has to be true:

• It has to be possible to use a *const T to a dropped (potentially custom, potentially thin) unsized tailed object to determine the layout (size/align) of the object. This is what is currently implemented (though with &T instead of &T). The validity of reading some location after it has been dropped is an open question IIUC (What about: use-after-move and (maybe) use-after-drop unsafe-code-guidelines#188) (except when the whole type is Copy, per drop_in_place's docs).
  In this design, custom DSTs would get a *mut T and use that to return layout, and must be able to do so while in the "zombie" (post-drop, pre-free) state.
• RcBox/ArcInner compute and store layout eagerly, so that they don't have to ask the type for its layout after dropping it.

Importantly, this is already true today, as you can construct Rc<DST>, create a Weak<DST>, and drop the Rc before the Weak. This PR is a strict improvement over the status quo, and the above question about potentially thin DSTs will need to be resolved by any custom DST proposal.