Do not create dangling &T in Weak<T>::drop #80488
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Rollup of 9 pull requests Successful merges: - rust-lang#78934 (refactor: removing library/alloc/src/vec/mod.rs ignore-tidy-filelength) - rust-lang#79479 (Add `Iterator::intersperse`) - rust-lang#80128 (Edit rustc_ast::ast::FieldPat docs) - rust-lang#80424 (Don't give an error when creating a file for the first time) - rust-lang#80458 (Some Promotion Refactoring) - rust-lang#80488 (Do not create dangling &T in Weak<T>::drop) - rust-lang#80491 (Miri: make size/align_of_val work for dangling raw ptrs) - rust-lang#80495 (Rename kw::Invalid -> kw::Empty) - rust-lang#80513 (Add regression test for rust-lang#80062) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
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Since at this point all strong pointers have been dropped, the wrapped
Thas also been dropped. As such, creating a&Tto the dropped place is negligent at best (language UB at worst). Since we haveLayout::for_value_rawnow, use that instead ofLayout::for_valueto 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:
*const Tto 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&Tinstead 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 isCopy, perdrop_in_place's docs).In this design, custom DSTs would get a
*mut Tand use that to return layout, and must be able to do so while in the "zombie" (post-drop, pre-free) state.RcBox/ArcInnercompute 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 aWeak<DST>, and drop theRcbefore theWeak. 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.