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As we did with `Box`, we can allocate an uninitialized `Rc` or `Arc`
beforehand, giving the optimizer a chance to skip the local value for
regular clones, or avoid any local altogether for `T: Copy`.
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For generic `T: Clone`, we can allocate an uninitialized box beforehand,
which gives the optimizer a chance to create the clone directly in the
heap. For `T: Copy`, we can go further and do a simple memory copy,
regardless of optimization level.
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Taken from #80293.
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BTreeMap: tougher checking on most uses of copy_nonoverlapping
Miri checks pointer provenance and destination, but we can check it in debug builds already.
Also, we can let Miri confirm we don't mistake imprints of moved keys and values as genuine.
r? `@Mark-Simulacrum`
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Hopefully this will prevent people from continuing to ask about this
over and over again :)
See [this Zulip discussion][1] for more.
[1]: https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Vec.3A.3Atruncate.20implementation
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As per T-lang consensus, this uses a branch to handle the dangling case.
The discussed optimization of only doing the branch in the T: ?Sized
case is left for a followup patch, as doing so is not trivial
(as it requires specialization for correctness, not just optimization).
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Fix missing link for "fully qualified syntax"
This issue can currently be seen at https://doc.rust-lang.org/stable/std/rc/index.html#toggle-all-docs:~:text=%5B-,fully%20qualified%20syntax
It originates from #76138, where the link was added to `library/alloc/src/sync.rs`, but not `library/alloc/src/rc.rs`.
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Mention Arc::make_mut and Rc::make_mut in the documentation of Cow
Following this discussion: https://users.rust-lang.org/t/should-the-cow-documentation-mention-arc/53341
_Rendered (the last paragraph is new):_
`@rustbot` modify labels: T-doc, T-libs
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remove allow(incomplete_features) from std
cc https://github.com/rust-lang/rust/pull/80349#issuecomment-753357123
> Now I am somewhat concerned that the standard library uses some of these features...
I think it is theoretically ok to use incomplete features in the standard library or the compiler if we know that there is an already working subset and we explicitly document what we have to be careful about. Though at that point it is probably better to try and split the incomplete feature into two separate ones, similar to `min_specialization`.
Will be interesting once `feature(const_evaluatable_checked)` works well enough to imo be used in the compiler but not yet well enough to be removed from `INCOMPLETE_FEATURES`.
r? `@RalfJung`
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Remove many unnecessary manual link resolves from library
Now that #76934 has merged, we can remove a lot of these! E.g, this is
no longer necessary:
[`Vec<T>`]: Vec
cc `@jyn514`
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Add fallible Box, Arc, and Rc allocator APIs
cc https://github.com/rust-lang/rust/issues/48043
It was suggested in https://github.com/rust-lang/rust/issues/48043#issuecomment-748008486 that `Box::try_*` follows the spirit of RFC 2116. This PR is an attempt to add the relevant APIs, tied to the same feature gate. Happy to make any changes or turn this into an RFC if necessary.
cc `@rust-lang/wg-allocators`
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Now that #76934 has merged, we can remove a lot of these! E.g, this is
no longer necessary:
[`Vec<T>`]: Vec
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The return of the GroupBy and GroupByMut iterators on slice
According to https://github.com/rust-lang/rfcs/pull/2477#issuecomment-742034372, I am opening this PR again, this time I implemented it in safe Rust only, it is therefore much easier to read and is completely safe.
This PR proposes to add two new methods to the slice, the `group_by` and `group_by_mut`. These two methods provide a way to iterate over non-overlapping sub-slices of a base slice that are separated by the predicate given by the user (e.g. `Partial::eq`, `|a, b| a.abs() < b.abs()`).
```rust
let slice = &[1, 1, 1, 3, 3, 2, 2, 2];
let mut iter = slice.group_by(|a, b| a == b);
assert_eq!(iter.next(), Some(&[1, 1, 1][..]));
assert_eq!(iter.next(), Some(&[3, 3][..]));
assert_eq!(iter.next(), Some(&[2, 2, 2][..]));
assert_eq!(iter.next(), None);
```
[An RFC](https://github.com/rust-lang/rfcs/pull/2477) was open 2 years ago but wasn't necessary.
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Rollup of 9 pull requests
Successful merges:
- #78934 (refactor: removing library/alloc/src/vec/mod.rs ignore-tidy-filelength)
- #79479 (Add `Iterator::intersperse`)
- #80128 (Edit rustc_ast::ast::FieldPat docs)
- #80424 (Don't give an error when creating a file for the first time)
- #80458 (Some Promotion Refactoring)
- #80488 (Do not create dangling &T in Weak<T>::drop)
- #80491 (Miri: make size/align_of_val work for dangling raw ptrs)
- #80495 (Rename kw::Invalid -> kw::Empty)
- #80513 (Add regression test for #80062)
Failed merges:
r? `@ghost`
`@rustbot` modify labels: rollup
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Do not create dangling &T in Weak<T>::drop
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 (https://github.com/rust-lang/unsafe-code-guidelines/issues/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.
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