Update unions for safe ManuallyDrop assignment. #912
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@RalfJung Can you review this? |
Havvy
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Dec 16, 2020
RalfJung
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Dec 19, 2020
src/types/union.md
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| implement [`Copy`]. See the [item] documentation for further details. | ||
| field. Since transmutes can cause unexpected or undefined behaviour, `unsafe` | ||
| is required to read from a union field, or to write to a field that doesn't | ||
| implement [`Copy`] or is a [`ManuallyDrop`] type. See the [item] documentation |
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a field that "is a ManuallyDrop type" makes little sense grammatically, IMO.
Suggested change
| implement [`Copy`] or is a [`ManuallyDrop`] type. See the [item] documentation | |
| implement [`Copy`] or has a [`ManuallyDrop`] type. See the [item] documentation |
RalfJung
reviewed
Dec 19, 2020
| these writes don't have to be placed in `unsafe` blocks | ||
| Writes to [`Copy`] or [`ManuallyDrop`][ManuallyDrop] union fields do not | ||
| require reads for running destructors, so these writes don't have to be placed | ||
| in `unsafe` blocks |
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The actual implementation goes a bit further, so u.f1.g2 = ... is safe if that assignment is at ManuallyDrop or impl Copy type... but on stable code that is impossible to observe since all fields must be ManuallyDrop or impl Copy. Not if the reference should even mention it, then.
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I think it's fine to leave out for now. Is it even possible on nightly to have a non-Copy/ManuallyDrop type?
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Yeah, with a feature flag you can use any type without drop glue.
In principle even those could be safe to assign, so we could remove this safety check entirely... for now I went with the smaller change that should just make the safety check unreachable on stable.
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Looks good, aside from the nit and the comment that I left. :) |
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GuillaumeGomez
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Update books ## nomicon 2 commits in d8383b65f7948c2ca19191b3b4bd709b403aaf45..a5a48441d411f61556b57d762b03d6874afe575d 2020-11-22 10:24:42 -0500 to 2020-12-06 10:39:41 +0900 - Update atomics.md (rust-lang/nomicon#249) - Rename `AllocRef` to `Allocator` and `(de)alloc` to `(de)allocate` (rust-lang/nomicon#248) ## reference 2 commits in a8afdca5d0715b2257b6f8b9a032fd4dd7dae855..b278478b766178491a8b6f67afa4bcd6b64d977a 2020-11-30 06:44:46 -0800 to 2020-12-21 18:18:03 -0800 - Update unions for safe ManuallyDrop assignment. (rust-lang/reference#912) - Removing ambiguity in type-layout.md (rust-lang/reference#911) ## book 25 commits in a190438d77d28041f24da4f6592e287fab073a61..5bb44f8b5b0aa105c8b22602e9b18800484afa21 2020-11-16 10:44:08 -0600 to 2020-12-18 20:07:31 -0500 - Make some further edits to rust-lang/book#2447 - Merge remote-tracking branch 'origin/pr/2447' - Remove copied and dangling link brackets - Merge remote-tracking branch 'origin/pr/2359' - Override toolchain to nightly for run lints action. (rust-lang/book#2528) - Remove an uneeded 'static lifetime (rust-lang/book#1752) - Fixes rust-lang/book#2330. Clarify why the lock is held too long - Update paragraph about rustfmt in Chapter 1.2 (rust-lang/book#2304) - Clarify language around further from rust-lang/book#2418 - Merge remote-tracking branch 'origin/pr/2418' - Merge remote-tracking branch 'origin/pr/2475' - Add some further edits to rust-lang/book#2433 - Merge remote-tracking branch 'origin/pr/2433' - Note all the method families to handle integer overflow - Merge remote-tracking branch 'origin/pr/2405' - Fix rust-lang/book#1855 - incorporate new reference cycle diagram - Make some further edits to the changes in rust-lang/book#1886 - Merge remote-tracking branch 'origin/pr/1886' - Make some further edits to rust-lang/book#1998 - Merge remote-tracking branch 'origin/pr/1998' - Update Rust version and output (rust-lang/book#2518) - Fix typo, regarding privileged ports being up to 1023 instead of 1024 (rust-lang/book#2509) - Change "appendixes" to "appendices" in intro. (rust-lang/book#2498) - Update 16-11 to use method call expression for `clone` (rust-lang/book#2511) - Correct chapter 20 final listing (rust-lang/book#2516) ## rust-by-example 7 commits in 236c734a2cb323541b3394f98682cb981b9ec086..1cce0737d6a7d3ceafb139b4a206861fb1dcb2ab 2020-11-30 14:05:49 -0300 to 2020-12-21 17:36:29 -0300 - Add book.description in book.toml (rust-lang/rust-by-example#1397) - Simplify the call of filter_map (rust-lang/rust-by-example#1396) - Update README.md (rust-lang/rust-by-example#1382) - Add missing main function in static life time example. (rust-lang/rust-by-example#1383) - Clarify first matching arm and all possible values (rust-lang/rust-by-example#1395) - Clarify distinction between for iter and into_iter (rust-lang/rust-by-example#1394) - Drop extern crate (rust-lang/rust-by-example#1393)
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Update for the change in rust-lang/rust#78068.