improve Vec example soundness in mem::transmute docs by llogiq · Pull Request #64436 · rust-lang/rust

llogiq · 2019-09-13T16:54:38Z

The previous version of the Vec example had a case of questionable soundness, because at one point v_orig was aliased.

r? @RalfJung

RalfJung · 2019-09-16T18:03:14Z

The previous version of the Vec example had a case of questionable soundness, because at one point v_orig was aliased.

Nice! Yes ManuallyDrop is certainly better.

However, the code is still somewhat confusing, with its three examples in one doctest and the clone that do not have anything to do with the actual operation.
Maybe it would be worth refactoring it to have one block per "way" of doing this transmute, and have each block look like

{let v = v_orig.clone();// Here comes the "way" do do stuff with `v`.}

That would make it more clear that the clone is part of the "environment" here, not part of what is really going on.

However, we can also land this PR without that change as it is a definite improvement.

RalfJung · 2019-09-17T07:30:36Z

Why not "clone the vector"?

RalfJung · 2019-09-17T07:32:09Z

Suggested change
/// let v_clone: &mut Vec<_> = &mut *std::mem::ManuallyDrop::new(v_clone);
/// let v_clone = std::mem::ManuallyDrop::new(v_clone);
This should work due to the Deref, right? And I think it is more readable.

llogiq · 2019-09-17T10:38:44Z

@RalfJung thanks for the review!

RalfJung · 2019-09-17T10:41:49Z

LGTM, thanks!

@bors r+ rollup

bors · 2019-09-17T10:41:50Z

📌 Commit 14bb7550cd65a90f38ae3f17c2e46a7bb17b47aa has been approved by RalfJung

rust-highfive · 2019-09-17T12:15:37Z

The job x86_64-gnu-llvm-6.0 of your PR failed (pretty log, raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

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2019-09-17T10:42:52.6564381Z ##[command]git remote add origin https://github.com/rust-lang/rust
2019-09-17T10:42:52.6760619Z ##[command]git config gc.auto 0
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2019-09-17T12:15:32.6901787Z 2019-09-17T12:15:32.6902154Z failures:
2019-09-17T12:15:32.6902335Z 2019-09-17T12:15:32.6903668Z ---- intrinsics.rs - intrinsics::transmute (line 846) stdout ----
2019-09-17T12:15:32.6903783Z error[E0596]: cannot borrow `v_clone` as mutable, as it is not declared as mutable
2019-09-17T12:15:32.6904420Z |
2019-09-17T12:15:32.6904420Z |
2019-09-17T12:15:32.6904468Z 35 | let v_clone = std::mem::ManuallyDrop::new(v_clone);
2019-09-17T12:15:32.6904804Z | ------- help: consider changing this to be mutable: `mut v_clone`
2019-09-17T12:15:32.6904865Z 36 | Vec::from_raw_parts(v_clone.as_mut_ptr() as *mut Option<&i32>,
2019-09-17T12:15:32.6904917Z | ^^^^^^^ cannot borrow as mutable
2019-09-17T12:15:32.6905016Z error: aborting due to previous error
2019-09-17T12:15:32.6905061Z 2019-09-17T12:15:32.6905317Z For more information about this error, try `rustc --explain E0596`.
2019-09-17T12:15:32.6905534Z Couldn't compile the test.
---
2019-09-17T12:15:32.6905980Z 2019-09-17T12:15:32.6945347Z error: test failed, to rerun pass '--doc'
2019-09-17T12:15:32.6962413Z 2019-09-17T12:15:32.6963304Z 2019-09-17T12:15:32.6964249Z command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "test" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "2" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace compiler-builtins-c" "--manifest-path" "/checkout/src/libtest/Cargo.toml" "-p" "core" "--" "--quiet"
2019-09-17T12:15:32.6965231Z 2019-09-17T12:15:32.6965371Z 2019-09-17T12:15:32.6977355Z failed to run: /checkout/obj/build/bootstrap/debug/bootstrap test
2019-09-17T12:15:32.6977600Z Build completed unsuccessfully in 1:25:49
2019-09-17T12:15:32.6977600Z Build completed unsuccessfully in 1:25:49
2019-09-17T12:15:32.7036969Z == clock drift check ==
2019-09-17T12:15:32.7051679Z local time: Tue Sep 17 12:15:32 UTC 2019
2019-09-17T12:15:32.8681083Z network time: Tue, 17 Sep 2019 12:15:32 GMT
2019-09-17T12:15:32.8681160Z == end clock drift check ==
2019-09-17T12:15:33.5216535Z ##[error]Bash exited with code '1'.
2019-09-17T12:15:33.5257944Z ##[section]Starting: Checkout
2019-09-17T12:15:33.5259781Z ==============================================================================
2019-09-17T12:15:33.5259833Z Task : Get sources
2019-09-17T12:15:33.5259894Z Description : Get sources from a repository. Supports Git, TfsVC, and SVN repositories.

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

RalfJung · 2019-09-17T12:46:15Z

@bors r- delegate+
Please fix the CI failure, wait for CI in the PR to be green, then do bors r=RalfJung

bors · 2019-09-17T12:46:18Z

✌️ @llogiq can now approve this pull request

llogiq · 2019-09-17T18:42:51Z

@bors r=RalfJung

bors · 2019-09-17T18:42:52Z

📌 Commit ab6e108 has been approved by RalfJung

@RalfJung

improve Vec example soundness in mem::transmute docs The previous version of the `Vec` example had a case of questionable soundness, because at one point `v_orig` was aliased. r? @RalfJung

@RalfJung

improve Vec example soundness in mem::transmute docs The previous version of the `Vec` example had a case of questionable soundness, because at one point `v_orig` was aliased. r? @RalfJung

@ghost

Rollup of 10 pull requests Successful merges: - #61626 (Get rid of special const intrinsic query in favour of `const_eval`) - #64283 (Updated RELEASES.md for 1.38.0) - #64394 (Shrink `SubregionOrigin`.) - #64429 (Fix failure note `to_str` implementation) - #64436 (improve Vec example soundness in mem::transmute docs) - #64502 (avoid duplicate issues for Miri build failures) - #64505 (Fix inconsistent link formatting) - #64529 (Add an example to Pin::as_mut) - #64541 (document Miri error categories) - #64544 (build-manifest: re-add some comments) Failed merges: r? @ghost

rust-highfive assigned RalfJung Sep 13, 2019

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Why not "clone the vector"?

RalfJung reviewed Sep 17, 2019
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improve Vec example soundness in mem::transmute docs
ab6e108

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tmandry mentioned this pull request Sep 17, 2019
Rollup of 8 pull requests #64560
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tmandry mentioned this pull request Sep 17, 2019
Rollup of 10 pull requests #64562
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bors merged commit ab6e108 into rust-lang:masterSep 18, 2019

llogiq deleted the transmute-docs branch September 18, 2019 04:31

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improve Vec example soundness in mem::transmute docs - #64436

improve Vec example soundness in mem::transmute docs#64436
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	/// let v_clone: &mut Vec<_> = &mut *std::mem::ManuallyDrop::new(v_clone);
	/// let v_clone = std::mem::ManuallyDrop::new(v_clone);

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