Add `MaybeUninit` method `array_assume_init` #80600

joboet · 2021-01-01T21:42:37Z

When initialising an array element-by-element, the conversion to the initialised array is done through mem::transmute, which is both ugly and does not work with const generics (see #61956). This PR proposes the associated method array_assume_init, matching the style of slice_assume_init_*:

unsafe fn array_assume_init<T, const N: usize>(array: [MaybeUninit<T>; N]) -> [T; N];

Example:

let mut array: [MaybeUninit<i32>; 3] = MaybeUninit::uninit_array();
array[0].write(0);
array[1].write(1);
array[2].write(2);

// SAFETY: Now safe as we initialised all elements
let array: [i32; 3] = unsafe {
     MaybeUninit::array_assume_init(array)
};

Things I'm unsure about:

Should this be a method of array instead?
Should the function be const?

rust-highfive · 2021-01-01T21:42:40Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @dtolnay (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

dtolnay

This looks good to me. Could you please open a tracking issue or pick one of the existing maybeuninit ones to track this under?

joboet · 2021-01-11T09:09:46Z

Here it is: #80908

dtolnay · 2021-01-11T09:13:40Z

@bors r

bors · 2021-01-11T09:13:42Z

📌 Commit dec8c03 has been approved by dtolnay

library/core/src/mem/maybe_uninit.rs

rust-log-analyzer · 2021-01-11T22:35:28Z

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   Compiling unwind v0.0.0 (/checkout/library/unwind)
error[E0308]: mismatched types
   --> library/core/src/mem/maybe_uninit.rs:843:13
    |
242 | impl<T> MaybeUninit<T> {
    |      - this type parameter
...
835 |     pub unsafe fn array_assume_init<const N: usize>(array: [Self; N]) -> [T; N] {
    |                                                                          ------ expected `[T; N]` because of return type
...
843 |             (&array as *const _ as *const T).read()
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected array, found type parameter `T`
    |
    = note:       expected array `[T; N]`
            found type parameter `T`
error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.
error: could not compile `core`
error: could not compile `core`

To learn more, run the command again with --verbose.
command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "check" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "16" "--release" "--color" "always" "--features" "panic-unwind backtrace compiler-builtins-c" "--manifest-path" "/checkout/library/test/Cargo.toml" "--message-format" "json-render-diagnostics"
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap check
Build completed unsuccessfully in 0:01:16

RalfJung · 2021-01-11T23:15:30Z

Okay then let's @bors r- until the new implementation is done.

dtolnay · 2021-01-12T00:43:15Z

@bors r

bors · 2021-01-12T00:43:16Z

📌 Commit 985071b has been approved by dtolnay

Rollup of 8 pull requests Successful merges: - rust-lang#79757 (Replace tabs earlier in diagnostics) - rust-lang#80600 (Add `MaybeUninit` method `array_assume_init`) - rust-lang#80880 (Move some tests to more reasonable directories) - rust-lang#80897 (driver: Use `atty` instead of rolling our own) - rust-lang#80898 (Add another test case for rust-lang#79808) - rust-lang#80917 (core/slice: remove doc comment about scoped borrow) - rust-lang#80927 (Replace a simple `if let` with the `matches` macro) - rust-lang#80930 (fix typo in trait method mutability mismatch help) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

ghost · 2021-01-26T13:09:03Z

library/core/src/mem/maybe_uninit.rs

+        // * MaybeUnint does not drop, so there are no double-frees
+        // And thus the conversion is safe
+        unsafe {
+            intrinsics::assert_inhabited::<T>();


Is this assertion really correct?

I tried this code:

#![feature(maybe_uninit_array_assume_init)] use std::mem::MaybeUninit; enum Uninhabited {} fn main() { unsafe { MaybeUninit::<Uninhabited>::array_assume_init([]); } }

And it panicked with attempted to instantiate uninhabited type `Uninhabited`.

Compiling playground v0.0.1 (/playground) Finished dev [unoptimized debuginfo] target(s) in 1.12s Running `target/debug/playground` thread 'main' panicked at 'attempted to instantiate uninhabited type `Uninhabited`', /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/core/src/mem/maybe_uninit.rs:842:13 stack backtrace: 0: rust_begin_unwind at /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/std/src/panicking.rs:493:5 1: core::panicking::panic_fmt at /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/core/src/panicking.rs:92:14 2: core::panicking::panic at /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/core/src/panicking.rs:50:5 3: core::mem::maybe_uninit::MaybeUninit<T>::array_assume_init at /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/core/src/mem/maybe_uninit.rs:842:13 4: playground::main at ./src/main.rs:9:9 5: core::ops::function::FnOnce::call_once at /rustc/f4eb5d9f719cd3c849befc8914ad8ce0ddcf34ed/library/core/src/ops/function.rs:227:5 note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

But it seems that creating an array of uninhabited type with length of zero is not UB (playground):

#![forbid(unsafe_code)] #[derive(Debug)] enum Uninhabited {} fn main() { let not_ub: [Uninhabited; 0] = []; dbg!(not_ub); }

Good point! This should probably be

if N > 0 { assert_inhabited(); }

I'll submit a PR to fix it.

…ertion, r=m-ou-se Fix assertion in `MaybeUninit::array_assume_init()` for zero-length arrays That assertion has a false positive ([playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=63922b8c897b04112adcdf346deb1d0e)): ```rust #![feature(maybe_uninit_array_assume_init)] use std::mem::MaybeUninit; enum Uninhabited {} fn main() { unsafe { // thread 'main' panicked at 'attempted to instantiate uninhabited type `Uninhabited`' MaybeUninit::<Uninhabited>::array_assume_init([]); } } ``` *Previously reported in rust-lang#80600 (comment) This PR makes it ignore zero-length arrays. cc rust-lang#80908

joboet added 2 commits January 1, 2021 22:03

Implement MaybeUninit::array_assume_init

826bc36

Add test for MaybeUninit::array_assume_init

0ff1e6c

rust-highfive assigned dtolnay Jan 1, 2021

rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 1, 2021

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Format code

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dtolnay reviewed Jan 11, 2021

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Tracking Issue for maybe_uninit_array_assume_init #80908

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Add tracking issue for array_assume_init

dec8c03

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 11, 2021

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library/core/src/mem/maybe_uninit.rs Outdated Show resolved Hide resolved

Simplify array_assume_init

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bors added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Jan 11, 2021

Fix implementation

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JohnTitor mentioned this pull request Jan 12, 2021

Rollup of 8 pull requests #80939

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bors merged commit babfdaf into rust-lang:master Jan 12, 2021

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ghost mentioned this pull request Jan 26, 2021

Fix assertion in MaybeUninit::array_assume_init() for zero-length arrays #81412

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clarfonthey mentioned this pull request Apr 16, 2022

Tracking Issue for MaybeUninit methods for arrays #96097

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Add `MaybeUninit` method `array_assume_init` #80600

Add `MaybeUninit` method `array_assume_init` #80600

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Add MaybeUninit method array_assume_init #80600

Add MaybeUninit method array_assume_init #80600

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Add `MaybeUninit` method `array_assume_init` #80600

Add `MaybeUninit` method `array_assume_init` #80600