Add slice::array_chunks_mut

[cuviper]

This follows array_chunks from #74373 with a mutable version, array_chunks_mut. The implementation is identical apart from mutability. The new tests are adaptations of the chunks_exact_mut tests, plus an inference test like the one for array_chunks.

I reused the unstable feature array_chunks and tracking issue #74985, but I can separate that if desired.

r? @withoutboats
cc @lcnr

[cuviper]

I'm willing to do reverse versions too -- array_rchunks[_mut]? That could be in this PR or a followup.

I also want to try array_windows, but I do think that deserves a separate feature and tracking issue.

[lcnr]

Can't say much about the aliasing stuff in array_chunks_mut, but the rest looks good.

Iirc @JulianKnodt wanted to look into implementing fn array_windows

[lcnr]

I think rchunks might be better with a different feature gate and in a separate PR, don't really care that much though

[cuviper]

Can't say much about the aliasing stuff in array_chunks_mut,

Yeah, I'm not certain that's the best way to avoid a mutable alias, but it seems simple enough.

Iirc @JulianKnodt wanted to look into implementing fn array_windows

OK, sure, I'll leave them to it.

[lcnr]

Afaik mutable aliases are fine as long as the aliasing mutable reference isn't used anymore, i.e. there is no ABAB access.

Does not using that explicit scope cause MIRI to fail?

[cuviper]

I thought the mere existence of aliases was a problem, but I could be wrong. I haven't tried with miri yet.

[cuviper]

Does not using that explicit scope cause MIRI to fail?

AFAICT, miri is fine with it either way. Should I simplify it then?

[lcnr]

i think so 😄

[Dylan-DPC-zz]

@withoutboats this is ready for review

[Dylan-DPC-zz]

r? @scottmcm

[scottmcm]

The code here looks right, and given that array_chunks exists, a _mut version seems totally reasonable.

@bors r+

[bors]

📌 Commit fca686f7e7da8fb971958b3928178cd229fa1620 has been approved by scottmcm

[bors]

⌛ Testing commit fca686f7e7da8fb971958b3928178cd229fa1620 with merge 321229650bb03b6b0edc8fd1f5056dc6fee9f7a9...

[rust-log-analyzer]

The job dist-x86_64-freebsd 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.

Click to expand the log.

   Compiling profiler_builtins v0.0.0 (/checkout/library/profiler_builtins)
[RUSTC-TIMING] build_script_build test:false 0.320
[RUSTC-TIMING] build_script_build test:false 0.344
[RUSTC-TIMING] build_script_build test:false 0.513
error[E0407]: method `get_unchecked` is not a member of trait `TrustedRandomAccess`
     |
     |
6062 | /     unsafe fn get_unchecked(&mut self, i: usize) -> &'a mut [T; N] {
6063 | |         unsafe { self.iter.get_unchecked(i) }
6064 | |     }
     | |_____^ not a member of trait `TrustedRandomAccess`
error: aborting due to previous error

For more information about this error, try `rustc --explain E0407`.
[RUSTC-TIMING] core test:false 8.219
[RUSTC-TIMING] core test:false 8.219
error: could not compile `core`.

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed
command did not execute successfully: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo" "build" "--target" "x86_64-unknown-linux-gnu" "-Zbinary-dep-depinfo" "-j" "16" "--release" "--locked" "--color" "always" "--features" "panic-unwind backtrace profiler compiler-builtins-c" "--manifest-path" "/checkout/library/test/Cargo.toml" "--message-format" "json-render-diagnostics"
failed to run: /checkout/obj/build/bootstrap/debug/bootstrap dist --host x86_64-unknown-freebsd --target x86_64-unknown-freebsd
Build completed unsuccessfully in 0:00:09
== clock drift check ==
  local time: Sat Sep  5 02:22:39 UTC 2020
  local time: Sat Sep  5 02:22:39 UTC 2020
  network time: Sat, 05 Sep 2020 02:22:39 GMT
== end clock drift check ==
##[error]Process completed with exit code 1.
Terminate orphan process: pid (4048) (node)
Terminate orphan process: pid (4057) (python)

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 @rust-lang/infra. (Feature Requests)

[bors]

💔 Test failed - checks-actions

[scottmcm]

Looks like this might be a real failure:

method `get_unchecked` is not a member of trait `TrustedRandomAccess`

@bors r-

[cuviper]

Ah, needed to catch up with #73565 -- plus tidy seems picker about unsafe comments now...

[scottmcm]

@bors r+

(Sorry for the delay here!)

[bors]

📌 Commit 86b9f71 has been approved by scottmcm

[bors]

⌛ Testing commit 86b9f71 with merge 8b6838b...

[bors]

☀️ Test successful - checks-actions, checks-azure
Approved by: scottmcm
Pushing 8b6838b to master...