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Allow casting mut array ref to mut ptr #81479

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merged 1 commit into from
Feb 13, 2021
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@osa1 osa1 commented Jan 28, 2021

Allow casting mut array ref to mut ptr

We now allow two new casts:

  • mut array reference to mut ptr. Example:

    let mut x: [usize; 2] = [0, 0];
    let p = &mut x as *mut usize;
    

    We allow casting const array references to const pointers so not
    allowing mut references to mut pointers was inconsistent.

  • mut array reference to const ptr. Example:

    let mut x: [usize; 2] = [0, 0];
    let p = &mut x as *const usize;
    

    This was similarly inconsistent as we allow casting mut references to
    const pointers.

Existing test 'vector-cast-weirdness' updated to test both cases.

Fixes #24151

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r? @lcnr

(rust-highfive has picked a reviewer for you, use r? to override)

@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jan 28, 2021
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LGTM

we don't have tests for &[T; N] as *mut T and &mut [T; N] as *const Tafaik. Both of these should error. Can you add them as well?

@lcnr lcnr added I-nominated T-lang Relevant to the language team, which will review and decide on the PR/issue. labels Jan 28, 2021
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lcnr commented Jan 28, 2021

Nominating for T-lang, this allows the cast &mut [T; N] as *mut T copying the already existing one cast &[T;N] as *const T.

Don't think this is controversial but still needs some T-lang signoff.

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osa1 commented Jan 28, 2021

Thanks for the quick review.

we don't have tests for &[T; N] as *mut T and &mut [T; N] as *const Tafaik. Both of these should error. Can you add them as well?

Should I do it in the same test file, or in a different file?

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osa1 commented Jan 28, 2021

Shouldn't &mut [T; N] as *const T be OK? In my changes I allow this cast currently.

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lcnr commented Jan 28, 2021

Should I do it in the same test file, or in a different file?

🤷 don't care, whatever you prefer

Shouldn't &mut [T; N] as *const T be OK? In my changes I allow this cast currently.

we do allow it for &mut T as *const T so imo it's fine. Afaict your changes don't allow this though?
You're using if m_expr.mutbl == m_cast.mutbl 🤔 Am I misunderstanding something here

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osa1 commented Jan 28, 2021

You're using if m_expr.mutbl == m_cast.mutbl thinking Am I misunderstanding something here

Ah, sorry, you're right. I'm confused..

I think we could allow it. I'll wait for the lang team's feedback.

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osa1 commented Jan 30, 2021

@lcnr just out of curiosity, when is this going to be discussed?

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lcnr commented Jan 30, 2021

the lang team meeting takes place every tuesday, so in 3 days

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osa1 commented Feb 4, 2021

Any news from the meeting, @lcnr ?

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osa1 commented Feb 4, 2021

I found the meeting notes: https://hackmd.io/_-pJa1UxTE6qO1JolgNVRQ I guess there wasn't time to discuss this PR.

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We discussed this in the meeting today and decided that we approve of this PR. =) Merge away!

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osa1 commented Feb 10, 2021

Thanks lang team for considering this.

I updated the PR:

  • I now allow mut-ref-to-const-ptr casts too
  • The existing test vector-cast-weirdness updated to test both mut-ref-to-mut-ptr and mut-ref-to-const-ptr cases. Because this test now tests all cases I removed the regression test.
  • Rebased the branch.

@lcnr I think this is ready for merging now.

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lcnr commented Feb 10, 2021

@osa1 do we already have a test checking that &[T; N] as *const T errors?

if/once that test exists r=me

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osa1 commented Feb 10, 2021

@lcnr

do we already have a test checking that &[T; N] as *const T errors?

I'm guessing you meant "&[T; N] as *mut T"? I added tests for that now.

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We now allow two new casts:

- mut array reference to mut ptr. Example:

      let mut x: [usize; 2] = [0, 0];
      let p = &mut x as *mut usize;

  We allow casting const array references to const pointers so not
  allowing mut references to mut pointers was inconsistent.

- mut array reference to const ptr. Example:

      let mut x: [usize; 2] = [0, 0];
      let p = &mut x as *const usize;

  This was similarly inconsistent as we allow casting mut references to
  const pointers.

Existing test 'vector-cast-weirdness' updated to test both cases.

Fixes rust-lang#24151
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Something I was wondering was whether the language reference, or some other documentation, would need to be updated. I couldn't find an obvious place when looking though. cc @rust-lang/lang-docs

JohnTitor added a commit to JohnTitor/rust that referenced this pull request Feb 11, 2021
Allow casting mut array ref to mut ptr

Allow casting mut array ref to mut ptr

We now allow two new casts:

- mut array reference to mut ptr. Example:

      let mut x: [usize; 2] = [0, 0];
      let p = &mut x as *mut usize;

  We allow casting const array references to const pointers so not
  allowing mut references to mut pointers was inconsistent.

- mut array reference to const ptr. Example:

      let mut x: [usize; 2] = [0, 0];
      let p = &mut x as *const usize;

  This was similarly inconsistent as we allow casting mut references to
  const pointers.

Existing test 'vector-cast-weirdness' updated to test both cases.

Fixes rust-lang#24151
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bors commented Feb 11, 2021

⌛ Testing commit d64b749 with merge 663d3127eb8812c66293464cf01bb7a2e5cf3763...

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bors commented Feb 12, 2021

💥 Test timed out

@bors bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Feb 12, 2021
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A job failed! Check out the build log: (web) (plain)

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

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lcnr commented Feb 12, 2021

probably a spurious error

@bors retry

@bors 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 Feb 12, 2021
Dylan-DPC-zz pushed a commit to Dylan-DPC-zz/rust that referenced this pull request Feb 12, 2021
Allow casting mut array ref to mut ptr

Allow casting mut array ref to mut ptr

We now allow two new casts:

- mut array reference to mut ptr. Example:

      let mut x: [usize; 2] = [0, 0];
      let p = &mut x as *mut usize;

  We allow casting const array references to const pointers so not
  allowing mut references to mut pointers was inconsistent.

- mut array reference to const ptr. Example:

      let mut x: [usize; 2] = [0, 0];
      let p = &mut x as *const usize;

  This was similarly inconsistent as we allow casting mut references to
  const pointers.

Existing test 'vector-cast-weirdness' updated to test both cases.

Fixes rust-lang#24151
bors added a commit to rust-lang-ci/rust that referenced this pull request Feb 13, 2021
Rollup of 10 pull requests

Successful merges:

 - rust-lang#79775 (Fix injected errors when running doctests on a crate named after a keyword)
 - rust-lang#81012 (Stabilize the partition_point feature)
 - rust-lang#81479 (Allow casting mut array ref to mut ptr)
 - rust-lang#81506 (HWAddressSanitizer support)
 - rust-lang#81741 (Increment `self.index` before calling `Iterator::self.a.__iterator_ge…)
 - rust-lang#81850 (use RWlock when accessing os::env)
 - rust-lang#81911 (GAT/const_generics: Allow with_opt_const_param to return GAT param def_id)
 - rust-lang#82022 (Push a `char` instead of a `str` with len one into a String)
 - rust-lang#82023 (Remove unnecessary lint allow attrs on example)
 - rust-lang#82030 (Use `Iterator::all` instead of open-coding it)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors merged commit fc93e26 into rust-lang:master Feb 13, 2021
@rustbot rustbot added this to the 1.52.0 milestone Feb 13, 2021
@osa1 osa1 deleted the issue24151 branch February 13, 2021 07:35
@lcnr lcnr added the relnotes Marks issues that should be documented in the release notes of the next release. label Feb 13, 2021
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lcnr commented Feb 13, 2021

it might make sense to mention this in the rel notes, not sure 🤔

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Havvy commented Feb 13, 2021

It's a change in the language, so yes.

osa1 added a commit to osa1/reference that referenced this pull request Mar 26, 2021
- Add reference-to-pointer casts
- Update array-to-pointer casts for [rust#81479][1]

[1]: rust-lang/rust#81479
wip-sync pushed a commit to NetBSD/pkgsrc-wip that referenced this pull request May 9, 2021
Package changes:
 * bump bootstraps to 1.51.0.
 * adjust patches and cargo checksums as required
 * 1.51 failed to build natively on 32-bit armv7, there is hope
   that this is fixed with 1.52.  (1.51 can be built with netbsd32
   emulation on a aarch64 system).

Upsteream changes:

Version 1.52.0 (2021-05-06)
============================

Language
--------
- [Added the `unsafe_op_in_unsafe_fn` lint, which checks whether the unsafe
  code in an `unsafe fn` is wrapped in a `unsafe` block.][79208] This lint
  is allowed by default, and may become a warning or hard error in a
  future edition.
- [You can now cast mutable references to arrays to a pointer of the same
  type as the element.][81479]

Compiler
--------
- [Upgraded the default LLVM to LLVM 12.][81451]

Added tier 3\* support for the following targets.

- [`s390x-unknown-linux-musl`][82166]
- [`riscv32gc-unknown-linux-musl` & `riscv64gc-unknown-linux-musl`][82202]
- [`powerpc-unknown-openbsd`][82733]

\* Refer to Rust's [platform support page][platform-support-doc] for more
information on Rust's tiered platform support.

Libraries
---------
- [`OsString` now implements `Extend` and `FromIterator`.][82121]
- [`cmp::Reverse` now has `#[repr(transparent)]` representation.][81879]
- [`Arc<impl Error>` now implements `error::Error`.][80553]
- [All integer division and remainder operations are now `const`.][80962]

Stabilised APIs
-------------
- [`Arguments::as_str`]
- [`char::MAX`]
- [`char::REPLACEMENT_CHARACTER`]
- [`char::UNICODE_VERSION`]
- [`char::decode_utf16`]
- [`char::from_digit`]
- [`char::from_u32_unchecked`]
- [`char::from_u32`]
- [`slice::partition_point`]
- [`str::rsplit_once`]
- [`str::split_once`]

The following previously stable APIs are now `const`.

- [`char::len_utf8`]
- [`char::len_utf16`]
- [`char::to_ascii_uppercase`]
- [`char::to_ascii_lowercase`]
- [`char::eq_ignore_ascii_case`]
- [`u8::to_ascii_uppercase`]
- [`u8::to_ascii_lowercase`]
- [`u8::eq_ignore_ascii_case`]

Rustdoc
-------
- [Rustdoc lints are now treated as a tool lint, meaning that lints are
  now prefixed with `rustdoc::`
  (e.g. `#[warn(rustdoc::non_autolinks)]`).][80527]

  Using the old style is still allowed, and will become a warning in
  a future release.
- [Rustdoc now supports argument files.][82261]
- [Rustdoc now generates smart punctuation for documentation.][79423]
- [You can now use "task lists" in Rustdoc Markdown.][81766] E.g.
  ```markdown
  - [x] Complete
  - [ ] Todo
  ```

Misc
----
- [You can now pass multiple filters to tests.][81356] E.g.
  `cargo test -- foo bar` will run all tests that match `foo` and `bar`.
- [Rustup now distributes PDB symbols for the `std` library on Windows,
  allowing you to see `std` symbols when debugging.][82218]

Internal Only
-------------
These changes provide no direct user facing benefits, but represent significant
improvements to the internals and overall performance of rustc and
related tools.

- [Check the result cache before the DepGraph when ensuring queries][81855]
- [Try fast_reject::simplify_type in coherence before doing full check][81744]
- [Only store a LocalDefId in some HIR nodes][81611]
- [Store HIR attributes in a side table][79519]

Compatibility Notes
-------------------
- [Cargo build scripts are now forbidden from setting `RUSTC_BOOTSTRAP`.]
  [cargo/9181]
- [Removed support for the `x86_64-rumprun-netbsd` target.][82594]
- [Deprecated the `x86_64-sun-solaris` target in favor of `x86_64-pc-solaris`.]
  [82216]
- [Rustdoc now only accepts `,`, ` `, and `\t` as delimiters for specifying
  languages in code blocks.][78429]
- [Rustc now catches more cases of `pub_use_of_private_extern_crate`][80763]
- [Changes in how proc macros handle whitespace may lead to panics when used
  with older `proc-macro-hack` versions. A `cargo update` should be sufficient
  to fix this in all cases.][84136]

[84136]: rust-lang/rust#84136
[80763]: rust-lang/rust#80763
[82166]: rust-lang/rust#82166
[82121]: rust-lang/rust#82121
[81879]: rust-lang/rust#81879
[82261]: rust-lang/rust#82261
[82218]: rust-lang/rust#82218
[82216]: rust-lang/rust#82216
[82202]: rust-lang/rust#82202
[81855]: rust-lang/rust#81855
[81766]: rust-lang/rust#81766
[81744]: rust-lang/rust#81744
[81611]: rust-lang/rust#81611
[81479]: rust-lang/rust#81479
[81451]: rust-lang/rust#81451
[81356]: rust-lang/rust#81356
[80962]: rust-lang/rust#80962
[80553]: rust-lang/rust#80553
[80527]: rust-lang/rust#80527
[79519]: rust-lang/rust#79519
[79423]: rust-lang/rust#79423
[79208]: rust-lang/rust#79208
[78429]: rust-lang/rust#78429
[82733]: rust-lang/rust#82733
[82594]: rust-lang/rust#82594
[cargo/9181]: rust-lang/cargo#9181
[`char::MAX`]: https://doc.rust-lang.org/std/primitive.char.html#associatedconstant.MAX
[`char::REPLACEMENT_CHARACTER`]: https://doc.rust-lang.org/std/primitive.char.html#associatedconstant.REPLACEMENT_CHARACTER
[`char::UNICODE_VERSION`]: https://doc.rust-lang.org/std/primitive.char.html#associatedconstant.UNICODE_VERSION
[`char::decode_utf16`]: https://doc.rust-lang.org/std/primitive.char.html#method.decode_utf16
[`char::from_u32`]: https://doc.rust-lang.org/std/primitive.char.html#method.from_u32
[`char::from_u32_unchecked`]: https://doc.rust-lang.org/std/primitive.char.html#method.from_u32_unchecked
[`char::from_digit`]: https://doc.rust-lang.org/std/primitive.char.html#method.from_digit
[`Peekable::next_if`]: https://doc.rust-lang.org/stable/std/iter/struct.Peekable.html#method.next_if
[`Peekable::next_if_eq`]: https://doc.rust-lang.org/stable/std/iter/struct.Peekable.html#method.next_if_eq
[`Arguments::as_str`]: https://doc.rust-lang.org/stable/std/fmt/struct.Arguments.html#method.as_str
[`str::split_once`]: https://doc.rust-lang.org/stable/std/primitive.str.html#method.split_once
[`str::rsplit_once`]: https://doc.rust-lang.org/stable/std/primitive.str.html#method.rsplit_once
[`slice::partition_point`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.partition_point
[`char::len_utf8`]: https://doc.rust-lang.org/stable/std/primitive.char.html#method.len_utf8
[`char::len_utf16`]: https://doc.rust-lang.org/stable/std/primitive.char.html#method.len_utf16
[`char::to_ascii_uppercase`]: https://doc.rust-lang.org/stable/std/primitive.char.html#method.to_ascii_uppercase
[`char::to_ascii_lowercase`]: https://doc.rust-lang.org/stable/std/primitive.char.html#method.to_ascii_lowercase
[`char::eq_ignore_ascii_case`]: https://doc.rust-lang.org/stable/std/primitive.char.html#method.eq_ignore_ascii_case
[`u8::to_ascii_uppercase`]: https://doc.rust-lang.org/stable/std/primitive.u8.html#method.to_ascii_uppercase
[`u8::to_ascii_lowercase`]: https://doc.rust-lang.org/stable/std/primitive.u8.html#method.to_ascii_lowercase
[`u8::eq_ignore_ascii_case`]: https://doc.rust-lang.org/stable/std/primitive.u8.html#method.eq_ignore_ascii_case
netbsd-srcmastr pushed a commit to NetBSD/pkgsrc that referenced this pull request May 31, 2021
Pkgsrc changes:
 * Bump bootstrap kit version to 1.51.0.
 * Adjust patches as needed.
 * Update checksum adjustments.
 * Fix syntax error in commands adjusting libserde_derive for Darwin

Upstream changes:

Version 1.52.1 (2021-05-10)
============================

This release disables incremental compilation, unless the user has explicitly
opted in via the newly added RUSTC_FORCE_INCREMENTAL=1 environment variable.

This is due to the widespread, and frequently occuring, breakage encountered by
Rust users due to newly enabled incremental verification in 1.52.0. Notably,
Rust users **should** upgrade to 1.52.0 or 1.52.1: the bugs that are detected by
newly added incremental verification are still present in past stable versions,
and are not yet fixed on any channel. These bugs can lead to miscompilation of
Rust binaries.

These problems only affect incremental builds, so release builds with Cargo
should not be affected unless the user has explicitly opted into incremental.
Debug and check builds are affected.

See [84970] for more details.

[84970]: rust-lang/rust#84970

Version 1.52.0 (2021-05-06)
============================

Language
--------
- [Added the `unsafe_op_in_unsafe_fn` lint, which checks whether
  the unsafe code in an `unsafe fn` is wrapped in a `unsafe`
  block.][79208] This lint is allowed by default, and may become
  a warning or hard error in a future edition.

- [You can now cast mutable references to arrays to a pointer of
  the same type as the element.][81479]

Compiler
--------
- [Upgraded the default LLVM to LLVM 12.][81451]

Added tier 3\* support for the following targets.

- [`s390x-unknown-linux-musl`][82166]
- [`riscv32gc-unknown-linux-musl` & `riscv64gc-unknown-linux-musl`][82202]
- [`powerpc-unknown-openbsd`][82733]

\* Refer to Rust's [platform support page][platform-support-doc] for more
information on Rust's tiered platform support.

Libraries
---------
- [`OsString` now implements `Extend` and `FromIterator`.][82121]
- [`cmp::Reverse` now has `#[repr(transparent)]` representation.][81879]
- [`Arc<impl Error>` now implements `error::Error`.][80553]
- [All integer division and remainder operations are now `const`.][80962]

Stabilised APIs
-------------
- [`Arguments::as_str`]
- [`char::MAX`]
- [`char::REPLACEMENT_CHARACTER`]
- [`char::UNICODE_VERSION`]
- [`char::decode_utf16`]
- [`char::from_digit`]
- [`char::from_u32_unchecked`]
- [`char::from_u32`]
- [`slice::partition_point`]
- [`str::rsplit_once`]
- [`str::split_once`]

The following previously stable APIs are now `const`.

- [`char::len_utf8`]
- [`char::len_utf16`]
- [`char::to_ascii_uppercase`]
- [`char::to_ascii_lowercase`]
- [`char::eq_ignore_ascii_case`]
- [`u8::to_ascii_uppercase`]
- [`u8::to_ascii_lowercase`]
- [`u8::eq_ignore_ascii_case`]

Rustdoc
-------
- [Rustdoc lints are now treated as a tool lint, meaning that
  lints are now prefixed with `rustdoc::` (e.g.
  `#[warn(rustdoc::non_autolinks)]`).][80527] Using the old style
  is still allowed, and will become a warning in a future release.
- [Rustdoc now supports argument files.][82261]
- [Rustdoc now generates smart punctuation for documentation.][79423]
- [You can now use "task lists" in Rustdoc Markdown.][81766] E.g.
  ```markdown
  - [x] Complete
  - [ ] Todo
  ```

Misc
----
- [You can now pass multiple filters to tests.][81356] E.g.
  `cargo test -- foo bar` will run all tests that match `foo` and `bar`.
- [Rustup now distributes PDB symbols for the `std` library on Windows,
  allowing you to see `std` symbols when debugging.][82218]

Internal Only
-------------
These changes provide no direct user facing benefits, but represent significant
improvements to the internals and overall performance of rustc and
related tools.

- [Check the result cache before the DepGraph when ensuring queries][81855]
- [Try fast_reject::simplify_type in coherence before doing full check][81744]
- [Only store a LocalDefId in some HIR nodes][81611]
- [Store HIR attributes in a side table][79519]

Compatibility Notes
-------------------
- [Cargo build scripts are now forbidden from setting
  `RUSTC_BOOTSTRAP`.][cargo/9181]
- [Removed support for the `x86_64-rumprun-netbsd` target.][82594]
- [Deprecated the `x86_64-sun-solaris` target in favor of
  `x86_64-pc-solaris`.][82216]
- [Rustdoc now only accepts `,`, ` `, and `\t` as delimiters for specifying
  languages in code blocks.][78429]
- [Rustc now catches more cases of `pub_use_of_private_extern_crate`][80763]
- [Changes in how proc macros handle whitespace may lead to panics
  when used with older `proc-macro-hack` versions. A `cargo update` should
  be sufficient to fix this in all cases.][84136]

[84136]: rust-lang/rust#84136
[80763]: rust-lang/rust#80763
[82166]: rust-lang/rust#82166
[82121]: rust-lang/rust#82121
[81879]: rust-lang/rust#81879
[82261]: rust-lang/rust#82261
[82218]: rust-lang/rust#82218
[82216]: rust-lang/rust#82216
[82202]: rust-lang/rust#82202
[81855]: rust-lang/rust#81855
[81766]: rust-lang/rust#81766
[81744]: rust-lang/rust#81744
[81611]: rust-lang/rust#81611
[81479]: rust-lang/rust#81479
[81451]: rust-lang/rust#81451
[81356]: rust-lang/rust#81356
[80962]: rust-lang/rust#80962
[80553]: rust-lang/rust#80553
[80527]: rust-lang/rust#80527
[79519]: rust-lang/rust#79519
[79423]: rust-lang/rust#79423
[79208]: rust-lang/rust#79208
[78429]: rust-lang/rust#78429
[82733]: rust-lang/rust#82733
[82594]: rust-lang/rust#82594
[cargo/9181]: rust-lang/cargo#9181
[`char::MAX`]: https://doc.rust-lang.org/std/primitive.char.html#associatedconstant.MAX
[`char::REPLACEMENT_CHARACTER`]: https://doc.rust-lang.org/std/primitive.char.html#associatedconstant.REPLACEMENT_CHARACTER
[`char::UNICODE_VERSION`]: https://doc.rust-lang.org/std/primitive.char.html#associatedconstant.UNICODE_VERSION
[`char::decode_utf16`]: https://doc.rust-lang.org/std/primitive.char.html#method.decode_utf16
[`char::from_u32`]: https://doc.rust-lang.org/std/primitive.char.html#method.from_u32
[`char::from_u32_unchecked`]: https://doc.rust-lang.org/std/primitive.char.html#method.from_u32_unchecked
[`char::from_digit`]: https://doc.rust-lang.org/std/primitive.char.html#method.from_digit
[`Peekable::next_if`]: https://doc.rust-lang.org/stable/std/iter/struct.Peekable.html#method.next_if
[`Peekable::next_if_eq`]: https://doc.rust-lang.org/stable/std/iter/struct.Peekable.html#method.next_if_eq
[`Arguments::as_str`]: https://doc.rust-lang.org/stable/std/fmt/struct.Arguments.html#method.as_str
[`str::split_once`]: https://doc.rust-lang.org/stable/std/primitive.str.html#method.split_once
[`str::rsplit_once`]: https://doc.rust-lang.org/stable/std/primitive.str.html#method.rsplit_once
[`slice::partition_point`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.partition_point
[`char::len_utf8`]: https://doc.rust-lang.org/stable/std/primitive.char.html#method.len_utf8
[`char::len_utf16`]: https://doc.rust-lang.org/stable/std/primitive.char.html#method.len_utf16
[`char::to_ascii_uppercase`]: https://doc.rust-lang.org/stable/std/primitive.char.html#method.to_ascii_uppercase
[`char::to_ascii_lowercase`]: https://doc.rust-lang.org/stable/std/primitive.char.html#method.to_ascii_lowercase
[`char::eq_ignore_ascii_case`]: https://doc.rust-lang.org/stable/std/primitive.char.html#method.eq_ignore_ascii_case
[`u8::to_ascii_uppercase`]: https://doc.rust-lang.org/stable/std/primitive.u8.html#method.to_ascii_uppercase
[`u8::to_ascii_lowercase`]: https://doc.rust-lang.org/stable/std/primitive.u8.html#method.to_ascii_lowercase
[`u8::eq_ignore_ascii_case`]: https://doc.rust-lang.org/stable/std/primitive.u8.html#method.eq_ignore_ascii_case
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CAD97 commented May 4, 2022

Note: &mut [T; n] as *const T is treated as a mutable use (thus getting read/write provenance and invalidating outstanding shared references), whereas &mut T as *const T is treated as an immutable use (getting read provenance and leaving outstanding shared references as valid). Zulip thread to discuss.

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&mut [T;N] as *mut T does not compile, but &[T;N] as *const T does
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