Implement Deref{Mut} for arrays

[Veykril]

Now that we have const generics, there shouldn't really be a reason for arrays not to implement Deref/DerefMut anymore (to my knowledge). So this PR is an attempt in making that work.

With this, arrays now participate in deref coercions.

Unfortunately we still need to special case arrays in method probing, as unsizing can be always done in a const context while deref cannot on stable and currently the compiler will prefer dereferencing over unsizing. Of note is that once Deref is stably const, the unsizing step can (probably, not tested) be fully removed, meaning Unsize adjustments no longer need to happen for arrays)

Fixes #62992

[rustbot]

r? @Nilstrieb

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[rustbot]

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If this PR contains changes to any rust-lang/rust public library APIs then please comment with @rustbot label +T-libs-api -T-libs to tag it appropriately. If this PR contains changes to any unstable APIs please edit the PR description to add a link to the relevant API Change Proposal or create one if you haven't already. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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• Changing observable runtime behavior of library APIs

[Noratrieb]

@compiler-errors on the compiler part, for the rest
r? libs-api

[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[Veykril]

Not sure if this might be a T-lang thing as well, since arrays now participate in deref coercions (and arrays being a language type opposed to library)

[compiler-errors]

I actually tried to implement this back in #92652. I don't think it makes sense to implement Deref in this case anymore, since array -> slice is an unsize coercion.

Regardless, needs an ACP from T-libs-api I think? https://std-dev-guide.rust-lang.org/feature-lifecycle/api-change-proposals.html

[Veykril]

Guess I wasn't looking thoroughly enough for a previous PR :) I can see the reasoning behind why not to add this from eddy's comment.

Turns out this is even the third PR trying to do this, I wonder if it makes sense to add a comment somewhere that states as to why we don't want this impl?

[rust-log-analyzer]

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Complete job name: PR (x86_64-gnu-llvm-14, false, ubuntu-20.04-xl)
git config --global core.autocrlf false
shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
env:
  CI_JOB_NAME: x86_64-gnu-llvm-14
---
Successfully built 1a4e86b77eb4
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Built container sha256:1a4e86b77eb4d273114a7fec4cee1862970c8aef0cd22ab0d3cd26294422753f
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upload failed: - to s3://rust-lang-ci-sccache2/docker/2d8760e15cd716e46ea98fab8e7af60c712922f5df298e54391d8a5a58bcaa16a7d6f2f012a2cb04e2fd5f51945d9112e87fb2970f7c4b52f59f1373bcbb7693 Unable to locate credentials
[CI_JOB_NAME=x86_64-gnu-llvm-14]
---
failures:

---- [ui] tests/ui/coercion/issue-73886.rs stdout ----

error: /checkout/tests/ui/coercion/issue-73886.rs:2: expected error not found: non-primitive cast: `&&[i32; 1]` as `&[_]`
error: 0 unexpected errors found, 1 expected errors not found
status: exit status: 1
status: exit status: 1
command: "/checkout/obj/build/x86_64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/coercion/issue-73886.rs" "-Zthreads=1" "--target=x86_64-unknown-linux-gnu" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Zdeduplicate-diagnostics=no" "-Cstrip=debuginfo" "--remap-path-prefix=/checkout/tests/ui=fake-test-src-base" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/coercion/issue-73886" "-A" "unused" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/x86_64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/x86_64-unknown-linux-gnu/test/ui/coercion/issue-73886/auxiliary"
    Error {
        line_num: 2,
        kind: Some(
            Error,
            Error,
        ),
        msg: "non-primitive cast: `&&[i32; 1]` as `&[_]`",
Some tests failed in compiletest suite=ui mode=ui host=x86_64-unknown-linux-gnu target=x86_64-unknown-linux-gnu
]

thread '[ui] tests/ui/coercion/issue-73886.rs' panicked at 'explicit panic', src/tools/compiletest/src/runtest.rs:1421:13
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace