[EXPERIMENTAL] Lower constant pattern subslices into valtrees

[jieyouxu]

🚧 This PR is experimental because this is the first time I touch MIR stuff :3

This PR attempts to revive #112370 and work towards fixing #110870.

Checklist

• Resolve unable to compare [T; N] == [T; N] via ~const PartialEq.
• Add before/after MIR tests to confirm this PR does indeed affect MIR build simplification in a desirable way.

[rustbot]

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[rust-log-analyzer]

The job x86_64-gnu-llvm-16 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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GITHUB_WORKSPACE=/home/runner/work/rust/rust
GOROOT_1_19_X64=/opt/hostedtoolcache/go/1.19.13/x64
---
#12 writing image sha256:f7d6f821a0698b4d03e3ad8230fe337a05af67cbca2c7e693c7e0ca1189dd3cc done
#12 naming to docker.io/library/rust-ci done
#12 DONE 10.0s
##[endgroup]
Setting extra environment values for docker:  --env ENABLE_GCC_CODEGEN=1 --env GCC_EXEC_PREFIX=/usr/lib/gcc/
[CI_JOB_NAME=x86_64-gnu-llvm-16]
##[group]Clock drift check
  local time: Sat Feb 24 10:10:06 UTC 2024
  network time: Sat, 24 Feb 2024 10:10:07 GMT
  network time: Sat, 24 Feb 2024 10:10:07 GMT
##[endgroup]
sccache: Starting the server...
##[group]Configure the build
configure: processing command line
configure: 
configure: build.configure-args := ['--build=x86_64-unknown-linux-gnu', '--llvm-root=/usr/lib/llvm-16', '--enable-llvm-link-shared', '--set', 'rust.thin-lto-import-instr-limit=10', '--set', 'change-id=99999999', '--enable-verbose-configure', '--enable-sccache', '--disable-manage-submodules', '--enable-locked-deps', '--enable-cargo-native-static', '--set', 'rust.codegen-units-std=1', '--set', 'dist.compression-profile=balanced', '--dist-compression-formats=xz', '--set', 'build.optimized-compiler-builtins', '--disable-dist-src', '--release-channel=nightly', '--enable-debug-assertions', '--enable-overflow-checks', '--enable-llvm-assertions', '--set', 'rust.verify-llvm-ir', '--set', 'rust.codegen-backends=llvm,cranelift,gcc', '--set', 'llvm.static-libstdcpp', '--enable-new-symbol-mangling']
configure: target.x86_64-unknown-linux-gnu.llvm-config := /usr/lib/llvm-16/bin/llvm-config
configure: llvm.link-shared     := True
configure: rust.thin-lto-import-instr-limit := 10
configure: change-id            := 99999999
---
   Compiling libc v0.2.153
   Compiling memchr v2.5.0
   Compiling std v0.0.0 (/checkout/library/std)
   Compiling compiler_builtins v0.1.108
error[E0277]: can't compare `[u8; 1]` with `[u8; 1]`
    |
    |
661 |             [10, ..] => true,
    |             ^^^^^^^^ no implementation for `[u8; 1] == [u8; 1]`
    |
    = help: the trait `~const PartialEq` is not implemented for `[u8; 1]`
    = help: the following other types implement trait `PartialEq<Rhs>`:
              <[T; N] as PartialEq<[U; N]>>
              <[T; N] as PartialEq<[U]>>
              <[T; N] as PartialEq<&[U]>>
              <[T; N] as PartialEq<&mut [U]>>
              <[T] as PartialEq<[U; N]>>
              <[T] as PartialEq<[U]>>
              <&[T] as PartialEq<[U; N]>>
              <&mut [T] as PartialEq<[U; N]>>

error[E0277]: can't compare `[u8; 2]` with `[u8; 2]`
    |
    |
663 |             [192, 168, ..] => true,
    |             ^^^^^^^^^^^^^^ no implementation for `[u8; 2] == [u8; 2]`
    |
    = help: the trait `~const PartialEq` is not implemented for `[u8; 2]`
    = help: the following other types implement trait `PartialEq<Rhs>`:
              <[T; N] as PartialEq<[U; N]>>
              <[T; N] as PartialEq<[U]>>
              <[T; N] as PartialEq<&[U]>>
              <[T; N] as PartialEq<&mut [U]>>
              <[T] as PartialEq<[U; N]>>
              <[T] as PartialEq<[U]>>
              <&[T] as PartialEq<[U; N]>>
              <&mut [T] as PartialEq<[U; N]>>

error[E0277]: can't compare `[u8; 2]` with `[u8; 2]`
    |
    |
688 |         matches!(self.octets(), [169, 254, ..])
    |                                 ^^^^^^^^^^^^^^ no implementation for `[u8; 2] == [u8; 2]`
    |
    = help: the trait `~const PartialEq` is not implemented for `[u8; 2]`
    = help: the following other types implement trait `PartialEq<Rhs>`:
              <[T; N] as PartialEq<[U; N]>>
              <[T; N] as PartialEq<[U]>>
              <[T; N] as PartialEq<&[U]>>
              <[T; N] as PartialEq<&mut [U]>>
              <[T] as PartialEq<[U; N]>>
              <[T] as PartialEq<[U]>>
              <&[T] as PartialEq<[U; N]>>
              <&mut [T] as PartialEq<[U; N]>>
For more information about this error, try `rustc --explain E0277`.
error: could not compile `core` (lib) due to 3 previous errors
Build completed unsuccessfully in 0:03:50
  local time: Sat Feb 24 10:14:33 UTC 2024

[michaelwoerister]

r? mir

[jieyouxu]

Going to abort this PR because:

• It's too specialized and only handles trivial const slices, does not handle nested subslice patterns
• It's so much easier/cleaner implementation-wise to delegate to ~const PartialEq once that is usable in libcore.