test: make <[T; N]>::default use const generics

[speedy-lex]

This is a test to check how much this would break crates. https://discord.com/channels/273534239310479360/957720175619215380/1416580604534980679

[speedy-lex]

cc @workingjubilee

[Zalathar]

@bors try

[rust-log-analyzer]

The job aarch64-gnu-llvm-19-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

[RUSTC-TIMING] rand test:false 1.005
[RUSTC-TIMING] allocbenches test:true 8.687
[RUSTC-TIMING] corebenches test:true 9.760
[RUSTC-TIMING] vec_deque_alloc_error test:true 0.282
error[E0599]: the function or associated item `default` exists for array `[DoesNotImplDefault; 0]`, but its trait bounds were not satisfied
##[error]   --> coretests/tests/array.rs:299:43
    |
297 |     struct DoesNotImplDefault;
    |     ------------------------- doesn't satisfy `DoesNotImplDefault: Default`
298 |
299 |     let _arr = <[DoesNotImplDefault; 0]>::default();
    |                                           ^^^^^^^ function or associated item cannot be called on `[DoesNotImplDefault; 0]` due to unsatisfied trait bounds
    |
    = note: the following trait bounds were not satisfied:
            `DoesNotImplDefault: Default`
            which is required by `[DoesNotImplDefault; 0]: Default`
help: consider annotating `DoesNotImplDefault` with `#[derive(Default)]`
    |
297 +     #[derive(Default)]
298 |     struct DoesNotImplDefault;
    |

[RUSTC-TIMING] alloctests test:true 14.903
[RUSTC-TIMING] alloctests test:true 13.941
For more information about this error, try `rustc --explain E0599`.
[RUSTC-TIMING] coretests test:true 57.300
error: could not compile `coretests` (test "coretests") due to 1 previous error
env -u RUSTC_WRAPPER CARGO_ENCODED_RUSTDOCFLAGS="-Csymbol-mangling-version=v0\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}--check-cfg=cfg(llvm_enzyme)\u{1f}-Dwarnings\u{1f}-Wrustdoc::invalid_codeblock_attributes\u{1f}--crate-version\u{1f}1.91.0-nightly\t(8f753a872\t2025-09-13)" CARGO_ENCODED_RUSTFLAGS="-Csymbol-mangling-version=v0\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}--check-cfg=cfg(llvm_enzyme)\u{1f}-Zmacro-backtrace\u{1f}-Csplit-debuginfo=off\u{1f}-Clink-arg=-L/usr/lib/llvm-19/lib\u{1f}-Cllvm-args=-import-instr-limit=10\u{1f}-Clink-args=-Wl,-z,origin\u{1f}-Clink-args=-Wl,-rpath,$ORIGIN/../lib\u{1f}-Alinker-messages\u{1f}--cap-lints=allow\u{1f}--cfg\u{1f}randomized_layouts" RUSTC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/dist/rustc-clif" RUSTDOC="/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/dist/rustdoc-clif" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo" "test" "--manifest-path" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/sysroot_tests/Cargo.toml" "--target-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/sysroot_tests_target" "--locked" "--target" "aarch64-unknown-linux-gnu" "-p" "coretests" "-p" "alloctests" "--tests" "--" "-q" exited with status ExitStatus(unix_wait_status(25856))
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo run --target aarch64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color always --release --manifest-path /checkout/compiler/rustc_codegen_cranelift/build_system/Cargo.toml -- test --download-dir /checkout/obj/build/cg_clif_download --out-dir /checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif --no-unstable-features --use-backend cranelift --sysroot llvm --skip-test testsuite.extended_sysroot [workdir=/checkout/compiler/rustc_codegen_cranelift]` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/test.rs:3662:25
Executed at: src/bootstrap/src/core/build_steps/test.rs:3707:26

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:20:51
  local time: Sun Sep 14 01:35:58 UTC 2025
  network time: Sun, 14 Sep 2025 01:35:59 GMT
##[error]Process completed with exit code 1.

[theemathas]

See #61415 for previous discussion

[Zalathar]

See also #145457.

[scimind2460]

Perhaps it's a good idea to remove this test for the PR?

[rust-bors]

☀️ Try build successful (CI)
Build commit: 17457aa (17457aab7004344a08bc08cfd53aa70cf02a73ef, parent: 02c7b1a7ac1d739663878030510508372e46f254)

[scimind2460]

Ok...

I don't know if I have permissions for this but I'll try...

@craterbot check

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[Noratrieb]

This is effecticely a duplicate of #145457 (comment), so there's no point in running crater again

[speedy-lex]

This is a duplicate we already have results here https://crater-reports.s3.amazonaws.com/pr-145457/index.html