Have arrays' drop_glue just unsize and call the slice version

[scottmcm]

It's silly to emit two loops (because of the drop ladder -- just one in panic=abort) for every array length that's dropped when we can just polymorphize to the slice version.

Built atop #154327 to avoid conflicts later, so draft for now.

r? @WaffleLapkin

[rustbot]

WaffleLapkin is not on the review rotation at the moment.
They may take a while to respond.

[scottmcm]

View changes since the review

Compare nightly https://rust.godbolt.org/z/5Wv1q86ja with a loop in each of the three.

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

##[endgroup]
Executing "/scripts/stage_2_test_set2.sh"
+ /scripts/stage_2_test_set2.sh
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip tidy'
PR_CI_JOB set; skipping tidy
+ ../x.py --stage 2 test --skip tidy --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
##[endgroup]
---
##[endgroup]
[JIT] mini_core_hello_world (skipped)
##[group][AOT] mini_core_hello_world
[AOT] mini_core_hello_world
##[error]error: internal compiler error: compiler/rustc_mir_transform/src/validate.rs:81:25: broken MIR in DropGlue(DefId(1:261 ~ mini_core[c82c]::drop_glue), Some([NoisyDropInner; 2_usize])) (after phase change to runtime-optimized) at bb0[0]:
                                Unsize coercion, but `*mut [NoisyDropInner; 2]` isn't coercible to `&mut [NoisyDropInner]`
   --> example/mini_core.rs:554:1
    |
554 | pub unsafe fn drop_glue<T: ?Sized>(_to_drop: *mut T) {
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


thread 'rustc' (37902) panicked at compiler/rustc_mir_transform/src/validate.rs:81:25:
Box<dyn Any>
stack backtrace:
   0: std::panicking::begin_panic::<rustc_errors::ExplicitBug>
   1: <rustc_errors::diagnostic::BugAbort as rustc_errors::diagnostic::EmissionGuarantee>::emit_producing_guarantee
   2: <rustc_errors::DiagCtxtHandle>::span_bug::<rustc_span::span_encoding::Span, alloc::string::String>
   3: rustc_middle::util::bug::opt_span_bug_fmt::<rustc_span::span_encoding::Span>::{closure#0}
   4: rustc_middle::ty::context::tls::with_opt::<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}
   5: rustc_middle::ty::context::tls::with_context_opt::<rustc_middle::ty::context::tls::with_opt<rustc_middle::util::bug::opt_span_bug_fmt<rustc_span::span_encoding::Span>::{closure#0}, !>::{closure#0}, !>
   6: rustc_middle::util::bug::span_bug_fmt::<rustc_span::span_encoding::Span>
   7: <rustc_mir_transform::validate::CfgChecker>::fail::<alloc::string::String>
   8: <rustc_mir_transform::validate::Validator as rustc_mir_transform::pass_manager::MirPass>::run_pass
   9: rustc_mir_transform::pass_manager::run_passes_inner
  10: rustc_mir_transform::shim::make_shim
      [... omitted 2 frames ...]
  11: <rustc_middle::ty::context::TyCtxt>::instance_mir
  12: rustc_monomorphize::collector::items_of_instance
      [... omitted 2 frames ...]
  13: rustc_monomorphize::collector::collect_items_rec
  14: rustc_monomorphize::collector::collect_items_rec
  15: rustc_monomorphize::collector::collect_items_root
  16: std::panicking::catch_unwind::<(), core::panic::unwind_safe::AssertUnwindSafe<rustc_data_structures::sync::parallel::par_for_each_in<rustc_middle::mono::MonoItem, alloc::vec::Vec<rustc_middle::mono::MonoItem>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure#0}>::{closure#0}::{closure#1}::{closure#0}>>
  17: <rustc_data_structures::sync::parallel::ParallelGuard>::run::<(), rustc_data_structures::sync::parallel::par_for_each_in<rustc_middle::mono::MonoItem, alloc::vec::Vec<rustc_middle::mono::MonoItem>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure#0}>::{closure#0}::{closure#1}::{closure#0}>
  18: <alloc::vec::into_iter::IntoIter<rustc_middle::mono::MonoItem> as core::iter::traits::iterator::Iterator>::fold::<(), core::iter::traits::iterator::Iterator::for_each::call<rustc_middle::mono::MonoItem, rustc_data_structures::sync::parallel::par_for_each_in<rustc_middle::mono::MonoItem, alloc::vec::Vec<rustc_middle::mono::MonoItem>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure#0}>::{closure#0}::{closure#1}>::{closure#0}>
  19: rustc_data_structures::sync::parallel::par_for_each_in::<rustc_middle::mono::MonoItem, alloc::vec::Vec<rustc_middle::mono::MonoItem>, rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}::{closure#0}>
  20: <rustc_session::session::Session>::time::<(), rustc_monomorphize::collector::collect_crate_mono_items::{closure#1}>
  21: rustc_monomorphize::collector::collect_crate_mono_items
  22: rustc_monomorphize::partitioning::collect_and_partition_mono_items
      [... omitted 2 frames ...]
  23: rustc_codegen_cranelift::driver::aot::run_aot
  24: <rustc_codegen_cranelift::CraneliftCodegenBackend as rustc_codegen_ssa::traits::backend::CodegenBackend>::codegen_crate
  25: <rustc_session::session::Session>::time::<alloc::boxed::Box<dyn core::any::Any>, rustc_interface::passes::start_codegen::{closure#0}>
  26: rustc_interface::passes::start_codegen
  27: <rustc_interface::queries::Linker>::codegen_and_build_linker
  28: <std::thread::local::LocalKey<core::cell::Cell<*const ()>>>::with::<rustc_middle::ty::context::tls::enter_context<<rustc_middle::ty::context::GlobalCtxt>::enter<rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}, core::option::Option<rustc_interface::queries::Linker>>::{closure#1}, core::option::Option<rustc_interface::queries::Linker>>::{closure#0}, core::option::Option<rustc_interface::queries::Linker>>
  29: <rustc_middle::ty::context::TyCtxt>::create_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_interface::passes::create_and_enter_global_ctxt<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>::{closure#2}>
  30: rustc_interface::passes::create_and_enter_global_ctxt::<core::option::Option<rustc_interface::queries::Linker>, rustc_driver_impl::run_compiler::{closure#0}::{closure#2}>
  31: std::panicking::catch_unwind::<(), core::panic::unwind_safe::AssertUnwindSafe<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}::{closure#0}>>
  32: rustc_interface::interface::run_compiler::<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}
  33: rustc_span::create_session_globals_then::<(), rustc_interface::util::run_in_thread_with_globals<rustc_interface::util::run_in_thread_pool_with_globals<rustc_interface::interface::run_compiler<(), rustc_driver_impl::run_compiler::{closure#0}>::{closure#1}, ()>::{closure#0}, ()>::{closure#0}::{closure#0}::{closure#0}>
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

note: using internal features is not supported and expected to cause internal compiler errors when used incorrectly

warning: the ICE couldn't be written to `/checkout/compiler/rustc_codegen_cranelift/rustc-ice-2026-04-12T06_12_18-37900.txt`: Read-only file system (os error 30)

note: rustc 1.97.0-nightly (d4435daab 2026-04-12) running on aarch64-unknown-linux-gnu

note: compiler flags: -C panic=abort -Z panic-abort-tests -Z codegen-backend=cranelift -C symbol-mangling-version=v0 -Z annotate-moves -Z randomize-layout -Z unstable-options -Z macro-backtrace -C split-debuginfo=off -C link-arg=-L/usr/lib/llvm-21/lib -C llvm-args=-import-instr-limit=10 -C link-args=-Wl,-z,origin -C link-args=-Wl,-rpath,$ORIGIN/../lib -C debuginfo=2 -C panic=abort -Z force-unstable-if-unmarked

query stack during panic:
#0 [mir_shims] generating MIR shim for `mini_core::drop_glue`, instance=DropGlue(DefId(1:261 ~ mini_core[c82c]::drop_glue), Some([NoisyDropInner; ValTree(Leaf(0x0000000000000002): usize)]))
#1 [items_of_instance] collecting items used by `mini_core::drop_glue::<[NoisyDropInner; ValTree(Leaf(0x0000000000000002): usize)]> - shim(Some([NoisyDropInner; ValTree(Leaf(0x0000000000000002): usize)]))`
#2 [collect_and_partition_mono_items] collect_and_partition_mono_items
end of query stack
error: aborting due to 1 previous error

"/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/dist/rustc-clif" "-Csymbol-mangling-version=v0" "-Zannotate-moves" "-Zrandomize-layout" "-Zunstable-options" "--check-cfg=cfg(bootstrap)" "-Zmacro-backtrace" "-Csplit-debuginfo=off" "-Clink-arg=-L/usr/lib/llvm-21/lib" "-Cllvm-args=-import-instr-limit=10" "-Clink-args=-Wl,-z,origin" "-Clink-args=-Wl,-rpath,$ORIGIN/../lib" "-L" "crate=/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/example" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2-codegen/cg_clif/build/example" "-Cdebuginfo=2" "--target" "aarch64-unknown-linux-gnu" "-Cpanic=abort" "--check-cfg=cfg(jit)" "--edition=2024" "example/mini_core_hello_world.rs" exited with status ExitStatus(unix_wait_status(25856))
Bootstrap failed while executing `--stage 2 test --skip tidy --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo run -Zwarnings --target aarch64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color=always --profile=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:3951:25
Executed at: src/bootstrap/src/core/build_steps/test.rs:3996:26

--- BACKTRACE vvv
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/std/src/../../backtrace/src/backtrace/libunwind.rs:117:9
   1: std::backtrace_rs::backtrace::trace_unsynchronized::<<std::backtrace::Backtrace>::create::{closure#0}>
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/std/src/../../backtrace/src/backtrace/mod.rs:66:14
   2: <std::backtrace::Backtrace>::create
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/std/src/backtrace.rs:331:13
   3: <bootstrap::utils::exec::DeferredCommand>::finish_process
             at /checkout/src/bootstrap/src/utils/exec.rs:939:17
   4: <bootstrap::utils::exec::DeferredCommand>::wait_for_output::<&bootstrap::utils::exec::ExecutionContext>
             at /checkout/src/bootstrap/src/utils/exec.rs:831:21
   5: <bootstrap::utils::exec::ExecutionContext>::run
             at /checkout/src/bootstrap/src/utils/exec.rs:741:45
   6: <bootstrap::utils::exec::BootstrapCommand>::run::<&bootstrap::core::builder::Builder>
             at /checkout/src/bootstrap/src/utils/exec.rs:339:27
   7: <bootstrap::core::build_steps::test::CodegenCranelift as bootstrap::core::builder::Step>::run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:3996:26
   8: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::test::CodegenCranelift>
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1579:36
   9: <bootstrap::core::build_steps::test::CodegenCranelift as bootstrap::core::builder::Step>::make_run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:3937:17
  10: <bootstrap::core::builder::StepDescription>::maybe_run
             at /checkout/src/bootstrap/src/core/builder/mod.rs:476:13
  11: bootstrap::core::builder::cli_paths::match_paths_to_steps_and_run
             at /checkout/src/bootstrap/src/core/builder/cli_paths.rs:141:22
  12: <bootstrap::core::builder::Builder>::run_step_descriptions
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1122:9
  13: <bootstrap::core::builder::Builder>::execute_cli
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1101:14
  14: <bootstrap::Build>::build
             at /checkout/src/bootstrap/src/lib.rs:799:25
  15: bootstrap::main
             at /checkout/src/bootstrap/src/bin/main.rs:130:11
  16: <fn() as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/core/src/ops/function.rs:250:5
  17: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/std/src/sys/backtrace.rs:166:18
  18: std::rt::lang_start::<()>::{closure#0}
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/std/src/rt.rs:206:18
  19: <&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/core/src/ops/function.rs:287:21
  20: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync, i32>
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/std/src/panicking.rs:581:40
  21: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync>
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/std/src/panicking.rs:544:19
  22: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync, i32>
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/std/src/panic.rs:359:14
  23: std::rt::lang_start_internal::{closure#0}
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/std/src/rt.rs:175:24
  24: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/ad726b5063362ec9897ef3d67452fc5606ee70fa/library/std/src/panicking.rs:581:40
---
  31: __libc_start_main
  32: _start


Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:25:38
  local time: Sun Apr 12 06:12:18 UTC 2026
  network time: Sun, 12 Apr 2026 06:12:19 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[WaffleLapkin]

💭 does it make sense to keep the array case separate for N <= 1 or something?