Revert const hacks and use const closures in std

[oli-obk]

This revealed some smaller bugs in stability checking that I fixed where needed:

• const closures use the const stability of their parent
• trait method default bodies use the const stability of their trait

Otherwise trivial reverts of the const hacks that were added

[rustbot]

r? @jhpratt

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The reviewer was selected based on:

• Owners of files modified in this PR: @scottmcm, libs
• @scottmcm, libs expanded to 8 candidates
• Random selection from Mark-Simulacrum, jhpratt, nia-e, scottmcm

[jhpratt]

r=me from a libs side with the one question resolved. I'm not a compiler reviewer, but the small change is obviously correct.

View changes since this review

[jhpratt]

Are these simply not needed any more?

[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)

Executing "/scripts/stage_2_test_set2.sh"
+ /scripts/stage_2_test_set2.sh
PR_CI_JOB set; skipping tidy
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip 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]
downloading https://static.rust-lang.org/dist/2026-04-14/rustfmt-nightly-aarch64-unknown-linux-gnu.tar.xz
---
[RUSTC-TIMING] vec_deque_alloc_error test:true 0.277
error[E0658]: const closures are experimental
##[error]   --> alloctests/../alloc/src/raw_vec/mod.rs:903:14
    |
903 |         .map(const |(layout, _pad)| layout)
    |              ^^^^^
    |
    = note: see issue #106003 <https://github.com/rust-lang/rust/issues/106003> for more information
    = help: add `#![feature(const_closures)]` to the crate attributes to enable
    = note: this compiler was built on 2026-04-29; consider upgrading it if it is out of date

error[E0658]: const closures are experimental
##[error]   --> alloctests/../alloc/src/raw_vec/mod.rs:904:18
    |
904 |         .map_err(const |_| CapacityOverflow.into())
    |                  ^^^^^
    |
    = note: see issue #106003 <https://github.com/rust-lang/rust/issues/106003> for more information
    = help: add `#![feature(const_closures)]` to the crate attributes to enable
    = note: this compiler was built on 2026-04-29; consider upgrading it if it is out of date

error: `Result::<T, E>::map` is not yet stable as a const fn
##[error]   --> alloctests/../alloc/src/raw_vec/mod.rs:901:5
    |
901 | /     elem_layout
902 | |         .repeat(cap)
903 | |         .map(const |(layout, _pad)| layout)
    | |___________________________________________^
    |
help: add `#![feature(const_result_trait_fn)]` to the crate attributes to enable
   --> alloctests/lib.rs:70:1
    |
 70 + #![feature(const_result_trait_fn)]
    |

error: `Result::<T, E>::map_err` is not yet stable as a const fn
##[error]   --> alloctests/../alloc/src/raw_vec/mod.rs:901:5
    |
901 | /     elem_layout
902 | |         .repeat(cap)
903 | |         .map(const |(layout, _pad)| layout)
904 | |         .map_err(const |_| CapacityOverflow.into())
    | |___________________________________________________^
    |
help: add `#![feature(const_result_trait_fn)]` to the crate attributes to enable
   --> alloctests/lib.rs:70:1
    |
---
warning: build failed, waiting for other jobs to finish...
[RUSTC-TIMING] corebenches test:true 7.935
[RUSTC-TIMING] alloctests test:true 14.879
[RUSTC-TIMING] coretests test:true 74.907
env -u RUSTC_WRAPPER CARGO_ENCODED_RUSTDOCFLAGS="-Zannotate-moves\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}-Wrustdoc::invalid_codeblock_attributes\u{1f}--crate-version\u{1f}1.97.0-nightly\t(a7694c44e\t2026-04-29)" CARGO_ENCODED_RUSTFLAGS="-Zannotate-moves\u{1f}-Zrandomize-layout\u{1f}-Zunstable-options\u{1f}--check-cfg=cfg(bootstrap)\u{1f}-Zmacro-backtrace\u{1f}-Csplit-debuginfo=off\u{1f}-Clink-arg=-L/usr/lib/llvm-21/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}--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))
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:3955:25
Executed at: src/bootstrap/src/core/build_steps/test.rs:4000:26

--- BACKTRACE vvv
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/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/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/../../backtrace/src/backtrace/mod.rs:66:14
   2: <std::backtrace::Backtrace>::create
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/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:4000:26
   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:3941:17
  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:803:25
  15: bootstrap::main
             at /checkout/src/bootstrap/src/bin/main.rs:130:11
  16: <fn() as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/core/src/ops/function.rs:250:5
  17: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/sys/backtrace.rs:166:18
  18: std::rt::lang_start::<()>::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/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/ef0fb8a2563200e322fa4419f09f65a63742038c/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/ef0fb8a2563200e322fa4419f09f65a63742038c/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/ef0fb8a2563200e322fa4419f09f65a63742038c/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/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panic.rs:359:14
  23: std::rt::lang_start_internal::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/rt.rs:175:24
  24: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/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 0:24:10
  local time: Wed Apr 29 07:40:07 UTC 2026
  network time: Wed, 29 Apr 2026 07:40:08 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[theemathas]

Does this fix #155781?

[jhpratt]

I'm pretty sure it will