Implement immediate-abort by hooking #[rustc_panic_entrypoint] and lint for functions that look like a panic entrypoint

[saethlin]

There are a handful of functions, mostly in core and alloc, which have a mess of attributes on them which try to ensure that they compile down to just intrinsics::abort() when cfg(panic = "immediate-abort"), and otherwise are outlined.

The goal of this change is to officially recognize this pattern: These are entry points to the panic runtime.

The "immediate aborting" of panic = "immediate-abort" is now implemented by replacing calls to entrypoint functions with calls to the intrinsics::abort, instead of manually ensuring that these functions are all #[inline] with bodies that only call intrinsics::abort, or that optimize to such a body.

TODO:

• Turn the new remap_mir function into a required MirPass.
• Should #[rustc_panic_entrypoint] imply #[cold] #[inline(never)] #[track_caller]? Is that even easy to implement?

[rust-log-analyzer]

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Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/lint-docs --build-rustc-stage 1 --src /checkout/compiler --out /checkout/obj/build/aarch64-unknown-linux-gnu/md-doc/rustc/src/lints --rustc /checkout/obj/build/aarch64-unknown-linux-gnu/stage1/bin/rustc --rustc-target aarch64-unknown-linux-gnu --validate` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1612:23
Executed at: src/bootstrap/src/core/build_steps/doc.rs:1414:13

Command has failed. Rerun with -v to see more details.
Build completed unsuccessfully in 0:51:29
  local time: Tue Dec 30 05:01:51 UTC 2025
  network time: Tue, 30 Dec 2025 05:01:51 GMT
##[error]Process completed with exit code 1.
##[group]Run echo "disk usage:"

[bors]

☔ The latest upstream changes (presumably #150386) made this pull request unmergeable. Please resolve the merge conflicts.

[bjorn3]

For cg_clif you can add fx.bcx.ins().trap(TrapCode::user(1 /* unreachable */).unwrap()); at

rust/compiler/rustc_codegen_cranelift/src/base.rs

Line 1084 in d465f99

and optionally (to skip codegening a bunch of instructions)

rust/compiler/rustc_codegen_cranelift/src/base.rs

Line 386 in d465f99

and

rust/compiler/rustc_codegen_cranelift/src/base.rs

Line 1055 in d465f99

let msg_ptr = crate::constant::pointer_for_anonymous_str(fx, msg_str);

[bjorn3]

Should #[rustc_panic_entrypoint] imply #[cold] #[inline(never)] #[track_caller]? Is that even easy to implement?

Should be possible to check #[rustc_panic_entrypoint] when computing the CodegenFnAttrs.