Auto merge of #104999 - saethlin:immediate-abort-inlining, r=thomcc

Adjust inlining attributes around panic_immediate_abort

The goal of `panic_immediate_abort` is to permit the panic runtime and formatting code paths to be optimized away. But while poking through some disassembly of a small program compiled with that option, I found that was not the case. Enabling LTO did address that specific issue, but enabling LTO is a steep price to pay for this feature doing its job.

This PR fixes that, by tweaking two things:
* All the slice indexing functions that we `const_eval_select` on get `#[inline]`. `objdump -dC` told me that originally some `_ct` functions could end up in an executable. I won't pretend to understand what's going on there.
* Normalize attributes across all `panic!` wrappers: use `inline(never) + cold` normally, and `inline` when `panic_immediate_abort` is enabled.

But also, with LTO and `panic_immediate_abort` enabled, this patch knocks ~709 kB out of the `.text` segment of `librustc_driver.so`. That is slightly surprising to me, my best theory is that this shifts some inlining earlier in compilation, enabling some subsequent optimizations. The size improvement of `librustc_driver.so` with `panic_immediate_abort` due to this patch is greater with LTO than without LTO, which I suppose backs up this theory.

I do not know how to test this. I would quite like to, because I think what this is solving was an accidental regression. This only works with `-Zbuild-std` which is a cargo flag, and thus can't be used in a rustc codegen test.

r? `@thomcc`

---

I do not seriously think anyone is going to use a compiler built with `panic_immediate_abort`, but I wanted a big complicated Rust program to try this out on, and the compiler is such.

1 files changed, 2 insertions, 2 deletions

diff --git a/library/std/src/panicking.rs b/library/std/src/panicking.rs
index d4976a469cc..1039835bbbd 100644
--- a/library/std/src/panicking.rs
+++ b/library/std/src/panicking.rs
@@ -594,8 +594,8 @@ pub fn begin_panic_handler(info: &PanicInfo<'_>) -> ! {
 // lang item for CTFE panic support
 // never inline unless panic_immediate_abort to avoid code
 // bloat at the call sites as much as possible
-#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never))]
-#[cold]
+#[cfg_attr(not(feature = "panic_immediate_abort"), inline(never), cold)]
+#[cfg_attr(feature = "panic_immediate_abort", inline)]
 #[track_caller]
 #[rustc_do_not_const_check] // hooked by const-eval
 pub const fn begin_panic<M: Any + Send>(msg: M) -> ! {