Auto merge of rust-lang#92535 - Amanieu:oom_hook_unwind, r=m-ou-se

Allow unwinding from OOM hooks

This is split off from rust-lang#88098 and contains just the bare minimum to allow specifying a custom OOM hook with `set_alloc_error_hook` which unwinds instead of aborting.

See rust-lang#88098 for an actual command-line flag which switches the default OOM behavior to unwind instead of aborting.

Previous perf results show a negligible impact on performance.

library/alloc/src/alloc.rs

@@ -348,7 +348,6 @@ extern "Rust" {
     // This is the magic symbol to call the global alloc error handler.  rustc generates
     // it to call `__rg_oom` if there is a `#[alloc_error_handler]`, or to call the
     // default implementations below (`__rdl_oom`) otherwise.
-    #[rustc_allocator_nounwind]
     fn __rust_alloc_error_handler(size: usize, align: usize) -> !;
 }
 
@@ -367,7 +366,6 @@ extern "Rust" {
 #[stable(feature = "global_alloc", since = "1.28.0")]
 #[rustc_const_unstable(feature = "const_alloc_error", issue = "92523")]
 #[cfg(all(not(no_global_oom_handling), not(test)))]
-#[rustc_allocator_nounwind]
 #[cold]
 pub const fn handle_alloc_error(layout: Layout) -> ! {
     const fn ct_error(_: Layout) -> ! {
@@ -398,13 +396,13 @@ pub mod __alloc_error_handler {
 
     // if there is no `#[alloc_error_handler]`
     #[rustc_std_internal_symbol]
-    pub unsafe extern "C" fn __rdl_oom(size: usize, _align: usize) -> ! {
+    pub unsafe extern "C-unwind" fn __rdl_oom(size: usize, _align: usize) -> ! {
         panic!("memory allocation of {} bytes failed", size)
     }
 
     // if there is an `#[alloc_error_handler]`
     #[rustc_std_internal_symbol]
-    pub unsafe extern "C" fn __rg_oom(size: usize, align: usize) -> ! {
+    pub unsafe extern "C-unwind" fn __rg_oom(size: usize, align: usize) -> ! {
         let layout = unsafe { Layout::from_size_align_unchecked(size, align) };
         extern "Rust" {
             #[lang = "oom"]

library/alloc/src/lib.rs

@@ -163,6 +163,7 @@
 #![cfg_attr(test, feature(test))]
 #![feature(unboxed_closures)]
 #![feature(unsized_fn_params)]
+#![feature(c_unwind)]
 //
 // Rustdoc features:
 #![feature(doc_cfg)]