Rollup merge of #124059 - RalfJung:default_alloc_error_hook, r=workingjubilee

default_alloc_error_hook: explain difference to default __rdl_oom in alloc

Though I'm not sure if that is really the reason that this code is duplicated. On no_std it may already be possible to call user-defined code on allocation failure.

library/std/src/alloc.rs

@@ -353,6 +353,12 @@ fn default_alloc_error_hook(layout: Layout) {
     if unsafe { __rust_alloc_error_handler_should_panic != 0 } {
         panic!("memory allocation of {} bytes failed", layout.size());
     } else {
+        // This is the default path taken on OOM, and the only path taken on stable with std.
+        // Crucially, it does *not* call any user-defined code, and therefore users do not have to
+        // worry about allocation failure causing reentrancy issues. That makes it different from
+        // the default `__rdl_oom` defined in alloc (i.e., the default alloc error handler that is
+        // called when there is no `#[alloc_error_handler]`), which triggers a regular panic and
+        // thus can invoke a user-defined panic hook, executing arbitrary user-defined code.
         rtprintpanic!("memory allocation of {} bytes failed\n", layout.size());
     }
 }