Auto merge of #50144 - sfackler:oom-lang-item, r=alexcrichton

Replace {Alloc,GlobalAlloc}::oom with a lang item.

The decision of what to do after an allocation fails is orthogonal to the decision of how to allocate the memory, so this PR splits them apart. `Alloc::oom` and `GlobalAlloc::oom` have been removed, and a lang item has been added:

```rust
#[lang = "oom"]
fn oom() -> !;
```

It is specifically a weak lang item, like panic_fmt, except that it is required when you depend on liballoc rather than libcore. libstd provides an implementation that aborts with the message `fatal runtime error: memory allocation failed`, matching the current behavior.

The new implementation is also significantly simpler - it's "just another weak lang item". [RFC 2070](https://github.com/rust-lang/rfcs/blob/master/text/2070-panic-implementation.md) specifies a path towards stabilizing panic_fmt, so any complexities around stable weak lang item definition are already being solved.

To bootstrap, oom silently aborts in stage0. alloc_system no longer has a bunch of code to print to stderr, and alloc_jemalloc no longer depends on alloc_system to pull in that code.

One fun note: System's GlobalAlloc implementation didn't override the default implementation of oom, so it currently aborts silently!

r? @alexcrichton

1 files changed, 11 insertions, 2 deletions

diff --git a/src/libstd/alloc.rs b/src/libstd/alloc.rs
index ff578ec42d2..a8578404467 100644
--- a/src/libstd/alloc.rs
+++ b/src/libstd/alloc.rs
@@ -13,10 +13,18 @@
 #![unstable(issue = "32838", feature = "allocator_api")]
 
 #[doc(inline)] #[allow(deprecated)] pub use alloc_crate::alloc::Heap;
-#[doc(inline)] pub use alloc_crate::alloc::Global;
+#[doc(inline)] pub use alloc_crate::alloc::{Global, oom};
 #[doc(inline)] pub use alloc_system::System;
 #[doc(inline)] pub use core::alloc::*;
 
+#[cfg(not(stage0))]
+#[cfg(not(test))]
+#[doc(hidden)]
+#[lang = "oom"]
+pub extern fn rust_oom() -> ! {
+    rtabort!("memory allocation failed");
+}
+
 #[cfg(not(test))]
 #[doc(hidden)]
 #[allow(unused_attributes)]
@@ -35,10 +43,11 @@ pub mod __default_lib_allocator {
         System.alloc(layout) as *mut u8
     }
 
+    #[cfg(stage0)]
     #[no_mangle]
     #[rustc_std_internal_symbol]
     pub unsafe extern fn __rdl_oom() -> ! {
-        System.oom()
+        super::oom()
     }
 
     #[no_mangle]