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

2 files changed, 3 insertions, 6 deletions

diff --git a/src/liballoc_jemalloc/Cargo.toml b/src/liballoc_jemalloc/Cargo.toml
index 02435170374..7986d5dd2eb 100644
--- a/src/liballoc_jemalloc/Cargo.toml
+++ b/src/liballoc_jemalloc/Cargo.toml
@@ -12,7 +12,6 @@ test = false
 doc = false
 
 [dependencies]
-alloc_system = { path = "../liballoc_system" }
 core = { path = "../libcore" }
 libc = { path = "../rustc/libc_shim" }
 compiler_builtins = { path = "../rustc/compiler_builtins_shim" }
diff --git a/src/liballoc_jemalloc/lib.rs b/src/liballoc_jemalloc/lib.rs
index 2b66c293f21..4b8755877de 100644
--- a/src/liballoc_jemalloc/lib.rs
+++ b/src/liballoc_jemalloc/lib.rs
@@ -14,7 +14,7 @@
             reason = "this library is unlikely to be stabilized in its current \
                       form or name",
             issue = "27783")]
-#![feature(alloc_system)]
+#![feature(core_intrinsics)]
 #![feature(libc)]
 #![feature(linkage)]
 #![feature(staged_api)]
@@ -23,15 +23,12 @@
 #![cfg_attr(not(dummy_jemalloc), feature(allocator_api))]
 #![rustc_alloc_kind = "exe"]
 
-extern crate alloc_system;
 extern crate libc;
 
 #[cfg(not(dummy_jemalloc))]
 pub use contents::*;
 #[cfg(not(dummy_jemalloc))]
 mod contents {
-    use core::alloc::GlobalAlloc;
-    use alloc_system::System;
     use libc::{c_int, c_void, size_t};
 
     // Note that the symbols here are prefixed by default on macOS and Windows (we
@@ -100,10 +97,11 @@ mod contents {
         ptr
     }
 
+    #[cfg(stage0)]
     #[no_mangle]
     #[rustc_std_internal_symbol]
     pub unsafe extern fn __rde_oom() -> ! {
-        System.oom()
+        ::core::intrinsics::abort();
     }
 
     #[no_mangle]