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, 0 insertions, 37 deletions

diff --git a/src/libcore/alloc.rs b/src/libcore/alloc.rs
index 54440eaa40f..674c4fb57c7 100644
--- a/src/libcore/alloc.rs
+++ b/src/libcore/alloc.rs
@@ -451,17 +451,6 @@ pub unsafe trait GlobalAlloc {
         }
         new_ptr
     }
-
-    /// Aborts the thread or process, optionally performing
-    /// cleanup or logging diagnostic information before panicking or
-    /// aborting.
-    ///
-    /// `oom` is meant to be used by clients unable to cope with an
-    /// unsatisfied allocation request, and wish to abandon
-    /// computation rather than attempt to recover locally.
-    fn oom(&self) -> ! {
-        unsafe { ::intrinsics::abort() }
-    }
 }
 
 /// An implementation of `Alloc` can allocate, reallocate, and
@@ -614,32 +603,6 @@ pub unsafe trait Alloc {
     ///   to allocate that block of memory.
     unsafe fn dealloc(&mut self, ptr: NonNull<Opaque>, layout: Layout);
 
-    /// Allocator-specific method for signaling an out-of-memory
-    /// condition.
-    ///
-    /// `oom` aborts the thread or process, optionally performing
-    /// cleanup or logging diagnostic information before panicking or
-    /// aborting.
-    ///
-    /// `oom` is meant to be used by clients unable to cope with an
-    /// unsatisfied allocation request, and wish to abandon
-    /// computation rather than attempt to recover locally.
-    ///
-    /// Implementations of the `oom` method are discouraged from
-    /// infinitely regressing in nested calls to `oom`. In
-    /// practice this means implementors should eschew allocating,
-    /// especially from `self` (directly or indirectly).
-    ///
-    /// Implementations of the allocation and reallocation methods
-    /// (e.g. `alloc`, `alloc_one`, `realloc`) are discouraged from
-    /// panicking (or aborting) in the event of memory exhaustion;
-    /// instead they should return an appropriate error from the
-    /// invoked method, and let the client decide whether to invoke
-    /// this `oom` method in response.
-    fn oom(&mut self) -> ! {
-        unsafe { ::intrinsics::abort() }
-    }
-
     // == ALLOCATOR-SPECIFIC QUANTITIES AND LIMITS ==
     // usable_size