rustc: Allow changing the default allocator

This commit is an implementation of [RFC 1183][rfc] which allows swapping out
the default allocator on nightly Rust. No new stable surface area should be
added as a part of this commit.

[rfc]: https://github.com/rust-lang/rfcs/pull/1183

Two new attributes have been added to the compiler:

* `#![needs_allocator]` - this is used by liballoc (and likely only liballoc) to
  indicate that it requires an allocator crate to be in scope.
* `#![allocator]` - this is a indicator that the crate is an allocator which can
  satisfy the `needs_allocator` attribute above.

The ABI of the allocator crate is defined to be a set of symbols that implement
the standard Rust allocation/deallocation functions. The symbols are not
currently checked for exhaustiveness or typechecked. There are also a number of
restrictions on these crates:

* An allocator crate cannot transitively depend on a crate that is flagged as
  needing an allocator (e.g. allocator crates can't depend on liballoc).
* There can only be one explicitly linked allocator in a final image.
* If no allocator is explicitly requested one will be injected on behalf of the
  compiler. Binaries and Rust dylibs will use jemalloc by default where
  available and staticlibs/other dylibs will use the system allocator by
  default.

Two allocators are provided by the distribution by default, `alloc_system` and
`alloc_jemalloc` which operate as advertised.

Closes #27389

1 files changed, 26 insertions, 0 deletions

diff --git a/src/test/compile-fail/allocator-rust-dylib-is-jemalloc.rs b/src/test/compile-fail/allocator-rust-dylib-is-jemalloc.rs
new file mode 100644
index 00000000000..224d9379ee1
--- /dev/null
+++ b/src/test/compile-fail/allocator-rust-dylib-is-jemalloc.rs
@@ -0,0 +1,26 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-msvc everything is the system allocator on msvc
+// aux-build:allocator-dylib2.rs
+// error-pattern: cannot link together two allocators
+
+// Ensure that rust dynamic libraries use jemalloc as their allocator, verifying
+// by linking in the system allocator here and ensuring that we get a complaint.
+
+#![feature(alloc_system)]
+
+extern crate allocator_dylib2;
+extern crate alloc_system;
+
+fn main() {
+    allocator_dylib2::foo();
+}
+