Auto merge of #42727 - alexcrichton:allocators-new, r=eddyb

rustc: Implement the #[global_allocator] attribute

This PR is an implementation of [RFC 1974] which specifies a new method of
defining a global allocator for a program. This obsoletes the old
`#![allocator]` attribute and also removes support for it.

[RFC 1974]: https://github.com/rust-lang/rfcs/pull/1974

The new `#[global_allocator]` attribute solves many issues encountered with the
`#![allocator]` attribute such as composition and restrictions on the crate
graph itself. The compiler now has much more control over the ABI of the
allocator and how it's implemented, allowing much more freedom in terms of how
this feature is implemented.

cc #27389

2 files changed, 23 insertions, 7 deletions

diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index 1eb749623d8..03e5ea529e2 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -249,6 +249,8 @@ pub trait AstBuilder {
                 name: Ident, attrs: Vec<ast::Attribute>,
                 items: Vec<P<ast::Item>>) -> P<ast::Item>;
 
+    fn item_extern_crate(&self, span: Span, name: Ident) -> P<ast::Item>;
+
     fn item_static(&self,
                    span: Span,
                    name: Ident,
@@ -1095,6 +1097,10 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         )
     }
 
+    fn item_extern_crate(&self, span: Span, name: Ident) -> P<ast::Item> {
+        self.item(span, name, Vec::new(), ast::ItemKind::ExternCrate(None))
+    }
+
     fn item_static(&self,
                    span: Span,
                    name: Ident,
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index 4b0ec1a20e3..cb625b1ac06 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -143,7 +143,6 @@ declare_features! (
     (active, placement_in_syntax, "1.0.0", Some(27779)),
     (active, unboxed_closures, "1.0.0", Some(29625)),
 
-    (active, allocator, "1.0.0", Some(27389)),
     (active, fundamental, "1.0.0", Some(29635)),
     (active, main, "1.0.0", Some(29634)),
     (active, needs_allocator, "1.4.0", Some(27389)),
@@ -366,6 +365,10 @@ declare_features! (
 
     // Allows unsized tuple coercion.
     (active, unsized_tuple_coercion, "1.20.0", Some(42877)),
+
+    // global allocators and their internals
+    (active, global_allocator, "1.20.0", None),
+    (active, allocator_internals, "1.20.0", None),
 );
 
 declare_features! (
@@ -385,6 +388,7 @@ declare_features! (
     // rustc internal
     (removed, unmarked_api, "1.0.0", None),
     (removed, pushpop_unsafe, "1.2.0", None),
+    (removed, allocator, "1.0.0", None),
 );
 
 declare_features! (
@@ -591,16 +595,22 @@ pub const BUILTIN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeG
                                              "the `#[rustc_on_unimplemented]` attribute \
                                               is an experimental feature",
                                              cfg_fn!(on_unimplemented))),
-    ("allocator", Whitelisted, Gated(Stability::Unstable,
-                                     "allocator",
-                                     "the `#[allocator]` attribute is an experimental feature",
-                                     cfg_fn!(allocator))),
+    ("global_allocator", Normal, Gated(Stability::Unstable,
+                                       "global_allocator",
+                                       "the `#[global_allocator]` attribute is \
+                                        an experimental feature",
+                                       cfg_fn!(global_allocator))),
+    ("default_lib_allocator", Whitelisted, Gated(Stability::Unstable,
+                                            "allocator_internals",
+                                            "the `#[default_lib_allocator]` \
+                                             attribute is an experimental feature",
+                                            cfg_fn!(allocator_internals))),
     ("needs_allocator", Normal, Gated(Stability::Unstable,
-                                      "needs_allocator",
+                                      "allocator_internals",
                                       "the `#[needs_allocator]` \
                                        attribute is an experimental \
                                        feature",
-                                      cfg_fn!(needs_allocator))),
+                                      cfg_fn!(allocator_internals))),
     ("panic_runtime", Whitelisted, Gated(Stability::Unstable,
                                          "panic_runtime",
                                          "the `#[panic_runtime]` attribute is \