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/197

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 412a3493208..2cfb1616927 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 df8ee189d21..0163bb3b1d0 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -137,7 +137,6 @@ 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)),
@@ -360,6 +359,10 @@ 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! (
@@ -379,6 +382,7 @@ 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! (
@@ -585,16 +589,22 @@ 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 \