Auto merge of #25898 - jdm:rustup, r=asajeffrey

Update to 3/4 nightly rustc.

The comment changes address a bunch of new warnings that look like this:
```
warning: unused doc comment
   --> components/script/dom/rtcpeerconnection.rs:449:5
    |
449 |     /// https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-signalingstatechange
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ rustdoc does not generate documentation for macros
    |
    = help: to document an item produced by a macro, the macro must produce the documentation as part of its expansion
```

1 files changed, 2 insertions, 2 deletions

diff --git a/components/script_plugins/lib.rs b/components/script_plugins/lib.rs
index 035ce326480..752a1343db0 100644
--- a/components/script_plugins/lib.rs
+++ b/components/script_plugins/lib.rs
@@ -20,14 +20,15 @@
 #![cfg(feature = "unrooted_must_root_lint")]
 
 extern crate rustc;
+extern crate rustc_ast;
 extern crate rustc_driver;
 extern crate rustc_hir;
 extern crate rustc_lint;
 extern crate rustc_session;
 extern crate rustc_span;
-extern crate syntax;
 
 use rustc::ty;
+use rustc_ast::ast::{AttrKind, Attribute};
 use rustc_driver::plugin::Registry;
 use rustc_hir::def_id::DefId;
 use rustc_hir::intravisit as visit;
@@ -38,7 +39,6 @@ use rustc_span::source_map;
 use rustc_span::source_map::{ExpnKind, MacroKind, Span};
 use rustc_span::symbol::sym;
 use rustc_span::symbol::Symbol;
-use syntax::ast::{AttrKind, Attribute};
 
 #[allow(deprecated)]
 #[plugin_registrar]