servo: Merge #18327 - Upgrade to rustc 1.21.0-nightly (7eeac1b81 2017-08-30) (from servo:rustup); r=nox

Fix this error in the new Rust Nightly:

```rust
   Compiling script v0.0.1 (file:///home/simon/servo2/components/script)
error[E0599]: no method named `trace` found for type `&fn(&dom::node::Node) -> u16` in the current scope
   --> /home/simon/servo2/components/script/dom/treewalker.rs:464:10
    |
464 | #[derive(JSTraceable)]
    |          ^^^^^^^^^^^
    |
    = note: JSTraceable is a function, perhaps you wish to call it
    = help: items from traits can only be used if the trait is implemented and in scope
    = note: the following trait defines an item `trace`, perhaps you need to implement it:
            candidate #1: `dom::bindings::trace::JSTraceable`

error: aborting due to previous error

error: Could not compile `script`.
```

~I’ve tried to reproduce it in a minimal test case, but `impl<A, B> SomeTrait for fn(A) -> B` did not apply for `fn(&u32) -> u32` even on Rust 1.19.0 stable. So I don’t know what changed.~ This is very likely https://github.com/servo/servo/pull/18327.

Source-Repo: https://github.com/servo/servo
Source-Revision: fd833d9f18cd9196502b9768ab0ac8c1498b2f68

servo/components/script/dom/bindings/trace.rs

@@ -425,6 +425,13 @@ unsafe impl<A, B> JSTraceable for fn(A) -> B {
     }
 }
 
+unsafe impl<'a, A, B> JSTraceable for fn(&A) -> B {
+    #[inline]
+    unsafe fn trace(&self, _: *mut JSTracer) {
+        // Do nothing
+    }
+}
+
 unsafe impl<T> JSTraceable for IpcSender<T> where T: for<'de> Deserialize<'de> + Serialize {
     #[inline]
     unsafe fn trace(&self, _: *mut JSTracer) {

servo/components/script_plugins/utils.rs

@@ -29,7 +29,7 @@ pub fn match_def_path(cx: &LateContext, def_id: DefId, path: &[&str]) -> bool {
 }
 
 pub fn in_derive_expn(span: Span) -> bool {
-    if let Some(i) = span.ctxt.outer().expn_info() {
+    if let Some(i) = span.ctxt().outer().expn_info() {
         if let ExpnFormat::MacroAttribute(n) = i.callee.format {
             n.as_str().contains("derive")
         } else {

servo/rust-toolchain

@@ -1 +1 @@
-nightly-2017-08-30
+nightly-2017-08-31