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 servo/servo#18327.

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

UltraBlame original commit: 2093d5141fb21195b1f84894b2921bcace8c3cfd

Author: marco-c

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

@@ -3287,6 +3287,48 @@ nothing
 unsafe
 impl
 <
+'
+a
+A
+B
+>
+JSTraceable
+for
+fn
+(
+&
+A
+)
+-
+>
+B
+{
+#
+[
+inline
+]
+unsafe
+fn
+trace
+(
+&
+self
+_
+:
+*
+mut
+JSTracer
+)
+{
+/
+/
+Do
+nothing
+}
+}
+unsafe
+impl
+<
 T
 >
 JSTraceable

servo/components/script_plugins/utils.rs

@@ -293,6 +293,8 @@ i
 span
 .
 ctxt
+(
+)
 .
 outer
 (

servo/rust-toolchain

@@ -4,4 +4,4 @@ nightly
 -
 08
 -
-30
+31