rustdoc: color the question mark operator

The idea was proposed by eddyb in: https://github.com/rust-lang/rust/issues/31436#issuecomment-247426582

src/librustdoc/html/highlight.rs

@@ -104,6 +104,7 @@ pub enum Class {
     Lifetime,
     PreludeTy,
     PreludeVal,
+    QuestionMark,
 }
 
 /// Trait that controls writing the output of syntax highlighting. Users should
@@ -237,8 +238,10 @@ fn write_token<W: Writer>(&mut self,
             token::Dot | token::DotDot | token::DotDotDot | token::Comma | token::Semi |
                 token::Colon | token::ModSep | token::LArrow | token::OpenDelim(_) |
                 token::CloseDelim(token::Brace) | token::CloseDelim(token::Paren) |
-                token::CloseDelim(token::NoDelim) |
-                token::Question => Class::None,
+                token::CloseDelim(token::NoDelim) => Class::None,
+
+            token::Question => Class::QuestionMark,
+
             token::Dollar => {
                 if self.lexer.peek().tok.is_ident() {
                     self.in_macro_nonterminal = true;
@@ -348,6 +351,7 @@ pub fn rustdoc_class(self) -> &'static str {
             Class::Lifetime => "lifetime",
             Class::PreludeTy => "prelude-ty",
             Class::PreludeVal => "prelude-val",
+            Class::QuestionMark => "question-mark"
         }
     }
 }

src/librustdoc/html/static/rustdoc.css

@@ -570,6 +570,9 @@ pre.rust .self, pre.rust .bool-val, pre.rust .prelude-val,
 pre.rust .attribute, pre.rust .attribute .ident { color: #C82829; }
 pre.rust .macro, pre.rust .macro-nonterminal { color: #3E999F; }
 pre.rust .lifetime { color: #B76514; }
+pre.rust .question-mark {
+    color: #c67e2d;
+}
 
 .rusttest { display: none; }
 pre.rust { position: relative; }