Fix lexing of attributes and doc comments in Rust lexer (#957)

A bug was causing files starting with attributes and doc comments to
produce incorrect error nodes. This commit fixes that bug while also
allowing attributes to occur in positions other than the start of a
line, bringing it in line with Rust's grammar.

lib/rouge/lexers/rust.rb

@@ -49,6 +49,7 @@ def macro_closed?
 
       start {
         @macro_delims = { ']' => 0, ')' => 0, '}' => 0 }
+        push :bol
       }
 
       delim_map = { '[' => ']', '(' => ')', '{' => '}' }
@@ -60,20 +61,21 @@ def macro_closed?
       )x
       size = /8|16|32|64/
 
-      state :start_line do
+      # Although not officially part of Rust, the rustdoc tool allows code in
+      # comments to begin with `#`. Code like this will be evaluated but not
+      # included in the HTML output produced by rustdoc. So that code intended
+      # for these comments can be higlighted with Rouge, the  Rust lexer needs
+      # to check if the beginning of the line begins with a `# `.
+      state :bol do
         mixin :whitespace
-        rule %r/\s+/, Text
-        rule %r/#\[/ do
-          token Name::Decorator; push :attribute
-        end
+        rule %r/#\s[^\n]*/, Comment::Special
         rule(//) { pop! }
-        rule %r/#\s[^\n]*/, Comment::Preproc
       end
 
       state :attribute do
         mixin :whitespace
         mixin :has_literals
-        rule %r/[(,)=]/, Name::Decorator
+        rule %r/[(,)=:]/, Name::Decorator
         rule %r/\]/, Name::Decorator, :pop!
         rule id, Name::Decorator
       end
@@ -85,8 +87,9 @@ def macro_closed?
       end
 
       state :root do
-        rule %r/\n/, Text, :start_line
+        rule %r/\n/, Text, :bol
         mixin :whitespace
+        rule %r/#!?\[/, Name::Decorator, :attribute
         rule %r/\b(?:#{Rust.keywords.join('|')})\b/, Keyword
         mixin :has_literals
 

spec/visual/samples/rust

@@ -1,3 +1,4 @@
+#![test(a::b)]
 fn f() {
     let a = String::new("hello");
     let b: &str = a;