remove StringReader::peek

The reader itself doesn't need ability to peek tokens, so it's better
if clients implement this functionality.

This hopefully becomes especially easy once we use iterator interface
for lexer, but this is not too easy at the moment, because of buffered
errors.

src/librustdoc/html/highlight.rs

@@ -79,6 +79,7 @@ pub fn render_with_highlighting(
 /// each span of text in sequence.
 struct Classifier<'a> {
     lexer: lexer::StringReader<'a>,
+    peek_token: Option<Token>,
     source_map: &'a SourceMap,
 
     // State of the classifier.
@@ -178,6 +179,7 @@ impl<'a> Classifier<'a> {
     fn new(lexer: lexer::StringReader<'a>, source_map: &'a SourceMap) -> Classifier<'a> {
         Classifier {
             lexer,
+            peek_token: None,
             source_map,
             in_attribute: false,
             in_macro: false,
@@ -187,10 +189,19 @@ impl<'a> Classifier<'a> {
 
     /// Gets the next token out of the lexer.
     fn try_next_token(&mut self) -> Result<Token, HighlightError> {
-        match self.lexer.try_next_token() {
-            Ok(token) => Ok(token),
-            Err(_) => Err(HighlightError::LexError),
+        if let Some(token) = self.peek_token.take() {
+            return Ok(token);
         }
+        self.lexer.try_next_token().map_err(|()| HighlightError::LexError)
+    }
+
+    fn peek(&mut self) -> Result<&Token, HighlightError> {
+        if self.peek_token.is_none() {
+            self.peek_token = Some(
+                self.lexer.try_next_token().map_err(|()| HighlightError::LexError)?
+            );
+        }
+        Ok(self.peek_token.as_ref().unwrap())
     }
 
     /// Exhausts the `lexer` writing the output into `out`.
@@ -234,7 +245,7 @@ impl<'a> Classifier<'a> {
             // reference or dereference operator or a reference or pointer type, instead of the
             // bit-and or multiplication operator.
             token::BinOp(token::And) | token::BinOp(token::Star)
-                if self.lexer.peek() != &token::Whitespace => Class::RefKeyWord,
+                if self.peek()? != &token::Whitespace => Class::RefKeyWord,
 
             // Consider this as part of a macro invocation if there was a
             // leading identifier.
@@ -257,7 +268,7 @@ impl<'a> Classifier<'a> {
             token::Question => Class::QuestionMark,
 
             token::Dollar => {
-                if self.lexer.peek().is_ident() {
+                if self.peek()?.is_ident() {
                     self.in_macro_nonterminal = true;
                     Class::MacroNonTerminal
                 } else {
@@ -280,9 +291,9 @@ impl<'a> Classifier<'a> {
                 // as an attribute.
 
                 // Case 1: #![inner_attribute]
-                if self.lexer.peek() == &token::Not {
+                if self.peek()? == &token::Not {
                     self.try_next_token()?; // NOTE: consumes `!` token!
-                    if self.lexer.peek() == &token::OpenDelim(token::Bracket) {
+                    if self.peek()? == &token::OpenDelim(token::Bracket) {
                         self.in_attribute = true;
                         out.enter_span(Class::Attribute)?;
                     }
@@ -292,7 +303,7 @@ impl<'a> Classifier<'a> {
                 }
 
                 // Case 2: #[outer_attribute]
-                if self.lexer.peek() == &token::OpenDelim(token::Bracket) {
+                if self.peek()? == &token::OpenDelim(token::Bracket) {
                     self.in_attribute = true;
                     out.enter_span(Class::Attribute)?;
                 }
@@ -341,7 +352,7 @@ impl<'a> Classifier<'a> {
                         if self.in_macro_nonterminal {
                             self.in_macro_nonterminal = false;
                             Class::MacroNonTerminal
-                        } else if self.lexer.peek() == &token::Not {
+                        } else if self.peek()? == &token::Not {
                             self.in_macro = true;
                             Class::Macro
                         } else {

src/libsyntax/parse/lexer/mod.rs

@@ -142,10 +142,6 @@ impl<'a> StringReader<'a> {
         buffer
     }
 
-    pub fn peek(&self) -> &Token {
-        &self.peek_token
-    }
-
     /// For comments.rs, which hackily pokes into next_pos and ch
     fn new_raw(sess: &'a ParseSess,
                source_file: Lrc<syntax_pos::SourceFile>,