Move token tree related lexer state to a separate struct

We only used a bunch of fields when tokenizing into a token tree,
so let's move them out of the base lexer

src/libsyntax/parse/lexer/mod.rs

@@ -66,15 +66,7 @@ pub struct StringReader<'a> {
     span: Span,
     /// The raw source span which *does not* take `override_span` into account
     span_src_raw: Span,
-    /// Stack of open delimiters and their spans. Used for error message.
-    open_braces: Vec<(token::DelimToken, Span)>,
-    crate unmatched_braces: Vec<UnmatchedBrace>,
-    /// The type and spans for all braces
-    ///
-    /// Used only for error recovery when arriving to EOF with mismatched braces.
-    matching_delim_spans: Vec<(token::DelimToken, Span, Span)>,
-    crate override_span: Option<Span>,
-    last_unclosed_found_span: Option<Span>,
+    override_span: Option<Span>,
 }
 
 impl<'a> StringReader<'a> {
@@ -254,11 +246,7 @@ impl<'a> StringReader<'a> {
             token: token::Eof,
             span: syntax_pos::DUMMY_SP,
             span_src_raw: syntax_pos::DUMMY_SP,
-            open_braces: Vec::new(),
-            unmatched_braces: Vec::new(),
-            matching_delim_spans: Vec::new(),
             override_span,
-            last_unclosed_found_span: None,
         }
     }