diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index d97d1e2f0f4..36ee1949ad5 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -1877,7 +1877,7 @@ impl<'a> Parser<'a> {
         Ok(MutTy { ty: t, mutbl: mutbl })
     }
 
-    fn is_named_argument(&mut self) -> bool {
+    fn is_named_argument(&self) -> bool {
         let offset = match self.token {
             token::Interpolated(ref nt) => match **nt {
                 token::NtPat(..) => return self.look_ahead(1, |t| t == &token::Colon),
@@ -2471,27 +2471,27 @@ impl<'a> Parser<'a> {
         })
     }
 
-    fn mk_expr(&mut self, span: Span, node: ExprKind, attrs: ThinVec<Attribute>) -> P<Expr> {
+    fn mk_expr(&self, span: Span, node: ExprKind, attrs: ThinVec<Attribute>) -> P<Expr> {
         P(Expr { node, span, attrs, id: ast::DUMMY_NODE_ID })
     }
 
-    fn mk_unary(&mut self, unop: ast::UnOp, expr: P<Expr>) -> ast::ExprKind {
+    fn mk_unary(&self, unop: ast::UnOp, expr: P<Expr>) -> ast::ExprKind {
         ExprKind::Unary(unop, expr)
     }
 
-    fn mk_binary(&mut self, binop: ast::BinOp, lhs: P<Expr>, rhs: P<Expr>) -> ast::ExprKind {
+    fn mk_binary(&self, binop: ast::BinOp, lhs: P<Expr>, rhs: P<Expr>) -> ast::ExprKind {
         ExprKind::Binary(binop, lhs, rhs)
     }
 
-    fn mk_call(&mut self, f: P<Expr>, args: Vec<P<Expr>>) -> ast::ExprKind {
+    fn mk_call(&self, f: P<Expr>, args: Vec<P<Expr>>) -> ast::ExprKind {
         ExprKind::Call(f, args)
     }
 
-    fn mk_index(&mut self, expr: P<Expr>, idx: P<Expr>) -> ast::ExprKind {
+    fn mk_index(&self, expr: P<Expr>, idx: P<Expr>) -> ast::ExprKind {
         ExprKind::Index(expr, idx)
     }
 
-    fn mk_range(&mut self,
+    fn mk_range(&self,
                     start: Option<P<Expr>>,
                     end: Option<P<Expr>>,
                     limits: RangeLimits)
@@ -2503,7 +2503,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn mk_assign_op(&mut self, binop: ast::BinOp,
+    fn mk_assign_op(&self, binop: ast::BinOp,
                         lhs: P<Expr>, rhs: P<Expr>) -> ast::ExprKind {
         ExprKind::AssignOp(binop, lhs, rhs)
     }
@@ -3840,7 +3840,7 @@ impl<'a> Parser<'a> {
     /// Produce an error if comparison operators are chained (RFC #558).
     /// We only need to check lhs, not rhs, because all comparison ops
     /// have same precedence and are left-associative
-    fn check_no_chained_comparison(&mut self, lhs: &Expr, outer_op: &AssocOp) {
+    fn check_no_chained_comparison(&self, lhs: &Expr, outer_op: &AssocOp) {
         debug_assert!(outer_op.is_comparison(),
                       "check_no_chained_comparison: {:?} is not comparison",
                       outer_op);
@@ -5137,7 +5137,7 @@ impl<'a> Parser<'a> {
         })
     }
 
-    fn is_async_block(&mut self) -> bool {
+    fn is_async_block(&self) -> bool {
         self.token.is_keyword(keywords::Async) &&
         (
             ( // `async move {`
@@ -5149,19 +5149,19 @@ impl<'a> Parser<'a> {
         )
     }
 
-    fn is_async_fn(&mut self) -> bool {
+    fn is_async_fn(&self) -> bool {
         self.token.is_keyword(keywords::Async) &&
             self.look_ahead(1, |t| t.is_keyword(keywords::Fn))
     }
 
-    fn is_do_catch_block(&mut self) -> bool {
+    fn is_do_catch_block(&self) -> bool {
         self.token.is_keyword(keywords::Do) &&
         self.look_ahead(1, |t| t.is_keyword(keywords::Catch)) &&
         self.look_ahead(2, |t| *t == token::OpenDelim(token::Brace)) &&
         !self.restrictions.contains(Restrictions::NO_STRUCT_LITERAL)
     }
 
-    fn is_try_block(&mut self) -> bool {
+    fn is_try_block(&self) -> bool {
         self.token.is_keyword(keywords::Try) &&
         self.look_ahead(1, |t| *t == token::OpenDelim(token::Brace)) &&
         self.span.rust_2018() &&
@@ -5183,7 +5183,7 @@ impl<'a> Parser<'a> {
         self.look_ahead(1, |t| t.is_keyword(keywords::Type))
     }
 
-    fn is_auto_trait_item(&mut self) -> bool {
+    fn is_auto_trait_item(&self) -> bool {
         // auto trait
         (self.token.is_keyword(keywords::Auto)
             && self.look_ahead(1, |t| t.is_keyword(keywords::Trait)))
@@ -5445,7 +5445,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Checks if this expression is a successfully parsed statement.
-    fn expr_is_complete(&mut self, e: &Expr) -> bool {
+    fn expr_is_complete(&self, e: &Expr) -> bool {
         self.restrictions.contains(Restrictions::STMT_EXPR) &&
             !classify::expr_requires_semi_to_be_stmt(e)
     }
@@ -6517,7 +6517,7 @@ impl<'a> Parser<'a> {
         Ok((id, generics))
     }
 
-    fn mk_item(&mut self, span: Span, ident: Ident, node: ItemKind, vis: Visibility,
+    fn mk_item(&self, span: Span, ident: Ident, node: ItemKind, vis: Visibility,
                attrs: Vec<Attribute>) -> P<Item> {
         P(Item {
             ident,
@@ -6549,7 +6549,7 @@ impl<'a> Parser<'a> {
 
     /// Returns `true` if we are looking at `const ID`
     /// (returns `false` for things like `const fn`, etc.).
-    fn is_const_item(&mut self) -> bool {
+    fn is_const_item(&self) -> bool {
         self.token.is_keyword(keywords::Const) &&
             !self.look_ahead(1, |t| t.is_keyword(keywords::Fn)) &&
             !self.look_ahead(1, |t| t.is_keyword(keywords::Unsafe))
@@ -6657,7 +6657,7 @@ impl<'a> Parser<'a> {
         })
     }
 
-    fn complain_if_pub_macro(&mut self, vis: &VisibilityKind, sp: Span) {
+    fn complain_if_pub_macro(&self, vis: &VisibilityKind, sp: Span) {
         match *vis {
             VisibilityKind::Inherited => {}
             _ => {
@@ -6686,7 +6686,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn missing_assoc_item_kind_err(&mut self, item_type: &str, prev_span: Span)
+    fn missing_assoc_item_kind_err(&self, item_type: &str, prev_span: Span)
                                    -> DiagnosticBuilder<'a>
     {
         let expected_kinds = if item_type == "extern" {