Parse paths and identifiers only once

In the old implementation each variable in an expression would be parsed
up to three times:

* Try to parse a path because it contains a leading double colon, or
  infix double colons.
* Try to parse it as path again by scanning for an identifier that
  contains an upper case character.
* Fall back to scanning for any identifier.

This PR turns all three steps into one, without the need for
backtracking.

1 files changed, 12 insertions, 3 deletions

diff --git a/askama_parser/src/node.rs b/askama_parser/src/node.rs
index 4c21ad1..a1aa7e2 100644
--- a/askama_parser/src/node.rs
+++ b/askama_parser/src/node.rs
@@ -3,15 +3,17 @@ use std::str;
 use nom::branch::alt;
 use nom::bytes::complete::{tag, take_until};
 use nom::character::complete::char;
-use nom::combinator::{complete, consumed, cut, eof, map, not, opt, peek, recognize, value};
+use nom::combinator::{
+    complete, consumed, cut, eof, map, map_res, not, opt, peek, recognize, value,
+};
 use nom::error::{Error, ErrorKind};
 use nom::multi::{fold_many0, many0, many1, separated_list0, separated_list1};
 use nom::sequence::{delimited, pair, preceded, terminated, tuple};
 use nom::{error_position, IResult};
 
 use super::{
-    bool_lit, char_lit, identifier, is_ws, keyword, num_lit, path, skip_till, str_lit, ws, Expr,
-    State,
+    bool_lit, char_lit, identifier, is_ws, keyword, num_lit, path_or_identifier, skip_till,
+    str_lit, ws, Expr, PathOrIdentifier, State,
 };
 
 #[derive(Debug, PartialEq)]
@@ -161,6 +163,13 @@ impl<'a> Target<'a> {
             return Ok((i, Self::Tuple(Vec::new(), targets)));
         }
 
+        let path = |i| {
+            map_res(path_or_identifier, |v| match v {
+                PathOrIdentifier::Path(v) => Ok(v),
+                PathOrIdentifier::Identifier(v) => Err(v),
+            })(i)
+        };
+
         // match structs
         let (i, path) = opt(path)(i)?;
         if let Some(path) = path {