Add look-ahead to custom syntax parser.

src/parser.rs

@@ -1831,15 +1831,17 @@ fn parse_custom_syntax(
     tokens.push(key.into());
 
     loop {
-        settings.pos = input.peek().unwrap().1;
+        let (fwd_token, fwd_pos) = input.peek().unwrap();
+        settings.pos = *fwd_pos;
         let settings = settings.level_up();
 
-        let required_token =
-            if let Some(seg) = parse_func(&segments).map_err(|err| err.0.into_err(settings.pos))? {
-                seg
-            } else {
-                break;
-            };
+        let required_token = if let Some(seg) = parse_func(&segments, fwd_token.syntax().as_ref())
+            .map_err(|err| err.0.into_err(settings.pos))?
+        {
+            seg
+        } else {
+            break;
+        };
 
         match required_token.as_str() {
             MARKER_IDENT => match input.next().unwrap() {

src/syntax.rs

@@ -26,11 +26,11 @@ pub type FnCustomSyntaxEval =
 /// A general expression parsing trait object.
 #[cfg(not(feature = "sync"))]
 pub type FnCustomSyntaxParse =
-    dyn Fn(&[ImmutableString]) -> Result<Option<ImmutableString>, ParseError>;
+    dyn Fn(&[ImmutableString], &str) -> Result<Option<ImmutableString>, ParseError>;
 /// A general expression parsing trait object.
 #[cfg(feature = "sync")]
 pub type FnCustomSyntaxParse =
-    dyn Fn(&[ImmutableString]) -> Result<Option<ImmutableString>, ParseError> + Send + Sync;
+    dyn Fn(&[ImmutableString], &str) -> Result<Option<ImmutableString>, ParseError> + Send + Sync;
 
 /// An expression sub-tree in an [`AST`][crate::AST].
 #[derive(Debug, Clone)]
@@ -185,7 +185,7 @@ impl Engine {
         self.register_custom_syntax_raw(
             key,
             // Construct the parsing function
-            move |stream| {
+            move |stream, _| {
                 if stream.len() >= segments.len() {
                     Ok(None)
                 } else {
@@ -213,7 +213,7 @@ impl Engine {
     pub fn register_custom_syntax_raw(
         &mut self,
         key: impl Into<ImmutableString>,
-        parse: impl Fn(&[ImmutableString]) -> Result<Option<ImmutableString>, ParseError>
+        parse: impl Fn(&[ImmutableString], &str) -> Result<Option<ImmutableString>, ParseError>
             + SendSync
             + 'static,
         new_vars: isize,