Rewind parser

src/combinator.rs

@@ -1022,6 +1022,55 @@ impl<I: Clone, O, A: Parser<I, O, Error = E>, U: Clone, E: Error<I>> Parser<I, U
     }
 }
 
+/// See [`Parser::rewind`].
+pub struct Rewind<A>(pub(crate) A);
+
+impl<I: Clone, O, E: Error<I>, A> Parser<I, O> for Rewind<A>
+where
+    A: Parser<I, O, Error = E>,
+{
+    type Error = E;
+
+    fn parse_inner<D: Debugger>(
+        &self,
+        debugger: &mut D,
+        stream: &mut StreamOf<I, Self::Error>,
+    ) -> PResult<I, O, Self::Error>
+    where
+        Self: Sized,
+    {
+        let rewind_from = stream.save();
+        match {
+            #[allow(deprecated)]
+            debugger.invoke(&self.0, stream)
+        } {
+            (errors, Ok((out, alt))) => {
+                stream.revert(rewind_from);
+                (errors, Ok((out, alt)))
+            }
+            (errors, Err(err)) => (errors, Err(err)),
+        }
+    }
+
+    fn parse_inner_verbose(
+        &self,
+        d: &mut Verbose,
+        s: &mut StreamOf<I, Self::Error>,
+    ) -> PResult<I, O, Self::Error> {
+        #[allow(deprecated)]
+        self.parse_inner(d, s)
+    }
+
+    fn parse_inner_silent(
+        &self,
+        d: &mut Silent,
+        s: &mut StreamOf<I, Self::Error>,
+    ) -> PResult<I, O, Self::Error> {
+        #[allow(deprecated)]
+        self.parse_inner(d, s)
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use error::Simple;

src/lib.rs

@@ -953,6 +953,29 @@ pub trait Parser<I: Clone, O> {
         }
     }
 
+    /// Parse something and then rewind the state so that it only looks ahead and does not consume input.
+    /// Chumsky's parsers are always rewind the state when they fail.
+    /// But this combinator makes a parsers to rewind the state whether it succeeds or fails.
+    /// A typical use-case of this is that you want to parse something which is not followed by something else.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use chumsky::prelude::*;
+    /// let just_numbers = text::digits::<_, Simple<char>>(10)
+    ///     .padded()
+    ///     .then_ignore(none_of("+-*/".chars()).rewind())
+    ///     .separated_by(just(','));
+    /// // 3 is not parsed because it's followed by '+'.
+    /// assert_eq!(just_numbers.parse("1, 2, 3 + 4"), Ok(vec!["1".to_string(), "2".to_string()]));
+    /// ```
+    fn rewind(self) -> Rewind<Self>
+    where
+        Self: Sized,
+    {
+        Rewind(self)
+    }
+
     /// Box the parser, yielding a parser that performs parsing through dynamic dispatch.
     ///
     /// Boxing a parser might be useful for: