fix: filter creates an error with an incorrect found (#839)

Author: Hedgehogo

src/combinator.rs

@@ -252,6 +252,7 @@ where
 {
     #[inline(always)]
     fn go<M: Mode>(&self, inp: &mut InputRef<'src, '_, I, E>) -> PResult<M, O> {
+        let found = inp.peek_maybe();
         let before = inp.cursor();
         // Remove the pre-inner alt, to be reinserted later so we always preserve it
         let old_alt = inp.errors.alt.take();
@@ -260,30 +261,29 @@ where
         let span = inp.span_since(&before);
         let new_alt = inp.errors.alt.take();
 
+        inp.errors.alt = old_alt;
         match res {
             Ok(out) => {
                 if (self.filter)(&out) {
                     // If successful, reinsert the original alt and then apply the new alt on top of it, since both are valid
-                    inp.errors.alt = old_alt;
                     if let Some(new_alt) = new_alt {
                         inp.add_alt_err(&new_alt.pos, new_alt.err);
                     }
                     Ok(M::bind(|| out))
                 } else {
                     // If unsuccessful, reinsert the original alt but replace the new alt with the "something else" error (since it overrides it)
                     let expected = [DefaultExpected::SomethingElse];
-                    let err = E::Error::expected_found(expected, None, span);
-                    inp.errors.alt = old_alt;
+                    // TODO: Use something more detailed than the next token as the found
+                    let err = E::Error::expected_found(expected, found, span);
                     inp.add_alt_err(&before.inner, err);
                     Err(())
                 }
             }
 
             Err(_) => {
-                inp.errors.alt = old_alt;
-                if let Some(new_alt) = new_alt {
-                    inp.add_alt_err(&new_alt.pos, new_alt.err);
-                }
+                // Can't fail!
+                let new_alt = new_alt.unwrap();
+                inp.add_alt_err(&new_alt.pos, new_alt.err);
                 Err(())
             }
         }

src/lib.rs

@@ -3840,6 +3840,17 @@ mod tests {
     fn filter() {
         use crate::{DefaultExpected, LabelError};
 
+        let parser = just::<_, _, extra::Err<Rich<_>>>("a").filter(|_| false);
+
+        assert_eq!(
+            parser.parse("a").into_result(),
+            Err(vec![LabelError::<&str, _>::expected_found(
+                [DefaultExpected::SomethingElse],
+                Some('a'.into()),
+                SimpleSpan::new((), 0..1)
+            ),])
+        );
+
         let parser = group((
             just("a").or_not(),
             just("b").filter(|_| false).or_not(),