lex: remove DollarBacktick token

can now read src to check dollar before backticks, so simpler to always
use backtick token

fixes several bugs, e.g.

| $`abc` | (previously did not get closed)

`$abc$` (previously did not get closed)

src/inline.rs

@@ -372,19 +372,38 @@ impl<'s> Parser<'s> {
                 self.push(EventKind::Str);
             };
             Some(Continue)
+        } else if matches!(first.kind, lex::Kind::Seq(Sequence::Backtick)) {
+            let len_opener = u8::try_from(first.len).ok()?;
+            let ty = if let Some(sp) = self
+                .events
+                .back()
+                .and_then(|e| matches!(&e.kind, EventKind::Str).then(|| e.span))
+                .filter(|sp| {
+                    sp.end() == self.input.span.start()
+                        && sp.of(self.input.src).as_bytes()[sp.len() - 1] == b'$'
+                        && sp
+                            .end()
+                            .checked_sub(2)
+                            .map_or(true, |i| self.input.src.as_bytes()[i] != b'\\')
+                }) {
+                let (ty, num_dollar) = if sp.len() > 1
+                    && sp.of(self.input.src).as_bytes()[sp.len() - 2] == b'$'
+                    && sp
+                        .end()
+                        .checked_sub(3)
+                        .map_or(true, |i| self.input.src.as_bytes()[i] != b'\\')
+                {
+                    (DisplayMath, 2)
                 } else {
-            let (ty, len_opener) = match first.kind {
-                lex::Kind::DollarBacktick(l) if first.len - l as usize == 1 => {
-                    Some((InlineMath, l))
-                }
-                lex::Kind::DollarBacktick(l) if first.len - l as usize == 2 => {
-                    Some((DisplayMath, l))
-                }
-                lex::Kind::Seq(Sequence::Backtick) if first.len < 256 => {
-                    Some((Verbatim, first.len as u8))
-                }
-                _ => None,
-            }?;
+                    (InlineMath, 1)
+                };
+                let border = sp.end() - num_dollar;
+                self.events.back_mut().unwrap().span = Span::new(sp.start(), border);
+                self.input.span = Span::new(border, self.input.span.end());
+                ty
+            } else {
+                Verbatim
+            };
             self.push_sp(EventKind::Placeholder, self.input.span.empty_before());
             self.verbatim = Some(VerbatimState {
                 event_opener: self.events.len(),
@@ -394,6 +413,8 @@ impl<'s> Parser<'s> {
             });
             self.attributes = None;
             self.push(EventKind::Enter(ty))
+        } else {
+            None
         }
     }
 

src/lex.rs

@@ -20,7 +20,6 @@ pub enum Kind {
     Close(Delimiter),
     Sym(Symbol),
     Seq(Sequence),
-    DollarBacktick(u8),
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -239,20 +238,6 @@ impl<'s> Lexer<'s> {
 
                     '`' => self.eat_seq(Backtick),
                     '.' => self.eat_seq(Period),
-                    '$' => {
-                        self.eat_while(|c| c == '$');
-                        let mut n_ticks: u8 = 0;
-                        self.eat_while(|c| {
-                            if c == '`' {
-                                if let Some(l) = n_ticks.checked_add(1) {
-                                    n_ticks = l;
-                                    return true;
-                                }
-                            }
-                            false
-                        });
-                        DollarBacktick(n_ticks)
-                    }
 
                     _ => Text,
                 }
@@ -312,7 +297,6 @@ fn is_special(c: char) -> bool {
             | ':'
             | '`'
             | '.'
-            | '$'
             | '\n'
     )
 }
@@ -418,11 +402,4 @@ mod test {
             Seq(Period).l(1),
         );
     }
-
-    #[test]
-    fn dollar_backtick() {
-        test_lex!("$`", DollarBacktick(1).l(2));
-        test_lex!("$$$`", DollarBacktick(1).l(4));
-        test_lex!("$$````", DollarBacktick(4).l(6));
-    }
 }