Fix a bug with how line/col positions are calculated

Positions were being calculated by looking at the *first* character,
which would cause the same character to be visited twice. Now, if the
character is a null byte (\0), it won't advance the position at all.

The null byte should be a safe sentinel value to use because it
(hypothetically) shouldn't be showing up in a source file. This will
probably cause a hang if a file that starts with null bytes, but that's
a problem for future me.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/syn/lexer.rs

@@ -33,12 +33,11 @@ pub struct Lexer<'t> {
 
 impl<'t> Lexer<'t> {
     pub fn new(source: impl ToString, text: &'t str) -> Self {
-        let c = text.chars().next().unwrap_or('\0');
         Self {
             source: Rc::new(source.to_string()),
             text,
-            start: Pos::new(c),
-            end: Pos::new(c),
+            start: Pos::new('\0'),
+            end: Pos::new('\0'),
         }
     }
 
@@ -89,7 +88,7 @@ impl<'t> Lexer<'t> {
             return Ok(None);
         }
 
-        if let Some(cap) = LEX_PAT.captures(&self.text[self.end.byte..]) {
+        if let Some(cap) = LEX_PAT.captures(&self.text[self.start.byte..]) {
             self.end = self.end.next_str(cap.get(0).unwrap().as_str());
             let sp_token = if let Some(_) = cap.name("assign") {
                 self.make_token(Token::Assign)

src/syn/span.rs

@@ -37,8 +37,15 @@ impl Pos {
     }
 
     pub fn next(self, c: char) -> Self {
-        let (line, col) = if self.c == '\n' {
-            (self.line + 1, 0)
+        // catch the "pre-scan" case
+        if c == '\0' {
+            let mut next = self;
+            next.c = c;
+            return next;
+        }
+
+        let (line, col) = if c == '\n' {
+            (self.line + 1, 1)
         } else {
             (self.line, self.col + 1)
         };
@@ -99,7 +106,11 @@ impl Span {
 impl Display for Span {
     fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
         if self.start.line == self.end.line {
-            write!(fmt, "line {} in {}", self.start.line, self.source)
+            write!(
+                fmt,
+                "line {}, column {} in {}",
+                self.start.line, self.start.col, self.source
+            )
         } else {
             write!(
                 fmt,