[fine] Parent pointers in trees

fine/src/tokens.rs

@@ -58,11 +58,18 @@ pub enum TokenKind {
     Yield,
 }
 
+// NOTE: Tokens are kinda big (like 40 bytes?) and AFAICT the only way to go
+//       smaller would be to stop using string pointers and use smaller
+//       sizes/offsets instead, e.g., 32b for offset and 32b for size, and
+//       stop tracking the position independently from the start, and then
+//       require the source text when converting to line/col. I'm unwilling to
+//       give up the ergonomics of &str and String right now, so we're just
+//       not doing it.
 #[derive(Debug, PartialEq, Eq, Clone)]
 pub struct Token<'a> {
     pub kind: TokenKind,
     pub start: usize,
-    value: Result<&'a str, String>,
+    value: Result<&'a str, Box<str>>,
 }
 
 impl<'a> Token<'a> {
@@ -78,7 +85,7 @@ impl<'a> Token<'a> {
         Token {
             kind: TokenKind::Error,
             start,
-            value: Err(message),
+            value: Err(message.into()),
         }
     }