feat(tazjin/rlox): Add initial support for strings

... including concatenation.

This diverges significantly from the book, as I'm using std::String
instead of implementing the book's whole heap object management
system.

It's possible that Lox in Rust actually doesn't need a GC and the
ownership model works just fine.

Change-Id: I374a0461d627cfafc26b2b54bfefac8b7c574d00
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2577
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/rlox/src/bytecode/compiler.rs

@@ -144,6 +144,10 @@ fn rule_for<T: Iterator<Item = Token>>(token: &TokenKind) -> ParseRule<T> {
             ParseRule::new(None, Some(Compiler::binary), Precedence::Comparison)
         }
 
+        TokenKind::String(_) => {
+            ParseRule::new(Some(Compiler::string), None, Precedence::None)
+        },
+
         _ => ParseRule::new(None, None, Precedence::None),
     }
 }
@@ -255,6 +259,18 @@ impl<T: Iterator<Item = Token>> Compiler<T> {
         Ok(())
     }
 
+    fn string(&mut self) -> LoxResult<()> {
+        match &self.previous().kind {
+            TokenKind::String(s) => {
+                let s = s.clone();
+                self.emit_constant(Value::String(s));
+            }
+            _ => unreachable!("only called for strings"),
+        }
+
+        Ok(())
+    }
+
     fn parse_precedence(&mut self, precedence: Precedence) -> LoxResult<()> {
         self.advance();
         let rule: ParseRule<T> = rule_for(&self.previous().kind);