feat: do not add duplicate string constants

This means the string_equality benchmark does not crash rlox anymore
(but it crashes clox).

lib/compiler/src/compiler.rs

@@ -1,4 +1,4 @@
-use std::{fmt::Display, iter::Peekable, mem::size_of, unreachable};
+use std::{fmt::Display, iter::Peekable, mem::size_of, ops::Deref, unreachable};
 
 use cursor::Line;
 use gc::{Chunk, Closure, ClosureRef, Function, Heap, Object, ObjectRef, Value};
@@ -1327,7 +1327,10 @@ impl<'a, 'b> Compiler<'a, 'b> {
 
     fn add_identifier_constant(&mut self, token: &Token<'a>) -> Result<u8> {
         let name = self.interner.intern(token.lexeme());
-        self.add_object_constant(name, token)
+        match self.current_chunk().constants().iter().position(|v| v.equals_string(&name)) {
+            Some(index) => Ok(index.try_into().unwrap()),
+            None => self.add_object_constant(name, token),
+        }
     }
 
     fn add_object_constant(&mut self, obj: impl Into<Object>, token: &Token<'a>) -> Result<u8> {

lib/gc/src/value.rs

@@ -12,7 +12,7 @@ mod closure;
 pub use closure::*;
 
 mod upvalue;
-use strings::string_interner::StringInterner;
+use strings::string_interner::{StringInterner, InternedString};
 pub use upvalue::*;
 
 mod class;
@@ -66,6 +66,16 @@ impl Value {
         }
     }
 
+    pub fn equals_string(&self, s: &InternedString) -> bool {
+        match self {
+            Value::Object(o) => match o.deref() {
+                Object::String(string) => string == s,
+                _ => false,
+            },
+            _ => false,
+        }
+    }
+
     pub fn less_than(&self, other: &Value) -> Option<Value> {
         match (self, other) {
             (Value::Number(a), Value::Number(b)) => Some(Value::Boolean(a < b)),