feat(tazjin/rlox): Implement early return from functions

In the book this is implemented via exceptions as control flow, and
I'm sticking somewhat closely to that by doing it via an error
variant.

Change-Id: I9c7b84d6bb28265ab94021ea681df84f16a53da2
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2395
Reviewed-by: tazjin <mail@tazj.in>
Tested-by: BuildkiteCI

users/tazjin/rlox/src/interpreter.rs

@@ -40,7 +40,18 @@ impl Callable {
                     fn_env.define(param, value)?;
                 }
 
-                lox.interpret_block(Rc::new(RwLock::new(fn_env)), &func.body)
+                let result = lox.interpret_block(Rc::new(RwLock::new(fn_env)), &func.body);
+
+                match result {
+                    // extract returned values if applicable
+                    Err(Error {
+                        kind: ErrorKind::FunctionReturn(value),
+                        ..
+                    }) => Ok(value),
+
+                    // otherwise just return the result itself
+                    _ => result,
+                }
             }
         }
     }
@@ -221,6 +232,12 @@ impl Interpreter {
             Statement::If(if_stmt) => return self.interpret_if(if_stmt),
             Statement::While(while_stmt) => return self.interpret_while(while_stmt),
             Statement::Function(func) => return self.interpret_function(func.clone()),
+            Statement::Return(ret) => {
+                return Err(Error {
+                    line: 0,
+                    kind: ErrorKind::FunctionReturn(self.eval(&ret.value)?),
+                })
+            }
         };
 
         Ok(value)