Context managers

Define __enter__ and __exit__ to make a class usable with with. Entry supplies the as value; exit takes only a mutable receiver and returns (). No inheritance or protocol declaration is needed.

class Message:
    text: str
    def __enter__(self: &mut Message) -> str:
        drop(print("enter"))
        self.text
    def __exit__(self: &mut Message) -> ():
        drop(print("exit"))

def main() -> Result[(), IoError]:
    with Message("hello") as text:
        print(text)?
    print("after")?
    Ok(())

enter
hello
exit
after

The manager moves into the block. Body locals and the entry value clean up before __exit__; the manager drops afterward, including on early exits. An owned entry value can move out, for example through return. The as binding is local to the body; omit it for unit entry methods.

with acquire()?: propagates acquisition failure without entering the context.

The lifecycle trace uses drop(print(...)) deliberately: cleanup cannot return an error, and a failed diagnostic write must not interrupt cleanup. For output that must succeed, provide an explicit fallible operation and propagate its result before leaving the scope.