Comment by tibordp

Python context managers are actually very similar to guard objects in C++ and Rust.

What I meant was something like this (could also be done with `contextlib`, but it's also verbose)

    seen_names = {}

    class EnsureUnique:
        def __init__(self, name: str):
            self.name = name
        
        def __enter__(self):
            if self.name in seen_names:
                raise ValueError(f"Duplicate name: {self.name}")
            seen_names.add(self.name)

        def __exit__(self, exc_type, exc_value, traceback):
            seen_names.remove(self.name)


    def bar():
        with EnsureUnique("foo"):
            do_something()
            ...

With defer this could be simplified to

    static seen_names: HashSet<&[u8]> = HashSet::new();

    fn bar() {
        if !seen_names.insert("foo") {
            panic!("Duplicate name: foo")
        }
        defer seen_names.remove("foo");

        do_something();
    }