Module scoped 

A Scoped Identifier

This works on two different principles.

The first is that HRTB (higher rank type bounds) produces fresh lifetimes on each call, and they can’t be unified against each other across multiple calls to Scoped::with.

This allows Scoped::with to prevent the following at compile time:

Scoped::with(|a: Scoped<'_>| {
    Scoped::with(|b: Scoped<'_>| {
        assert_eq!(a, b);
    })
})

This means that all Scoped types created with Scoped::with are guaranteed to be unique! If you can’t compare them, then they can never compare equal.

The second principle is based off of generativity where macros are used to generate the boilerplate to create unique lifetimes. You can read about the details in generativity’s documentation.

This allows [scope] to produce unique Scopeds.

scope!(a);
scope!(b);
assert_eq!(a, b);

How to decide which to use

Scoped::with generally produces better error messages, and tends to be easier to reason about. So it should be the first choice. However, if you are using some strange control flow or need early returns, and it would be easier to embed a Scoped directly in the current scope, then you should use the scope macro.

Structs

Scoped

A scoped Identifier

ScopedToken

The Trivial Token generated by Scoped