[Pitch] Formally defining consuming and nonconsuming argument type modifiers

[Saklad5 (Jeremy Saklad)]

@John_McCall

Well, I don’t know what a local ref means because that’s not concretely proposed.

It’s from the performance roadmap.

@Joe_Groff

Borrow variables

When working with deep object graphs, it’s natural to want to assign a local variable to a property deeply nested within the graph:

let greatAunt = mother.father.sister
greatAunt.sayHello()
greatAunt.sayGoodbye()

[…]
We can do this by introducing a new kind of local variable binding that binds to the value in place without copying, while asserting a borrow over the objects necessary to access the value in place:

// `ref` comes from C#, as a strawman starting point for syntax.
// (It admittedly isn't a perfect name, since unlike C#'s ref, this would
// actively prevent mutation of stuff borrowed to form the reference, and
// if the right hand side involves computed properties and such, it may not
// technically be a reference)
ref greatAunt = mother.father.sister
greatAunt.sayHello()
mother.father.sister = otherGreatAunt // error, can't mutate `mother.father.sister` while `greatAunt` borrows it
greatAunt.sayGoodbye()