Syntax extension to generate unique identifiers

[sfackler]

When writing a macro, it's common to use a couple of temporary variables, for example m in https://gist.github.com/luqmana/6219956. However, if you do something like

let m = 10;
let h = hashmap! {
    "foo" => m
};

The macro's internal m ends up being inserted, creating a type of infinite size. You can try to get around this issue by changing m to something like __hashmap_temp_var_, but you can still run into issues if, for example, you have nested invocations of a macro or if someone decides to be a jerk and define a variable called __hashmap_temp_var_.

It'd be nice to have a syntax extension that will generate identifiers that are guaranteed to be unique. I'd imagine it'd be used something like

macro_rules! my_macro (
    ($a:expr) => (
        {
            let unique_ident!(m) = HashMap::new();
            let unique_ident!(v) = ~[];
            unique_ident!(v).push($a);
            unique_ident!(m).insert("foo", $a);
            (unique_ident!(v), unique_ident!(m))
        }
    )
)

The implementation would be interesting, since you'd have to make sure that unique_ident!(m) expands to the same thing within a given macro expansion, but to a different thing within any nested expansions. You'd also want to make sure it doesn't shadow any other variables in scope, maybe by using a name that would be illegal to manually define.

[huonw]

Is this required with @jbclements' wonderful hygiene work?

[sfackler]

Oh cool, I didn't realize that #9026 covers this!