How can I create hygienic identifiers in code generated by procedural macros?

Summary: you can't yet use hygienic identifiers with proc macros on stable Rust. Your best bet is to use a particularly ugly name such as __your_crate_your_name.

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You are creating identifiers (in particular, f) by using quote!. This is certainly convenient, but it's just a helper around the actual proc macro API the compiler offers. So let's take a look at that API to see how we can create identifiers! In the end we need a TokenStream, as that's what our proc macro returns. How can we construct such a token stream?

We can parse it from a string, e.g. "let f = 3;".parse::<TokenStream>(). But this was basically an early solution and is discouraged now. In any case, all identifiers created this way behave in a non-hygienic manner, so this won't solve your problem.

The second way (which quote! uses under the hood) is to create a TokenStream manually by creating a bunch of TokenTrees. One kind of TokenTree is an Ident (identifier). We can create an Ident via new:

fn new(string: &str, span: Span) -> Ident

The string parameter is self explanatory, but the span parameter is the interesting part! A Span stores the location of something in the source code and is usually used for error reporting (in order for rustc to point to the misspelled variable name, for example). But in the Rust compiler, spans carry more than location information: the kind of hygiene! We can see two constructor functions for Span:

• fn call_site() -> Span: creates a span with call site hygiene. This is what you call "unhygienic" and is equivalent to "copy and pasting". If two identifiers have the same string, they will collide or shadow each other.

• fn def_site() -> Span: this is what you are after. Technically called definition site hygiene, this is what you call "hygienic". The identifiers you define and the ones of your user live in different universes and won't ever collide. As you can see in the docs, this method is still unstable and thus only usable on a nightly compiler. Bummer!

There are no really great workarounds. The obvious one is to use a really ugly name like __your_crate_some_variable. To make it a bit easier for you, you can create that identifier once and use it within quote! (slightly better solution here):

let ugly_name = quote! { __your_crate_some_variable };
quote! {
    let #ugly_name = 3;
    println!("{}", #ugly_name);
}

Sometimes you can even search through all identifiers of the user that could collide with yours and then simply algorithmically chose an identifier that does not collide. This is actually what we did for auto_impl, with a fallback super ugly name. This was mainly to improve the generated documentation from having super ugly names in it.

Apart from that, I'm afraid you cannot really do anything.

You can thanks to a UUID:

fn generate_unique_ident(prefix: &str) -> Ident {
    let uuid = uuid::Uuid::new_v4();
    let ident = format!("{}_{}", prefix, uuid).replace('-', "_");

    Ident::new(&ident, Span::call_site())
}