commit ab91236d9517d41ef05b0a3ac76b681297de5e59
author Devin Jeanpierre <jeanpierreda@google.com> Thu Jun 09 16:49:52 2022 -0700
committer Copybara-Service <copybara-worker@google.com> Thu Jun 09 16:50:48 2022 -0700
tree 267aa6e14eb966297f032d17b72e3f9acd10076b
parent 3a2026b39ae3b7d6cceef25b8c19a1708f19918c

Support `#[recursively_pinned]` generics.

This is mostly only important for its interaction with lifetimes. We don't expect to support generic types in C++ interop, not as Rust generics -- but we do expect, I think, to support C++ classes with lifetime parameters, which would become generic parameters.

I had put off implementing it because it introduces the possibility of name collision, but now is as good a time as any to implement it. (It will help fix up tests in one of the other CLs I'm working on.)

I'm actually really impressed by the thoroughness of `syn` here. Even if it doesn't automatically give me hygiene, it does have all the facilities I need. It looks ugly, but nonetheless I am impressed by e.g. https://docs.rs/syn/latest/syn/struct.Generics.html#method.split_for_impl .

---

P.S. I have *no idea* how to implement `add_lifetime()` in a nice, copy-free, readable way. I know how I'd do it in Python, but...

This CL contains my best attempt (using `Cow`). Alternate idea was to move the logic into a function, something like:

```rs
fn add_lifetime(generics: &mut syn::Generics, prefix: &str) -> proc_macro2::TokenStream {
    let taken_lifetimes: HashSet<&syn::Lifetime> = generics.lifetimes().map(|def| &def.lifetime).collect();
    let try_lifetime = |name: &str| -> Option<syn::Lifetime> {
        let lifetime = syn::Lifetime::new(name, Span::call_site());
        if taken_lifetimes.contains(&lifetime) { None } else { Some(lifetime) }
    };

    let lifetime = try_lifetime(prefix).unwrap_or_else(|| {
        (2..).map(|n| format!("{prefix}_{n}")).find_map(|s| try_lifetime(&s)).unwrap()
    });
    let quoted_lifetime = quote! {#lifetime};
    generics.params.push(syn::GenericParam::Lifetime(syn::LifetimeDef::new(lifetime)));
    quoted_lifetime
}
```

But this is repellant to me, tbh.

PiperOrigin-RevId: 454042762