Fix "higher kinded" -> "higher ranked" in RFC 3498

[traviscross]

In RFC 3498 "Lifetime Capture Rules 2024" we specify that lifetime parameters from for<..> binders are captured under the rules.

Currently opaque types in Rust do not support capturing these. In early drafts of the document, we had called the missing feature "higher ranked lifetime bounds on nested opaque types". However, we had then noticed that the relevant error message in rustc called these "higher kinded" instead:

error: higher kinded lifetime bounds on nested opaque types are not supported yet

We changed later drafts to follow that language. But that language was wrong. These are definitely "higher ranked" lifetime bounds, not "higher kinded" ones, whatever that might mean. We've now fixed the error message emitted by rustc in:

• Better comment for implicit captures in RPITIT rust#122100

Correspondingly, let's now fix this error in the RFC.

Rendered

cc @oli-obk @compiler-errors

[fmease]

Thanks for cleaning this up! 🙏 Higher ranked is definitely the correct term here.

A higher kinded lifetime bound would be something of kind K -> Constraint with K in Type, Region or Const, haha, if we assume an outlives-bound. E.g., a higher-kinded B where B<'a> normalizes to outlives-bound 'a: 'b with lifetime 'a variable, 'b fixed or a higher-kinded P where P<T> normalizes to outlives-bound T: 'a with T variable, 'a fixed.