Neither, I was disagreeing with the comparison to Go and generics.
Generics in Go don't add anything beyond what generics already do in other languages, so the challenge with bringing generics to Go is "how do we adapt the language to support a feature that already exists in other languages and is generally well understood". Bringing generics to a language that wasn't designed with them in mind has often resulted in a sub-optimal implementation (eg. Java vs C#).
On the other hand, "safe custom allocators" are not a feature that any language (to my knowledge) has solved. It's not as though this was an oversight in Rust's initial design: using a custom allocator in an unsafe context has always been possible in Rust, and it's too early to say whether bringing this feature into the language later will result in a similarly sub-optimal design: in order to be sub-optimal there would have to exist some better solution out there, and there currently doesn't.
Generics in Go don't add anything beyond what generics already do in other languages, so the challenge with bringing generics to Go is "how do we adapt the language to support a feature that already exists in other languages and is generally well understood". Bringing generics to a language that wasn't designed with them in mind has often resulted in a sub-optimal implementation (eg. Java vs C#).
On the other hand, "safe custom allocators" are not a feature that any language (to my knowledge) has solved. It's not as though this was an oversight in Rust's initial design: using a custom allocator in an unsafe context has always been possible in Rust, and it's too early to say whether bringing this feature into the language later will result in a similarly sub-optimal design: in order to be sub-optimal there would have to exist some better solution out there, and there currently doesn't.