> if you provide your own thin abstractions

Without generics these abstractions are hard to work with and hard to make work across libraries. I am reminded of https://thefeedbackloop.xyz/i-bet-that-almost-works/

> Take the rust compiler itself: AFAIK it's now implemented in C++

The compiler is written in Rust. It uses LLVM, which is in C++, for codegen, but in the future we might be able to turn that off too (https://internals.rust-lang.org/t/possible-alternative-compi...)

Besides, the implementation language of a compiler does not make the compiler an "abstraction" over the language. Sort of, but not really.