If you think of typescript as "tooling" that makes sense.

But if you think of it as compiler, or transpiler, not so much. Python, Ruby etc "compilers" (runtimes, really) aren't written in Python or Ruby. Or rather, there are alternatives around, but they aren't the fastest nor the best. Such "tools" are built in C. As are compilers for C++, Rust or most languages really.

I understand your point. Compilers and interpreters are traditionally written in C/C++, but we don't have to do the same than before.

What's the conversion rate on that though?

For smaller languages that are much less supported that makes much more sense.

For bigger languages we treat it much more as a black box.

> but typescript developers will not learn rust to improve the typescript compiler.

Why not? TS has much more in common with Rust than with JS, in terms of typing. Compiler engineers are not that prevalent in the community, but they are much more prevalent in the systems engineering world which Rust is a part of, so I'd assume it's actually more beneficial to have a compiler in Rust than in TS because it would mean more people interested in contributing.

> TS has much more in common with Rust than with JS, in terms of typing.

TS has much more in common with JS than it does with Rust in every other comparison

Every TS dev will know at least a little bit of JS. Not every TS dev will know Rust at all. They're very different languages.

Compiler engineers will be far more familiar with Rust, C++, C and other similar languages than with JS, so it doesn't really matter whether TS and JS are similar as long as compiler engineers can use their preferred language to build the compiler.