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Reddit is not my natural enviornment, but I occasionally hover around its various programming communities. This is a personal archive of an answer to a question about the Deno, the new “run javascript on your own computer” project.
I’m far from an expert in all the real details here, but I’ll give this a go.
Deno has not re-implemented the V8 engine in rust. Deno still uses the C++ code for the V8 javascript engine. A javascript engine in an important part of a system like deno or node, but it’s not the only part of a system like deno or node.
For example — as a javascript programmer, you may think of setTimeout as a part of javascript — but the code that schedules and manages all the different bits of javascript that you’ve asked setTimeout to run is not part of V8.
Another example — if you want your javascript program to open a file, the underlying code that open a file is not a part of V8.
Details on how node itself is built aren’t as abundant as details on how to use node — but this learning-nodejs repo has a decent introduction, and there’s lots of good details in this How Node.js Bootstraps Itself: 2019 Edition talk from Node Interactive 2019. The short version: In addition to V8 Engine — node itself gets a lot of its functionality from a plain C library called libuv, and a lot of C++ glue code maintained by the NodeJS project itself.
Deno’s manual has an architecture section, including a diagram at the bottom. Again, I’m far from an expert here, but based on this document it seems like the deno executable is a rust program. There’s something called libdeno that’s implemented in C++ (although I don’t see any C++ files in the repository, so I’m unsure what this is). Deno also uses the tokio project, which is (roughly) a rust equivalent to libuv. It also looks like the code for “io” (file reading/writing, printing things, etc), TCP Networking, and managing child processes is implemented via rust.
So your question, What Rust has to do with Deno?, is a good one, but it’s also a pretty broad one, and hard to answer succinctly (especially since I’m not a rust programmer) — but I hope this at least helps you get pointed in the right direction.