cargo should be the main build frontend

[ry]

Background: Deno has a relatively complex build system. It involves building various programs, running them, and using their output in further steps (V8 snapshots). We use the GN/Ninja build system to define this both because it solves these tasks well and because V8 uses it. Over time we have integrated more tightly with the Rust ecosystem by making "cargo check", "cargo build", and "cargo publish" work - but only superficially. When you run "cargo build", it calls into GN/Ninja for the bulk of the work, and only Rust compilation and final linking is done by cargo. The default build frontend is tools/build.py, which calls GN/Ninja directly. build.py is what we develop with and what we use to produce the release. When you call build.py, the rust is compiled and linked by our code.

We could simplify our code base and better integrate with the Rust ecosystem, if we could remove the GN/Ninja/build.py frontend and instead only use "cargo build". This is easier said than done. I'm starting this issue to track progress towards this eventual goal.

Problems:

• "cargo build" does not properly do incremental builds when a file such as //js/main.ts is modified. Better incremental builds with 'cargo build' #2609
• nowhere in the CI do we run the tests after "cargo build"
• hyper_hello is not built by cargo hyper_hello should be in its own crate #2641
• "cargo test" does not work without having tools/http_server.py running in the background.
• "cargo build" (debug mode) does not work on windows due to this
• the snapshot creator program should probably be in rust Snapshot creator in Rust #2617
• Use proc macros for the snapshot Build snapshot directly from build.rs rather than through GN #2711

I've started a project called cargo_gn to better abstract this integration... But it might make sense to first iterate on the more deno-specific tools/gn.rs first.

When complete:

• //build_extra/rust can be removed
• //third_party/rust_crates can be removed
• we don't need to rely on https://github.com/piscisaureus/gnargo
• Native plugins don't need BUILD.gn files (Feature: Native plugins #2385)