Enabling Rust support for Temporal

[Manishearth]

Hi!

I've been working on implementing ECMAScript Temporal upstream in V8. It's being implemented based on the temporal_rs and icu4x libraries, which are in Rust.

As such, it needs Rust support to work (initial V8 CL for adding Rust to DEPS.

When first working on this feature V8-in-Node CI was failing due to the lack of these vendored deps. So V8 now has a v8_enable_temporal_support that is enabled by default¹ but disabled in some builds, including the Node build.

Presumably, NodeJS will eventually want to have support for Temporal. It's probably worth figuring out what is needed to get there even though there's still a fair amount of time for us to be ready to ship.

Opening this issue to start the conversation; I'm happy to help out upstream if needed.

A challenge that may crop up is that Node vendors v8 which means that it will have to vendor third_party/rust and the toolchain: currently V8's third_party/rust is the same as that of Chrome, which includes a lot of extra Rust deps that Chrome has (so far this hasn't been a problem). If Node doesn't wish to pull all of that in, it may need to do something build-system wise to download those deps but not check them in to the tree.

Footnotes

1. Temporal is still disabled at runtime by default (the implementation is not ready); but it still gets built. ↩

[Renegade334]

Note that there are existing discussions tracking future Temporal support: #57127 for V8 support, #57891 for API uptake.

[joyeecheung]

cc @nodejs/build @targos

which means that it will have to vendor third_party/rust and the toolchain: currently V8's third_party/rust is the same as that of Chrome, which includes a lot of extra Rust deps that Chrome has (so far this hasn't been a problem). If Node doesn't wish to pull all of that in, it may need to do something build-system wise to download those deps but not check them in to the tree.

IMO the latter probably makes more sense at least for the time being - Node.js currently also relies on toolchains installed in the system so instead of using third_party/libc++¹ and third_party/llvm-build etc, it looks up the compilers from the system from the configure script. There's not a DEPS manifest and build dependencies are just documented in BUILDING.md, generally the GYP files /Python scripts just figures out what is the path to the toolchain somehow and then use them in actions, like what we do with code that need to be processed by Python/assembled.

Footnotes

1. some discussions about using libc++ on Windows somehow which might or might not lead to vendoring it in https://github.com/nodejs/node/issues/58123 ↩

[Manishearth]

@joyeecheung would that be the case for third_party/rust as well? That is not a system dep, it is a clone of https://chromium.googlesource.com/chromium/src/third_party/rust placed in the appropriate directory.

[joyeecheung]

IIUC those are crates depended by V8's Temporal implementation? In that case it seems to make more sense to copy them as part of the V8 upgrades, and then compile them using the found toolchain(we have a similar situation with most things in deps/v8/third_party except zlib/simdutf which Node.js also uses, Node.js currently doesn't have rust dependencies though)

[Manishearth]

@joyeecheung Yes, but there are also a lot of crates in there that are depended on by Chromium things (not just V8 temporal). We haven't felt the need to slice it further, and it's tricky to do so since the whole BUILD.gn/import generation code for this is managed by chrome.

So if you copy them into tree, it will be a lot of irrelevant files being copied in as well. Which might be okay. You may also be able to do an allowlisted copy, I can help provide a set of folders that matter.

[joyeecheung]

The V8 upgrades are automated via git node v8: https://github.com/nodejs/node/blob/main/doc/contributing/maintaining/maintaining-V8.md (for example this is the code used to remove third_party/eu-strip)

I wonder if there can be a text file/JSON in the V8 directory somewhere with the allowlist for third_party/rust, and then git node v8 can just read that list and only copy relevant folders/remove unnecessary files for the rust dependencies when it does the upgrade. @targos may know better.