[Rust] Frontmatter syntax support

[epage]

What happened?

RFC #3503 adds support for a new frontmatter syntax. The contained content format is defined by the calling tool with a tool identifier.

In RFC #3502, the cargo infostring identifier would be used to signify the block uses the Cargo.toml format/schema. Cargo will assume that an empty infostring is meant to be used with it.

Note: These are not stable. Stabilization is being tracked in:

• Tracking Issue for frontmatter rust-lang/rust#136889
• Tracking Issue for cargo-script RFC 3424 rust-lang/cargo#12207

e.g.

#!/usr/bin/env cargo

---
[dependencies]
regex = "1"
---

fn main() {
}

[FichteFoll]

Thanks for the heads-up. Note that we generally err on the side of caution for supporting unstable/experimental syntax, depending on how much it breaks the rest of highlighting, and seeing how your example snippet does not seem to break, we will most likely hold off on supporting it for a while.

A potentially larger issue is that the frontmatter uses TOML and that we do not currently have a TOML syntax in the default packages. This is something that I have had on my radar for a while but I never rose to action since the third-party syntax was decent enough. However, TOML usage is spreading more and more, with it being for Python and Rust packaging metadata, so I personally feel pretty strongly about having a syntax in the default packages anyway, after which it will also be possible to embed it into Rust for frontmatter highlighting. (See also #453.)

That said, TOML is a different issue and an initial support of frontmatter in Rust is not blocked by it.

[epage]

I fully understand not supporting it yet. I'm opening issues like this on different rust grammar repos more as a heads up that this is coming than wanting it now.