Adds rustStringIdentifier region

[sanpii]

The RFC implicit named arguments for formatting macros is merged since rust 1.59, I think it’s more readable to highlight the identifier.

Before:

After:

It’s my first vim syntax contribution, it’s probably perfectible.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @chris-morgan (or someone else) soon.

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[chris-morgan]

I’ve thought about this sort of thing before (prototyped it a few years ago), but overall I don’t like it because it’s too faulty: the use of curly braces don’t mean that it’s a format string. If this were applied only to certain whitelisted macros (e.g. format, print, eprintln, panic, todo, assert’s second argument which is not terribly easy to express in Vim syntax highlighting, and in practice not even worth trying), there would be cause to do it, but more generally I don’t think it’s a good idea.

On the specific highlighting, I’d want at least { and } to be something that defaulted to Special, and very probably the format specifier from the colon onwards; {{ and }} would also need to be Special, as an escape sequence. And I don’t think Identifier is the right group to use at all. Special is probably the least bad choice for a default highlight link.

(Incidentally, I’d say RFC 2795 is a red herring here.)