Add new keywords

[hcpl]

• async from async/await notation for ergonomic asynchronous IO rfcs#2394;
• existential from Named existentials and impl Trait variable declarations rfcs#2071.

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[hcpl]

cc @da-x

[da-x]

Hi,

existential is a weak keyword, meaning it has special meaning only in specific contexts. In other contexts it can be a valid variable name. So we should identify these contexts and highlight it only then. Can you please make that change?

[hcpl]

Hi, @da-x!

existential is a weak keyword, meaning it has special meaning only in specific contexts. In other contexts it can be a valid variable name.

I didn't know this was a thing. I searched for places where "existential is a weak keyword" was mentioned in https://github.com/rust-lang/rust and https://github.com/rust-lang/rfcs, but couldn't find any. Could you provide the source?

[da-x]

@hcpl: https://github.com/rust-lang/rust/blob/abe19a730576cf7ead1bf7995271b53d551ea37f/src/libsyntax_pos/symbol.rs#L417

[dlrobertson]

You could treat existential like union (which is also a weak keyword) and add a match line that only highlights it if it is followed by a typedef such that the following works.

existential type Adder: Fn(usize) -> usize; // highlighted
let existential = 42; // not highlighted

Feel free to use #284 as inspiration 😄

[hcpl]

Thanks @dlrobertson! I was thinking to learn more vimscript on this weekend to understand advanced concepts involved in rust.vim, but your PR showed be that it's pretty easy to begin with and you've already done all the hard work anyway. 🎉

[da-x]

Thanks!