Improve Rust syntax highlighting

[chbk]

Release Notes:

• Improved Rust syntax highlighting.

Zed 0.174.6	With this PR

• identifier: variable

let identifier = true;
const IDENTIFIER: i32 = 3;

[everdrone]

I think I have made some of these changes in another PR #25338

[maxdeviant]

I'm not sure I understand the intent of this change.

Rust only has one kind of constants: constants.

So why do we need to denote them as @constant.variable and not just @constant?

This is the only place in the Rust highlights we use @constant, so what does @constant.variable give us?

[chbk]

The issue is that we only have two scopes to differentiate three different elements:

constant.builtin: for builtin constants, or builtin literal values, as in Java.
constant: for constant variables, as in C, but the concept is similar in Rust.

Then how do we scope other literal values?

My solution to this is to use constant.variable for constants and constant for literals.

[maxdeviant]

The issue is that we only have two scopes to differentiate three different elements:

constant.builtin: for builtin constants, or builtin literal values, as in Java. constant: for constant variables, as in C, but the concept is similar in Rust.

Then how do we scope other literal values?

My solution to this is to use constant.variable for constants and constant for literals.

Let's get concrete here and provide examples of what would fall into each case.

We already have @constant.builtin for built-in constants:

undefined
null

And right now in Rust we use @constant for constants:

const MESSAGE: &str = "Hello";

What is the third type of constant?

I know in TypeScript there are two kinds of "constants" that have different semantic meaning:

const FIVE = 5;

const foo = bar();

Is this what you're referring to?

[chbk]

This is the conflict I am trying to solve. In Java:

final int x = 3; // x is a constant -> scoped constant
String y = null; // null is a literal -> scoped constant.builtin

Then let's jump to CSS:

bold, red /* literal values -> scoped constant, get same color as constant variables... */

As a solution:

• we differentiate literals from constants respectively with constant and constant.variable
• or we add literal and literal.builtin scopes for literal values

[maxdeviant]

It seems like that should just be something that is fixed in the CSS highlights?

I don't see why we need to introduce a new @constant.variable everywhere and use it as opposed to just changing the CSS selectors to use @constant.builtin (which we already have precedent for), or maybe some other selector that is CSS-specific.

The name constant.variable just doesn't seem like a good fit either, as if something is constant it is, by definition, not variable. But naming aside, it just seems like we don't need to make this change to the Rust highlights at all.

[chbk]

I agree the naming is odd, but they are called constant variables in C, so this was my attempt at homogenizing the language highlights. In that case, I will rework the CSS PR. Thank you for your feedback!

[maxdeviant]

Thanks!