Repeat expressions

[mohammadfawaz]

Closes #28670
Also closes #28699

This PR adds repeat expression as in [3u32; 4] which is equivalent to [3u32, 3u32, 3u32, 3u32]. Similarly to array type, the length (or count) is an expression that should evaluate to a constant at some point.

Most change in this PR are fairly clear and straightforward. I still made the interpreter work with repeat expressions even though it's getting replaced soon.

[mikebenfield]

Suggested change

	// Handle array with multiple elements: [expr1, expr2, ...] or single element with a trailing comma: [expr,]
	if self.eat(&Token::Comma) {
	let sep = Some(Token::Comma);
	let mut elements = vec![first_expr];
	while !self.check(&close) {
	// Allow parser recovery by accepting optional expressions.
	if let Some(elem) = self.parse_expression().map(Some)? {
	elements.push(elem);
	}
	// If the separator is present, consume it; otherwise, exit loop.
	if sep.as_ref().filter(|sep| !self.eat(sep)).is_some() {
	break;
	}
	}
	let span = open_span + self.expect(&close)?;
	return Ok(ArrayExpression { elements, span, id: self.node_builder.next_id() }.into());
	}
	// Single-element array with no trailing comma: [expr]
	let span = open_span + self.expect(&close)?;
	Ok(ArrayExpression { elements: vec![first_expr], span, id: self.node_builder.next_id() }.into())
	// Handle array with multiple elements: [expr1, expr2, ...] or single element with or without trailing comma: [expr,]
	let mut elements = vec![first_expr];
	while self.eat(&Token::Comma) && !self.check(&close) {
	elements.push(self.parse_expression()?);
	}
	let span = open_span + self.expect(&close)?;
	Ok(ArrayExpression { elements, span, id: self.node_builder.next_id() }.into())

[mikebenfield]

This is the only place unknown repeat counts are disallowed, so if I just have something like this:

    transition main(x : u32) -> u32 {
        let arr = [3u32; x];
        return arr[0u32];
    }

This expect will be triggered.

I think probably the right way forward is for ConstPropagationOutput to have a repeat_count_not_evaluated item just like it has array_length_not_evaluated and so on, and for the ConstPropagationAndUnrollingPass to check it when it hits a fixed point just as it does for the others. And we should have a test with something like the main function I wrote above that triggers this error.

[mohammadfawaz]

Oh absolutely, I did that for array types but forgot to do it here. Thanks for catching this.

[mohammadfawaz]

@mikebenfield requested changes addressed in this commit: 1bb79ed