[wgsl-in] Phony assignment

[hanawatson]

Hi, phony assignments in WGSL input seem to not be recognised by naga currently

code example (slightly altered copy of an example in the spec https://gpuweb.github.io/gpuweb/wgsl/#phony-assignment-section, since as this is an example use it should presumably definitely work):

var <private> counter: i32;

fn increment_and_yield_previous() -> i32 {
	let previous = counter;
	counter = counter + 1;
	return previous;
}

@stage(compute) @workgroup_size(1)
fn compute_main(
) {
	_ = increment_and_yield_previous();
}

resultant error:

error: unknown local function `_`
   ┌─ phonytest.wgsl:12:2
   │
12 │     _ = increment_and_yield_previous();
   │     ^ unknown local function

Could not parse WGSL

[teoxoy]

We currently allow the _ identifier for let and var assignments and this works like phony assignment but according to the spec (and this issue) we shouldn't require let/var since the _ is not supposed to be an identifier and therefore not able to be declared via let/var.

[stshine]

It is strange for phony assignment not requiring let.

[teoxoy]

@stshine see gpuweb/gpuweb#2127 for the reasons behind it