Not sure if this is the exact same issue but it seems module scope constants are explicitly allowed for workgroup_size, but naga only accepts literals.

Would be nice to do something like this:

let WORKGROUP_SIZE: u32 = 256u;
[[stage(compute), workgroup_size(WORKGROUP_SIZE)]]
fn main([[builtin(global_invocation_id)]] global_id: vec3<u32>, [[builtin(num_workgroups)]] num_workgroups: vec3<u32>) {
    let total_particles = num_workgroups[0] * num_workgroups[1] * num_workgroups[2] * WORKGROUP_SIZE;
...

Would it work to add in a branch that uses parse_const_expression in addition to parse_generic_non_negative_int_literal? or is that more complicated than it seems

Yeah, this can be considered a part of the issue. Technically speaking, there should be no place in code where only literals are supported. If literals are supported, then constants as well.
This part of WGSL has been evolving recently, so we couldn't know about this when just implementing the code originally. We need to update the code now.

This is the relevant line for specifying workgroup_size now (also see: [1] [2] [3])

It looks like expressions support overrides, it's just that they don't get compiled to const expressions in [1] since Module requires ast::EntryPoint to resolve into u32s for crate::EntryPoint.workgroup_size by then.

Maybe we can add a workgroup_size_overrides: Option<[Option<Handle<Expression>>; 3]> attribute to crate::EntryPoint that gets evaluated in [1] and initialize the u32s to zeros in [2] if size_expr is an ExpressionKind::Override.

I'll give it a try sometime this week.