Assignment to swizzle produces unhelpful error message

[jimblandy]

Given the program:

fn f() {
   var v = vec2(0);
   v.xy = vec2(1);
}

Naga complains:

error: the left-hand side of an assignment must be a reference
  ┌─ assign-to-swizzle.wgsl:2:20
  │  
2 │      var v = vec2(0);
  │ ╭───────────────────^
3 │ │    v.xy = vec2(1);
  │ ╰───────^ expression is not a reference

Could not parse WGSL

This leads to questions in chat like:

Hi. What's the equivalent WGSL code to this?

p.xy = f(p.xy)

I get this error and have no idea how to write this nicely...

The error message did not convey to the user that swizzle assignment is simply unsupported.

[JCapucho]

We could bake in some detection for this case and add it as note to the diagnostic, it would look something like this:

error: the left-hand side of an assignment must be a reference
  ┌─ test.wgsl:3:4
  │
3 │    v.xy = vec2(1);
  │    ^^^^ expression is not a reference
  │
  = You seem to be trying to assign to a swizzle
    wgsl doesn't support assignments to swizzles

If we wanted something fancier we could try adding suggested fixes like rustc has, but that would be more complicated

error: the left-hand side of an assignment must be a reference
  ┌─ test.wgsl:3:4
  │
3 │    v.xy = vec2(1);
  │    ^^^^ expression is not a reference
  │
  = You seem to be trying to assign to a swizzle
    wgsl doesn't support assignments to swizzles
help: try using assigning each component individually
  ┌─ test.wgsl:3:4
  │
3 │    let a = vec2(1);
4 │    v.xy = a.x;
5 │    v.xy = a.y;
  │    ^^^
  │