Add a proposal for uniformity assertions - #10959

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1. Should the proposal include attributes to facilitate library writers?

I wanted to point out a potential alternative path, inspired by the extensive const-eval capabilities of WGSL and C++ concepts.

WGSL function signatures today can already have calls to @const functions. If there were an explicit space in the syntax for that, then the following would be possible

fn foo(a: bool, b: vec2f) -> bool 
  where assertUniform(b) // part of the function's signature
{
  const threshold = 3;
  let cond = a || assertUniform(dpdx(b)).x > threshold;
  return cond;
}

However, I have not thought through the implications of such a mechanism. Neither have I researched which other parts of WGSL or which WGSL proposals could benefit from this.