glsl-in: Fix missing stores for local declarations

[adeline-sparks]

Previously, if a local variable was declared with a constant value, we
would elide the store and instead give the variable an initial value (as
if it was a global variable). This caused variables to not be
re-initialized each time through a loop.

Fixes #1935, but the issue seems broad and might fix others.

[JCapucho]

There are some problems with how the problem is handled but the overall approach is good, you should also change add_local_var to make the init field always None

# src/front/glsl/variables.rs:646
-                init: decl.init,
+                init: None,

[JCapucho]

This check should be moved before the constant is solved, no point in wasting time processing it if it isn't going to be used

[JCapucho]

This will prevent non constant globals from being evaluated, try this instead:

let maybe_constant = if ctx.external {
  match parser.solve_constant(ctx.ctx, root, meta) {
      Ok(res) => Some(res),
      // If the declaration is external (global scope) and is constant qualified
      // then the initializer must be a constant expression
      Err(err) is_const => return Err(err),
      _ => None,
  }
} else {
  None
};

[adeline-sparks]

Ah, thanks! There's a lot of cases here and I still didn't have it clear in my head.

[adeline-sparks]

Actually, if we have a non-constant qualified global with an initializer that can't be solved, should that also be an error?

[JCapucho]

No glsl allows globals to refer to other globals which can't be constified

[adeline-sparks]

Fascinating, ok! I will go with this. Thanks again!

[JCapucho]

Thanks for your contribution