impr: Another approach to resolving externals

[aleksanderkatan]

Right now, the following resolves to hello.bound.

const layout = tgpu.bindGroupLayout({
  count: { uniform: d.f32 }
});
const shader = tgpu.resolve({
  template: `
    hello.bound
  `,
  externals: {
    hello: layout,
  },
});

It is only after actually having a resolvable value like hello.bound.count, that the resolution works:

@group(0) @binding(0) var<uniform> count_0: f32;
    count_0

When the value is not resolvable, we should throw instead of ignoring it. It may be done by first searching for the external key, and then following the chain of property accesses.

[aleksanderkatan]

Some thoughts.

• Current approach: for every external, if it is resolvable, replace occurrences with resolved, otherwise check what props are called in the wgsl and call the procedure recursively on ${external}.${prop}.
• We are inconsistent in reporting unused externals. externals: { unused } will be reported, but externals: { buffers: { unused } } won't be.
• On one side, warns are useful for accidental inclusions and misspellings in wgsl, but unused imports can also be intended when using layouts that contain more than the current function needs.
• The approach of "find and external and follow .prop.prop without thinking" does not work, since an external can be a buffer used with prop accesses. Therefore, we have to follow the external prop chain one by one, and this is basically what we do at this moment.

We could instead follow ${external}.${prop} for props that exist on the external, instead of props that are called. Or do both, to report both non-existing props, and unused props. The question is whether we want to do this at all, since the user might be overwhelmed with warns they cannot get rid of.