[wgsl-in] allow functions to be declared literally after entry point

[vicary]

This is discovered in Deno when using tensorflow.js with its WebGPU backend, when tfjs declares a function fn main after the entry point fn _start. The generated WGSL works in Chrome Canary 108.0.5329.0 (V8 10.8.79) but fails in Deno, error details at tensorflow/tfjs#6842 (comment).

Experiments in tensorflow/tfjs#6842 (comment) showed that in naga, invoked functions have to be declared literally before the entry point and the @compute directive.

The current WGSL specification (20220922) seems to have function scoping, replacing the lexical scoping back in 2020.

A function declaration must only occur at module scope. A function name is in scope for the entire program.

Given the generated script shader.wgsl, should the function main be available in scope when _start is invoked?

[SparkyPotato]

This is a limitation of the current WGSL front end.
It will be fixed with #2065.

[teoxoy]

Closing as duplicate of #1745.