[glsl-in] Struct layout handling is overly simplistic

[fintelia]

Because naga assumes that structs have layout std140, this shader:

#version 460 core

struct A {
    float x[2];
};

layout(std430, set = 0, binding = 0) buffer DataBuffer {
    A a;
};

void main() {
    A b = a;
}

Produces this error:

error: 
   ┌─ shader.vert:11:1
   │
11 │ void main() {
   │ ^^^^^^^^^^^ naga::Function [1]
12 │     A b = a;
   │       ^^^^^
   │       │   │
   │       │   naga::Expression [3]
   │       naga::Expression [4]

Function [1] 'main' is invalid: 
    Store of [3] into [4] doesn't have matching types

[fintelia]

Note that it isn't enough to infer whether std140 or std430 layouts are needed for a given local variable, GLSL actually allows the layout to vary based on conditionals...

#version 460 core

struct A {
    float b[2];
};

layout(std140, set = 0, binding = 0) uniform Data {
    A a;
    bool flag;
};

layout(std430, set = 0, binding = 1) buffer DataBuffer {
    A b;
};

void main() {
    A c = flag ? a : b;
}

[teoxoy]

The solution here is to generate the structs with the correct layout as needed, unfortunately this also requires auto casting between the different layouts in some situations (for example a function picked to accept std430 structs and you pass it a std140 struct).

quoting @JCapucho in https://github.com/gfx-rs/naga/pull/2212#issuecomment-1386072710