add support for unions

[39ali]

Rust unions require reinterpreting the bits of one type as another — e.g. reading [f32; 5] as struct { a: i32, b: [f32; 3], c: u32 }. In SPIR-V this would naively lower to an OpBitcast between two composite types, which is illegal by the spec: OpBitcast only accepts scalar, vector, or pointer operands, never structs or arrays.

previously, any union field access involving layout-compatible but differently-typed SPIR-V composites would either produce invalid SPIR-V or fail to compile.

solution:

implement two new codegen paths in builder_methods.rs that handle type-mismatched copies without relying on composite OpBitcast:

memcpy_const_size — used by the memcpy / OpCopyMemory path (pointer-to-pointer copies).

try_store_composite_scalar_leaves — used by the store path (value-to-pointer writes).

Both work by:

decomposing both the source and destination types recursively into scalar leaves (collect_scalar_leaves), collecting the OpAccessChain index path, SPIR-V type, and bit width for each leaf.

grouping leaves by equal total bit widths (group_leaves_by_bits), this handles both 1:1 (same scalar count) and N:M (different counts, same total bits) cases.

emitting element-wise copies per group:

1→1: OpLoad + optional OpBitcast + OpStore
N→1 (pack): load N scalars, OpCompositeConstruct into an intermediate uN×k vector, OpBitcast to the wider dst type, OpStore
1→N (unpack): OpLoad, OpBitcast to an intermediate uN×k vector, OpCompositeExtract each component, OpStore each

solves #241

[Firestar99]

I like the simplicity of the implementation, this was pretty easy to review, but it does ring "AI impl" bells.

[Firestar99]

What's the difference between memcpy_const_size() and try_store_composite_scalar_leaves()? Both functions seem to be almost the exact same, apart from memcpy's arg being src: SpirvValue as a pointer and the other one val: SpirvValue as a value.

[Firestar99]

I see an implementation for 1 => 1, 1 => N, N => 1 but no N => M?

This compiletest fails:

// build-pass
// compile-flags: -C target-feature=+Int64

use spirv_std::spirv;

union MyUnion {
    a: (u32, u64),
    b: (u64, u32),
}

#[spirv(fragment)]
pub fn union_mixed_types(
    out: &mut u64,
) {
    let bla = MyUnion { a: (42, 123) };
    *out = unsafe { bla.b }.0;
}