Naga's typechecker ignores scalar kind when casting matrices

[cogumbreiro]

Description

I think that there's a bug in the naga/src/proc/typifier.rs. Notably, when casting with As the new scalar kind, variable kind, is not propagated to the output scalar, only the width.

                Ti::Matrix {
                    columns,
                    rows,
                    mut scalar,
                } => {
                    if let Some(width) = convert {
                        scalar.width = width;
                    }
                    TypeResolution::Value(Ti::Matrix {
                        columns,
                        rows,
                        scalar,
                    })
                }

Note a correct behavior on vectors where the kind is propagated to the generated vector:

                Ti::Vector {
                    size,
                    scalar: crate::Scalar { kind: _, width },
                } => TypeResolution::Value(Ti::Vector {
                    size,
                    scalar: crate::Scalar {
                        kind,
                        width: convert.unwrap_or(width),
                    },
                }), 

Source:

wgpu/naga/src/proc/typifier.rs

Lines 881 to 884 in f669024

	TypeResolution::Value(Ti::Matrix {
	columns,
	rows,
	scalar,

[cogumbreiro]

I've added an example that crashes in current naga.

[cogumbreiro]

I just realized that according to the spec, a matrix can have f32 or f16. However, since naga doesn't support f16, then perhaps this is not really a bug as of now.

However, here's a minimal example for posterity which I'm guessing would break once support for f16 is implemented:

@compute @workgroup_size(256, 1, 1) fn computeSomething() {
  let m1: mat2x2<f32> = mat2x2<f32>(
      vec2<f32>(1, 2),
      vec2<f32>(3, 4)
  );

  let m2: mat2x2<f16> = mat2x2<f16>(m1);
}

[jimblandy]

Since matrices are enforced to be f32 by the validator, this isn't really a bug we can hit at the moment. When we add f16 matrix support, this will become relevant.