Function param input modifiers `out` and `inout` are unexpectedly strict about slang

[csyonghe]

This will be one of the cases we will NOT follow dxc's behavior because that behavior is fundamentally flawed.

In HLSL, resource types are just handles.

If a function has an inout Texture2D, it doesn't mean the texture itself is modifiable, instead it just means the function may return another texture handle through that function parameter. This means that the caller needs to pass in a l-valued texture handle. Since global shader parameters are not l-values, passing them to an out parameter is incorrect from the type system's point of view, and this behavior is consistent and desired.

The HLSL language has made various mistakes confusing the mutability of a handle vs. the things pointed to by the handle. An example is the OutputIndices type of mesh shaders. They should have been treated as an handle through which modifications are made, therefore the handle itself should just be "in", instead of "out" in the entry point signature.

In slang we care a lot about type system consistency and we try to avoid confusing mistakes in HLSL.

[csyonghe]

I will close this issue because I feel strong enough that this isn't something we should "fix" in Slang.