Currently, the direction of function arguments in a structure that implements an interface is not checked if it is the same as in the interface declaration. Therefore a function can modify a variable that is expected to be only an input and do not modify a variable that is expected to be an output.

Current result:

public interface ITest {
    public void testIn(int a);
    public void testOut(out int b);
};

public struct TestImpl : ITest {
    public void testIn(out int a) {
        a = 5;
 }
    public void testOut(int b) {
        b = 6;
 }
}

RWStructuredBuffer<int> output;

void doSomething<T>(T data) where T : ITest {
    int a = 516;
    data.testIn(a);
    int b = 687;
    data.testOut(b);

    output[0] = a;
    output[1] = b;
}

[shader("compute")]
[numthreads(1,1,1)]
void computeMain()
{
    TestImpl data;
    doSomething(data);
}

This shader is successfully compiled and after the invocation of this code, output buffer contains [5, 687], which is not expected if we look at the interface declaration.

Tested on binaries from the latest release (v2024.17) and a build from the current master (89dd2b1).

Expected result:

Slang will write an error about the difference in an argument direction. This can be quite useful in the link-time specialization where the entire shader works with an unknown type based on a known interface.