Improve static_assert

[skallweitNV]

static_assert usage is very limited right now as it only works in function scope.

static_assert must be available in any scope.

Also, the error message argument should be optional.

[csyonghe]

@skallweitNV can you share more details on how you want to use static_assert?

Since slang has much more complicated semantics around what is compile-time (link time constants, specialization constants), it is more important for us to understand different kinds of needs.

[skallweitNV]

To elaborate a bit. This works, the static_assert is reported correctly:

struct S
{
    uint value;
};

void func()
{
    static_assert(sizeof(S) == 2, "some message"); // <-- REPORTS FAILED ASSERT
}

[shader("compute")]
void main() {
    func();
}

However, if func is not called, the static_assert is not checked:

struct S
{
    uint value;
};

void func()
{
    static_assert(sizeof(S) == 2, "some message"); // <-- NOT CHECKED
}

[shader("compute")]
void main() {
    // func();
}

If we take the static_assert out of function scope to global scope, the program doesn't compile anymore (but it should IMO):

struct S
{
    uint value;
};

static_assert(sizeof(S) == 2, "some message"); // <-- COMPILATION ERROR

[shader("compute")]
void main() {
}

The error is:

USER error: /user.slang(6): error 20001: unexpected '(', expected ')'
static_assert(sizeof(S) == 2, "some message");

In addition to making static_assert work in global scope, it would be a nice addition to make the string message argument optional:

static_assert(sizeof(S) == 2, "some message");
static_assert(sizeof(S) == 2); // <-- THIS SHOULD WORK TOO

So, this issues isn't really about what can go into the expression (any constant/compile-time expression should work) but in what scope static_assert can be used (and the compiler actually checks the assert).