Fix string handling in pushAny - #231

why is this here? Lua itself obviously works with C pointers all the time?

Quoting the Zig documentation: "The only valid reason for using a C pointer is in auto-generated code from translating C code."
I see no reason why they should be supported. The caller should know the type.

It is possible that the value comes from another C library. I have recently been doing work using Lua and sqlite and I need to work with C strings occasionally. I think C pointers should be supported

In that case, you would either know the length or whether it has a null terminator, then you could use either ptr[0..len] or std.mem.span(ptr).

If we supported C pointers with pushAny, it would be making the assumption that it has a null terminator, which results in UB when there isn't one.

Side note: I did not remove support for C pointers in this change. I only changed the error message.

I just tested with this PR. This fails

const c_str: [*c]const u8 = "hello zig";
try lua.pushAny(c_str);
try testing.expectEqualStrings("hello zig", try lua.toString(-1));

and both of these work

const c_str: [*c]const u8 = "hello zig";
try lua.pushAny(std.mem.span(c_str));
try testing.expectEqualStrings("hello zig", try lua.toString(-1));

const c_str: [*c]const u8 = "hello zig";
const ptr: [*:0]const u8 = c_str;
try lua.pushAny(ptr);
try testing.expectEqualStrings("hello zig", try lua.toString(-1));

I gave it more thought and I think I'm actually okay with this change. If someone has a [*c] pointer, they can use span or coerce to a null-terminated many-item pointer.