remove const qualifier if required

[axdank]

fix:

[natecraddock]

@VisenDev what do you think of these changes?

[VisenDev]

I generally oppose using const cast unless completely necessary. However, from what I can tell it does seem to be legal behavior to mutate the result of lua_tolstring.

If mutating the result of a lua_tolstring is defined behavior, then perhaps the correct this to do here is remove the const qualifier from this function

    pub fn toString(lua: *Lua, index: i32) ![:0]const u8 {

Rather than const casting its result.

[natecraddock]

What I read on this page https://www.lua.org/manual/5.4/manual.html#lua_tolstring (look for section 4.1.3), it is safe to mutate the result of lua_tolstring but only for a short time (roughly within a function call if I'm reading things right).

I think I'd rather allow the caller to use @constCast() as needed, or provide another function that returns a []u8 with a comment about when it is safe to use.

[natecraddock]

@axdank do you have any thoughts? Based on the lua reference manual 4.1.3, this could be a possible footgun depending on how the returned string would be used. The safest way would be to duplicate a const string if the string needs to be stored longer than Lua would keep that pointer valid.

I can imagine that someone could use toAny passing a struct with a []u8 in it and then run into issues if they didn't understand that string references can become invalid.

Maybe this is a case for toAnyAlloc? so the allocator can duplicate the const string?

[axdank]

@natecraddock How about these changes? I removed the use of @castConst, so if any structure needs a mutable string field, a copy of it is created.

[natecraddock]

Thank you for working on this!