We need something like source_location in D

[Nicholas Wilson]

On Wednesday, 31 July 2019 at 01:39:05 UTC, Andrej Mitrovic wrote:
> From C++20: 
> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1208r5.pdf
>
> Instead of having to constantly declare "file = __FILE__, line 
> = __LINE__" everywhere, it would be so much more useful to just 
> have a single built-in type which we can default initialize . 
> For example:
>
> -----
> import std.algorithm;
>
> void checkAndThrow (lazy bool expression, Loc loc = 
> __LOCATION__)
> {
>     if (!expression)
>         throw new Exception("Failed", loc.file, loc.line);
> }
>
> void main ()
> {
>     auto arr = [1, 1, 1, 1];
>     checkAndThrow(arr.sum == 4);
>
>     // should throw with file+line of this statement
>     checkAndThrow(arr.sum == 5);
> }
> -----
>
> This also makes it very easy to extend in the future, because 
> we don't have to pollute the namespace with a lot of "__"-style 
> symbols.
>
> The actual naming of the location symbol is something we can 
> bikeshed over, that's fine.
>
> If you grep for `string file = __FILE__, size_t line = 
> __LINE__` or `string file = __FILE__, int line = __LINE__`, it 
> is everywhere in Druntime and Phobos. I think it's time we got 
> rid of this repetition.
>
> I know __traits(getLocation) got implemented recently, but it's 
> not very useful in this context because it returns a tuple 
> (https://github.com/dlang/dmd/pull/10013). I don't see an easy 
> way to use it in function parameters.

It returning a tuple is not the problem because they auto expand :

void foo(string a, int b, int c)
{
     import std.stdio;
     writeln(a, " ", b);
}
void main()
{
     foo(__traits(getLocation,foo));
}

That you can't pass the caller loc is.

A DIP will take a long time, I don't think the addition of 
__SYMBOLS__ require one. The addition of a type will possibly be 
problematic, but if  `Loc` were a tuple that would be fine if we 
could do

void foo(auto a = 1);

since it would be just as silly to have to type

void foo(typeof(__LOCATION__) loc =  __LOCATION__);