[Issue 17593] __POS__ magical constant like __FILE__ and friends

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=17593

--- Comment #2 from Eyal <eyal at weka.io> ---
I tested the mixin approach before filing this. It doesn't work:

struct Pos { size_t line; }
void foo(size_t line=__LINE__, Pos pos=Pos(__LINE__))() { .. }

line is caller's line. pos.line is foo's decl line.

The reason I want this enhancement is that we have a logging framework that
uses the line numbers in compile-time (to make fast logs that log very
minimally at runtime).

I often want to create log-line wrappers for specific cases.

Currently I have to do this:

void myLogger(string fmt, string file=__FILE__, string mod=__MODULE__, size_t
line=__LINE__, Args...)(auto ref Args args) {
    log!("my extra stuff" ~ fmt, file, mod, line)(myExtraArgs, args);
}

And this has to be repeated, verbatim, for every log wrapper in existence.

It is quite a big discouragement against writing functions that work and pass
on code positions.

Much nicer:

void myLogger(string fmt, Pos pos=__POS__) {
    log!("my extra stuff" ~ fmt, pos)(myExtraArgs, args);
}

Almost as nice:

void myLogger(string fmt, Pos pos=mixin(POS)) ...

So alternatively to adding __POS__, dmd could be fixed so that __LINE__ is
based on its *lexical* position. i.e: same value for __LINE__ in a default CT
param value and when it is given to Pos() inside a default CT param value.

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