compile-time checked format strings

[Adam D. Ruppe]

On Saturday, 13 January 2018 at 19:15:49 UTC, kdevel wrote:
> dmd checks the types but does not count the arguments.

so note that dmd doesn't actually do any checks here - it is all 
done in library code.

The implementation is amusingly simple:
https://github.com/dlang/phobos/blob/master/std/format.d#L5803

try format(...) catch(Exception) trigger compile time error.


But, since float format doesn't work at compile time, this method 
doesn't actually work to catch any float-related problems except 
mismatching format characters!

For ints, it catches all that, but for float, it just bails out 
of the check as soon as it actually *succeeds* - because that 
kills CTFE.

We should just put in a fake stub float printer to hack this past 
for checking purposes.