Checking function parameters in Phobos

[Brad Anderson]

On Wednesday, 20 November 2013 at 00:48:40 UTC, bearophile wrote:
> [snip]
> Often I have genomic data or other text data that is surely 
> ASCII (and I can accept a run-time exception at loading time if 
> it's not ASCII). Once such text is in memory I'd like to not 
> pay for UTF on it. Sometimes you can do this with 
> std.string.representation, but there is no opposite function 
> (http://d.puremagic.com/issues/show_bug.cgi?id=10162 ). Also in 
> Phobos there are several string/char functions that could be 
> made faster if the input is assumed to be ASCII. To solve this 
> problem in languages as Haskell they usually introduce a new 
> type like AsciiString. In past I have suggested to introduce 
> such string wrapper in Phobos.
>

Is that not what phobo's AsciiString is? 
http://dlang.org/phobos/std_encoding.html#.AsciiString