[Issue 16487] Add function to obtain the available disk space

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=16487

--- Comment #12 from b2.temp at gmx.com ---
std.file makes more sense. User wants available disk space to know if it can
create a file or how many byte in disk a cache can use (or whatever else).
Much less useful in std.path.

ideally there would be:

std.filesystem.file
std.filesystem.path
std.filesystem.package <-- i would put it here

but since it's not the case std.file is the best ooption.

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