foreach() behavior on ranges

[Steven Schveighoffer]

On 8/25/21 6:06 AM, Joseph Rushton Wakeling wrote:
> On Tuesday, 24 August 2021 at 09:15:23 UTC, bauss wrote:
>> A range should be a struct always and thus its state is copied when 
>> the foreach loop is created.
> 
> That's quite a strong assumption, because its state might be a reference 
> type, or it might not _have_ state in a meaningful sense -- consider an 
> input range that wraps reading from a socket, or that just reads from 
> `/dev/urandom`, for two examples.
> 
> Deterministic copying per foreach loop is only guaranteed for forward 
> ranges.

structs still provide a mechanism (postblit/copy ctor) to properly save 
a forward range when copying, even if the guts need copying (unlike 
classes). In general, I think it was a mistake to use `.save` as the 
mechanism, as generally `.save` is equivalent to copying, so nobody does 
it, and code works fine for most ranges.

What should have happened is that input-only ranges should not have been 
copyable, and copying should have been the save mechanism. Then it 
becomes way way more obvious what is happening. Yes, this means forgoing 
classes as ranges.

-Steve