make Skip a DoubleEndedIterator

[oli-obk]

Playground Example

[rust-highfive]

r? @alexcrichton

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[bluss]

Implementation lgtm, but is it too surprising that it does all the skip steps when the back element is taken?

[oli-obk]

not doing that would require ExactSizeIterator

[alexcrichton]

This also looks good to me, but this is touching a pretty core iterator so I'm going to tag as T-libs just to be sure we discuss it a bit.

[alexcrichton]

We discussed this in the libs team meeting and the conclusions we reached were:

• The order of iteration here is probably a little too surprising. Mainly a Skip iterator drained via next_back would be consumed differently than if the underlying iterator were drained via next_back.
• We may be able to implement this, however, via an ExactSizeIterator bound.
• There's precedent for double-ended iteration requiring an exact size bound via the rposition method.

@oli-obk would you be ok updating this to also include an ExactSizeIterator bound? That way the next_back method could avoid skipping on the front unless necessary.

[oli-obk]

It's always backwards compatible to remove the bound in the future, so adding the bound now is fine with me.

Sadly this makes it useless for iterating over str slices by chars

[oli-obk]

updated and added more tests

[alexcrichton]

@bors: r+ 25e5de3

Thanks @oli-obk!

[bors]

⌛ Testing commit 25e5de3 with merge 07a1803...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt