Eliminate more excessive null-checks from slice iterators

[dotdash]

This adds the assume() calls back that got lost when rebasing #21886.

[rust-highfive]

r? @pcwalton

(rust_highfive has picked a reviewer for you, use r? to override)

[dotdash]

r? @huonw -- you had r-ed #21886 before

[huonw]

@bors r+ e457

[alexcrichton]

I think that unfortunately this doesn't work on windows: http://buildbot.rust-lang.org/builders/auto-win-32-nopt-t/builds/3435/steps/compile/logs/stdio

[dotdash]

Oh, #22395 accidently reverted LLVM to an older version that doesn't contain my fix to make assume() work correctly :-(

[dotdash]

@bors r-

r- for now until the llvm fix has landed (again)

[alexcrichton]

Ach sorry about that :( Feel free to re-include the update in this PR and send it back to bors.

[dotdash]

I'll just wait for the new llvmup PR. It will merge eventually, I suppose ;-)

[dotdash]

@bors r=huonw e457

[bors]

⌛ Testing commit e457328 with merge 779490e...

[bors]

💔 Test failed - auto-win-32-nopt-t

[dotdash]

Ah, due to the previous revert to the old llvm version, the snapshot uses the old llvm again and so it crashes in stage0 sigh

[Manishearth]

Now that new llvm landed, should this be working now?

[dotdash]

It would, if guarded by if cfg!(not(stage0)). As is, it would need a new snapshot that has the updated LLVM.

[dotdash]

@bors retry

Snapshot landed

[bors]

⌛ Testing commit e457328 with merge ef5b15c...

[bors]

💔 Test failed - auto-linux-64-x-android-t

[alexcrichton]

@bors: retry

[bors]

⚡ Previous build results are reusable. Rebuilding only auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt...

[bors]

💔 Test failed - auto-win-32-opt

[alexcrichton]

@bors: retry clean

[bors]

⌛ Testing commit e457328 with merge 1c93934...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt