Use len instead of size_hint where appropiate

[tbu-]

This makes it clearer that we're not just looking for a lower bound but
rather know that the iterator is an ExactSizeIterator.

[rust-highfive]

r? @alexcrichton

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[nagisa]

Doesn’t len have an assertion which size_hint doesn’t? I guess I’m being too paranoid, but there certainly must be some extra overhead which warrants some benchmarks… e.g. how does the performance of iterating though CharIndices change?

[tbu-]

If that were the case, we should rather modify len than use size_hint in places where len would be more appropiate.

But it looks like LLVM can see right through it:

use std::slice; pub fn slice_len(iter: &slice::Iter<u8>) -> usize { iter.len() } pub fn slice_len2(iter: &slice::Iter<u8>) -> usize { iter.size_hint().0 } 

produces the same bytecode

; ModuleID = 'a.0.rs' target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128" target triple = "x86_64-unknown-linux-gnu" %"2.std::slice::Iter<u8>" = type { i8*, i8*, %"2.std::marker::PhantomData<&'static u8>" } %"2.std::marker::PhantomData<&'static u8>" = type {} ; Function Attrs: norecurse readonly uwtable define i64 @_ZN1a9slice_len17hc2890fcf572cc394E(%"2.std::slice::Iter<u8>"* noalias nocapture readonly dereferenceable(16)) unnamed_addr #0 { entry-block: %1 = getelementptr inbounds %"2.std::slice::Iter<u8>", %"2.std::slice::Iter<u8>"* %0, i64 0, i32 1 %2 = bitcast i8** %1 to i64* %3 = load i64, i64* %2, align 8, !alias.scope !0, !noalias !5 %4 = bitcast %"2.std::slice::Iter<u8>"* %0 to i64* %5 = load i64, i64* %4, align 8, !alias.scope !0, !noalias !5 %6 = sub i64 %3, %5 ret i64 %6 } ; Function Attrs: norecurse nounwind readonly uwtable define i64 @_ZN1a10slice_len217h415029022c569338E(%"2.std::slice::Iter<u8>"* noalias nocapture readonly dereferenceable(16)) unnamed_addr #1 { entry-block: %1 = getelementptr inbounds %"2.std::slice::Iter<u8>", %"2.std::slice::Iter<u8>"* %0, i64 0, i32 1 %2 = bitcast i8** %1 to i64* %3 = load i64, i64* %2, align 8, !alias.scope !7, !noalias !10 %4 = bitcast %"2.std::slice::Iter<u8>"* %0 to i64* %5 = load i64, i64* %4, align 8, !alias.scope !7, !noalias !10 %6 = sub i64 %3, %5 ret i64 %6 } attributes #0 = { norecurse readonly uwtable } attributes #1 = { norecurse nounwind readonly uwtable } !0 = !{!1, !3} !1 = distinct !{!1, !2, !"_ZN79_$LT$std..slice..Iter$LT$$u27$a$C$$u20$T$GT$$u20$as$u20$std..iter..Iterator$GT$9size_hint17h39fecfdf3007b42aE: argument 1"} !2 = distinct !{!2, !"_ZN79_$LT$std..slice..Iter$LT$$u27$a$C$$u20$T$GT$$u20$as$u20$std..iter..Iterator$GT$9size_hint17h39fecfdf3007b42aE"} !3 = distinct !{!3, !4, !"_ZN4core4iter6traits17ExactSizeIterator3len17hb31ac8a36a76a083E: argument 0"} !4 = distinct !{!4, !"_ZN4core4iter6traits17ExactSizeIterator3len17hb31ac8a36a76a083E"} !5 = !{!6} !6 = distinct !{!6, !2, !"_ZN79_$LT$std..slice..Iter$LT$$u27$a$C$$u20$T$GT$$u20$as$u20$std..iter..Iterator$GT$9size_hint17h39fecfdf3007b42aE: argument 0"} !7 = !{!8} !8 = distinct !{!8, !9, !"_ZN79_$LT$std..slice..Iter$LT$$u27$a$C$$u20$T$GT$$u20$as$u20$std..iter..Iterator$GT$9size_hint17h39fecfdf3007b42aE: argument 1"} !9 = distinct !{!9, !"_ZN79_$LT$std..slice..Iter$LT$$u27$a$C$$u20$T$GT$$u20$as$u20$std..iter..Iterator$GT$9size_hint17h39fecfdf3007b42aE"} !10 = !{!11} !11 = distinct !{!11, !9, !"_ZN79_$LT$std..slice..Iter$LT$$u27$a$C$$u20$T$GT$$u20$as$u20$std..iter..Iterator$GT$9size_hint17h39fecfdf3007b42aE: argument 0"} 

[alexcrichton]

Yes most of these look like it's trivial for LLVM to see through as the size_hint method is just returning (a, Some(a)) which is easy to cause the assertion that (Some(low) == hi) to get optimized away. Looks fine to me.

@bors: r+ 8ff5c43

[bors]

⌛ Testing commit 8ff5c43 with merge ad07411...

[bors]

💔 Test failed - auto-mac-64-opt

[alexcrichton]

@bors: retry

On Thu, Jun 23, 2016 at 4:00 PM, bors notifications@github.com wrote:

💔 Test failed - auto-mac-64-opt
http://buildbot.rust-lang.org/builders/auto-mac-64-opt/builds/9566

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[bors]

⌛ Testing commit 8ff5c43 with merge 90943a5...

[alexcrichton]

@bors: retry force clean

• restarted buildbot

[bors]

⌛ Testing commit 8ff5c43 with merge 4b89deb...

[bors]

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