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Weaken guarantee around advancing underlying iterators in zip #83791
Weaken guarantee around advancing underlying iterators in zip #83791
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r? @m-ou-se (rust-highfive has picked a reviewer for you, use r? to override) |
@rustbot label T-libs |
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#83796 can be used for a crater run and serves as an example what we might get rid off. |
@@ -500,8 +500,8 @@ pub trait Iterator { | |||
/// In other words, it zips two iterators together, into a single one. | |||
/// | |||
/// If either iterator returns [`None`], [`next`] from the zipped iterator | |||
/// will return [`None`]. If the first iterator returns [`None`], `zip` will | |||
/// short-circuit and `next` will not be called on the second iterator. | |||
/// will return [`None`]. If the zipped iterator has no more elements to return then attempts |
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no more elements to return
What would this mean for non-fused iterators? What if both iterators return None
at some point, but both start giving Some(..)
again some time later? Are you sayingg that the Zip
iterator might just stop forever at that point (fusing the iterator), or are you just talking about iterators that we know will give None
forever because of our specializations?
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A hypothetical implementation could query size_hint()
of one or both iterators on every call to next()
and short-circuit when the upper bound of either is 0. That should be safe and not require any specialization. Whether that could be considered as fusing the iterator would depend on how the non-fused one calculates its size hint.
It's intentionally kept vague to leave more flexibility for optimizations while at the same time telling the users that the underlying iterators' next()
might not be called. Previously it only said that the 2nd iterator's might not be called, now it may call neither.
marking this as blocked till #83796 crater run is complete and triaged. |
@rustbot label -S-waiting-on-crater +S-waiting-on-review The crater run in #83796 completed, I went through the results in #83796 (comment) and wrote my conclusion in #83796 (comment) Possible routes that can be taken from here: A)
B)
C) Middle ground between A) and B): We could remove the TrustedRandomAccess specialization for D) Somehow extend TrustedRandomAccess to fix this case too? @steffahn hinted at some ideas in #85873 (comment) something a bit more concrete would help to fill in this option |
Yeah, I’m working on an approach that’ll need two or three additional methods for So the new methods would be just for triggering side effects. Maybe extra Such a proposal would also come with a list of recommendations/requirements around using these methods (not safety requirements) that you’ll need to follow if you want to not mess with an iterator’s order of side effects. Two or three, because a third method to be called before iteration from the front starts could be added for symmetry and e.g. to make I’d expect to be able to provide more details within 2 weeks. |
Interesting. Another idea I want to toss in, it might help to pass the nominal direction of iteration with each call to get_unchecked. This may not be relevant for current issues but it could help with implementing TRA for
While technically this is fine it seems aesthetically unpleasing. I wonder if we can somehow bundle this stuff into a helper trait/struct/associated type to separate it more cleanly. Or maybe we should use multiple impl blocks and collect them into a separate file. I think it'll make future improvements easier. |
Updated, please check if the change is now sufficiently small. It should still solve #82303 by saying that the 1st iterator is advanced zero or one times without touching the guarantee about the 2nd iterator. |
/// will return [`None`]. | ||
/// If the zipped iterator has no more elements to return then each further attempt to advance | ||
/// it will advance the first iterator zero or one times and not call [`next`] on the second | ||
/// iterator. |
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If the zipped iterator has no more elements to return[,] then each further attempt to advance it will […] not call
next
on the second iterator.
This is not correct. This only applies to the case where the first iterator has no more elements. If the first iterator is non-empty but the second one is empty, of course next
will (or at least can) be called on the second iterator.
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Right, how about?
/// If the zipped iterator has no more elements to return then each further attempt to advance
/// it will first try to advance the first iterator at most one time and if it still yielded an item
/// try to advance the second iterator at most one time.
The current guarantee is too strong as it would prevent adapters from exploiting knowledge about the iterator length and using counted loops for example because they would stop calling `next()` before it ever returned `None`. Additionally several nested zip iterators already fail to uphold this. This doesn't remove any of the specialization code that tries (and sometimes fails) to uphold the guarantee for `next()` because removing it would also affect `next_back()` in more surprising ways.
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@bors r+ rollup |
📌 Commit 2ff677d has been approved by |
…ntee, r=dtolnay Weaken guarantee around advancing underlying iterators in zip The current guarantee (introduced in rust-lang#52279) is too strong as it prevents adapters from exploiting knowledge about the iterator length and using counted loops for example because they would stop calling `next()` before it ever returned `None`. Additionally several nested zip iterators already fail to uphold this. This does not yet remove any of the specialization code that tries (and sometimes fails) to uphold the guarantee for `next()` because removing it would also affect `next_back()` in more surprising ways. The intent is to be able to remove for example this branch https://github.com/rust-lang/rust/blob/36bcf4069717b9dff90270d13b53a3b130329960/library/core/src/iter/adapters/zip.rs#L234-L243 or this test https://github.com/rust-lang/rust/blob/36bcf4069717b9dff90270d13b53a3b130329960/library/core/tests/iter/adapters/zip.rs#L177-L188 Solves rust-lang#82303 by declaring it a non-issue.
…askrgr Rollup of 6 pull requests Successful merges: - rust-lang#83791 (Weaken guarantee around advancing underlying iterators in zip) - rust-lang#90995 (Document non-guarantees for Hash) - rust-lang#91057 (Expand `available_parallelism` docs in anticipation of cgroup quota support) - rust-lang#91062 (rustdoc: Consolidate static-file replacement mechanism) - rust-lang#91208 (Account for incorrect `where T::Assoc = Ty` bound) - rust-lang#91266 (Use non-generic inner function for pointer formatting) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup
Pkgsrc changes: * Bump available bootstraps to 1.58.1. * Adjust one patch (and checksum) so that it still applies. Upstream changes: Version 1.59.0 (2022-02-24) ========================== Language -------- - [Stabilize default arguments for const generics][90207] - [Stabilize destructuring assignment][90521] - [Relax private in public lint on generic bounds and where clauses of trait impls][90586] - [Stabilize asm! and global_asm! for x86, x86_64, ARM, Aarch64, and RISC-V][91728] Compiler -------- - [Stabilize new symbol mangling format, leaving it opt-in (-Csymbol-mangling-version=v0)][90128] - [Emit LLVM optimization remarks when enabled with `-Cremark`][90833] - [Fix sparc64 ABI for aggregates with floating point members][91003] - [Warn when a `#[test]`-like built-in attribute macro is present multiple times.][91172] - [Add support for riscv64gc-unknown-freebsd][91284] - [Stabilize `-Z emit-future-incompat` as `--json future-incompat`][91535] - [Soft disable incremental compilation][94124] This release disables incremental compilation, unless the user has explicitly opted in via the newly added RUSTC_FORCE_INCREMENTAL=1 environment variable. This is due to a known and relatively frequently occurring bug in incremental compilation, which causes builds to issue internal compiler errors. This particular bug is already fixed on nightly, but that fix has not yet rolled out to stable and is deemed too risky for a direct stable backport. As always, we encourage users to test with nightly and report bugs so that we can track failures and fix issues earlier. See [94124] for more details. [94124]: rust-lang/rust#94124 Libraries --------- - [Remove unnecessary bounds for some Hash{Map,Set} methods][91593] Stabilized APIs --------------- - [`std::thread::available_parallelism`][available_parallelism] - [`Result::copied`][result-copied] - [`Result::cloned`][result-cloned] - [`arch::asm!`][asm] - [`arch::global_asm!`][global_asm] - [`ops::ControlFlow::is_break`][is_break] - [`ops::ControlFlow::is_continue`][is_continue] - [`TryFrom<char> for u8`][try_from_char_u8] - [`char::TryFromCharError`][try_from_char_err] implementing `Clone`, `Debug`, `Display`, `PartialEq`, `Copy`, `Eq`, `Error` - [`iter::zip`][zip] - [`NonZeroU8::is_power_of_two`][is_power_of_two8] - [`NonZeroU16::is_power_of_two`][is_power_of_two16] - [`NonZeroU32::is_power_of_two`][is_power_of_two32] - [`NonZeroU64::is_power_of_two`][is_power_of_two64] - [`NonZeroU128::is_power_of_two`][is_power_of_two128] - [`DoubleEndedIterator for ToLowercase`][lowercase] - [`DoubleEndedIterator for ToUppercase`][uppercase] - [`TryFrom<&mut [T]> for [T; N]`][tryfrom_ref_arr] - [`UnwindSafe for Once`][unwindsafe_once] - [`RefUnwindSafe for Once`][refunwindsafe_once] - [armv8 neon intrinsics for aarch64][stdarch/1266] Const-stable: - [`mem::MaybeUninit::as_ptr`][muninit_ptr] - [`mem::MaybeUninit::assume_init`][muninit_init] - [`mem::MaybeUninit::assume_init_ref`][muninit_init_ref] - [`ffi::CStr::from_bytes_with_nul_unchecked`][cstr_from_bytes] Cargo ----- - [Stabilize the `strip` profile option][cargo/10088] - [Stabilize future-incompat-report][cargo/10165] - [Support abbreviating `--release` as `-r`][cargo/10133] - [Support `term.quiet` configuration][cargo/10152] - [Remove `--host` from cargo {publish,search,login}][cargo/10145] Compatibility Notes ------------------- - [Refactor weak symbols in std::sys::unix][90846] This may add new, versioned, symbols when building with a newer glibc, as the standard library uses weak linkage rather than dynamically attempting to load certain symbols at runtime. - [Deprecate crate_type and crate_name nested inside `#![cfg_attr]`][83744] This adds a future compatibility lint to supporting the use of cfg_attr wrapping either crate_type or crate_name specification within Rust files; it is recommended that users migrate to setting the equivalent command line flags. - [Remove effect of `#[no_link]` attribute on name resolution][92034] This may expose new names, leading to conflicts with preexisting names in a given namespace and a compilation failure. - [Cargo will document libraries before binaries.][cargo/10172] - [Respect doc=false in dependencies, not just the root crate][cargo/10201] - [Weaken guarantee around advancing underlying iterators in zip][83791] - [Make split_inclusive() on an empty slice yield an empty output][89825] - [Update std::env::temp_dir to use GetTempPath2 on Windows when available.][89999] - [unreachable! was updated to match other formatting macro behavior on Rust 2021][92137] Internal Changes ---------------- These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools. - [Fix many cases of normalization-related ICEs][91255] - [Replace dominators algorithm with simple Lengauer-Tarjan][85013] - [Store liveness in interval sets for region inference][90637] - [Remove `in_band_lifetimes` from the compiler and standard library, in preparation for removing this unstable feature.][91867] [91867]: rust-lang/rust#91867 [83744]: rust-lang/rust#83744 [83791]: rust-lang/rust#83791 [85013]: rust-lang/rust#85013 [89825]: rust-lang/rust#89825 [89999]: rust-lang/rust#89999 [90128]: rust-lang/rust#90128 [90207]: rust-lang/rust#90207 [90521]: rust-lang/rust#90521 [90586]: rust-lang/rust#90586 [90637]: rust-lang/rust#90637 [90833]: rust-lang/rust#90833 [90846]: rust-lang/rust#90846 [91003]: rust-lang/rust#91003 [91172]: rust-lang/rust#91172 [91255]: rust-lang/rust#91255 [91284]: rust-lang/rust#91284 [91535]: rust-lang/rust#91535 [91593]: rust-lang/rust#91593 [91728]: rust-lang/rust#91728 [91878]: rust-lang/rust#91878 [91896]: rust-lang/rust#91896 [91926]: rust-lang/rust#91926 [91984]: rust-lang/rust#91984 [92020]: rust-lang/rust#92020 [92034]: rust-lang/rust#92034 [92483]: rust-lang/rust#92483 [cargo/10088]: rust-lang/cargo#10088 [cargo/10133]: rust-lang/cargo#10133 [cargo/10145]: rust-lang/cargo#10145 [cargo/10152]: rust-lang/cargo#10152 [cargo/10165]: rust-lang/cargo#10165 [cargo/10172]: rust-lang/cargo#10172 [cargo/10201]: rust-lang/cargo#10201 [cargo/10269]: rust-lang/cargo#10269 [cstr_from_bytes]: https://doc.rust-lang.org/stable/std/ffi/struct.CStr.html#method.from_bytes_with_nul_unchecked [muninit_ptr]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.as_ptr [muninit_init]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.assume_init [muninit_init_ref]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.assume_init_ref [unwindsafe_once]: https://doc.rust-lang.org/stable/std/sync/struct.Once.html#impl-UnwindSafe [refunwindsafe_once]: https://doc.rust-lang.org/stable/std/sync/struct.Once.html#impl-RefUnwindSafe [tryfrom_ref_arr]: https://doc.rust-lang.org/stable/std/convert/trait.TryFrom.html#impl-TryFrom%3C%26%27_%20mut%20%5BT%5D%3E [lowercase]: https://doc.rust-lang.org/stable/std/char/struct.ToLowercase.html#impl-DoubleEndedIterator [uppercase]: https://doc.rust-lang.org/stable/std/char/struct.ToUppercase.html#impl-DoubleEndedIterator [try_from_char_err]: https://doc.rust-lang.org/stable/std/char/struct.TryFromCharError.html [available_parallelism]: https://doc.rust-lang.org/stable/std/thread/fn.available_parallelism.html [result-copied]: https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.copied [result-cloned]: https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.cloned [asm]: https://doc.rust-lang.org/stable/core/arch/macro.asm.html [global_asm]: https://doc.rust-lang.org/stable/core/arch/macro.global_asm.html [is_break]: https://doc.rust-lang.org/stable/std/ops/enum.ControlFlow.html#method.is_break [is_continue]: https://doc.rust-lang.org/stable/std/ops/enum.ControlFlow.html#method.is_continue [try_from_char_u8]: https://doc.rust-lang.org/stable/std/primitive.char.html#impl-TryFrom%3Cchar%3E [zip]: https://doc.rust-lang.org/stable/std/iter/fn.zip.html [is_power_of_two8]: https://doc.rust-lang.org/stable/core/num/struct.NonZeroU8.html#method.is_power_of_two [is_power_of_two16]: https://doc.rust-lang.org/stable/core/num/struct.NonZeroU16.html#method.is_power_of_two [is_power_of_two32]: https://doc.rust-lang.org/stable/core/num/struct.NonZeroU32.html#method.is_power_of_two [is_power_of_two64]: https://doc.rust-lang.org/stable/core/num/struct.NonZeroU64.html#method.is_power_of_two [is_power_of_two128]: https://doc.rust-lang.org/stable/core/num/struct.NonZeroU128.html#method.is_power_of_two [stdarch/1266]: rust-lang/stdarch#1266
Pkgsrc changes: * Bump available bootstraps to 1.58.1. * Adjust one patch (and checksum) so that it still applies. Upstream changes: Version 1.59.0 (2022-02-24) ========================== Language -------- - [Stabilize default arguments for const generics][90207] - [Stabilize destructuring assignment][90521] - [Relax private in public lint on generic bounds and where clauses of trait impls][90586] - [Stabilize asm! and global_asm! for x86, x86_64, ARM, Aarch64, and RISC-V][91728] Compiler -------- - [Stabilize new symbol mangling format, leaving it opt-in (-Csymbol-mangling-version=v0)][90128] - [Emit LLVM optimization remarks when enabled with `-Cremark`][90833] - [Fix sparc64 ABI for aggregates with floating point members][91003] - [Warn when a `#[test]`-like built-in attribute macro is present multiple times.][91172] - [Add support for riscv64gc-unknown-freebsd][91284] - [Stabilize `-Z emit-future-incompat` as `--json future-incompat`][91535] - [Soft disable incremental compilation][94124] This release disables incremental compilation, unless the user has explicitly opted in via the newly added RUSTC_FORCE_INCREMENTAL=1 environment variable. This is due to a known and relatively frequently occurring bug in incremental compilation, which causes builds to issue internal compiler errors. This particular bug is already fixed on nightly, but that fix has not yet rolled out to stable and is deemed too risky for a direct stable backport. As always, we encourage users to test with nightly and report bugs so that we can track failures and fix issues earlier. See [94124] for more details. [94124]: rust-lang/rust#94124 Libraries --------- - [Remove unnecessary bounds for some Hash{Map,Set} methods][91593] Stabilized APIs --------------- - [`std::thread::available_parallelism`][available_parallelism] - [`Result::copied`][result-copied] - [`Result::cloned`][result-cloned] - [`arch::asm!`][asm] - [`arch::global_asm!`][global_asm] - [`ops::ControlFlow::is_break`][is_break] - [`ops::ControlFlow::is_continue`][is_continue] - [`TryFrom<char> for u8`][try_from_char_u8] - [`char::TryFromCharError`][try_from_char_err] implementing `Clone`, `Debug`, `Display`, `PartialEq`, `Copy`, `Eq`, `Error` - [`iter::zip`][zip] - [`NonZeroU8::is_power_of_two`][is_power_of_two8] - [`NonZeroU16::is_power_of_two`][is_power_of_two16] - [`NonZeroU32::is_power_of_two`][is_power_of_two32] - [`NonZeroU64::is_power_of_two`][is_power_of_two64] - [`NonZeroU128::is_power_of_two`][is_power_of_two128] - [`DoubleEndedIterator for ToLowercase`][lowercase] - [`DoubleEndedIterator for ToUppercase`][uppercase] - [`TryFrom<&mut [T]> for [T; N]`][tryfrom_ref_arr] - [`UnwindSafe for Once`][unwindsafe_once] - [`RefUnwindSafe for Once`][refunwindsafe_once] - [armv8 neon intrinsics for aarch64][stdarch/1266] Const-stable: - [`mem::MaybeUninit::as_ptr`][muninit_ptr] - [`mem::MaybeUninit::assume_init`][muninit_init] - [`mem::MaybeUninit::assume_init_ref`][muninit_init_ref] - [`ffi::CStr::from_bytes_with_nul_unchecked`][cstr_from_bytes] Cargo ----- - [Stabilize the `strip` profile option][cargo/10088] - [Stabilize future-incompat-report][cargo/10165] - [Support abbreviating `--release` as `-r`][cargo/10133] - [Support `term.quiet` configuration][cargo/10152] - [Remove `--host` from cargo {publish,search,login}][cargo/10145] Compatibility Notes ------------------- - [Refactor weak symbols in std::sys::unix][90846] This may add new, versioned, symbols when building with a newer glibc, as the standard library uses weak linkage rather than dynamically attempting to load certain symbols at runtime. - [Deprecate crate_type and crate_name nested inside `#![cfg_attr]`][83744] This adds a future compatibility lint to supporting the use of cfg_attr wrapping either crate_type or crate_name specification within Rust files; it is recommended that users migrate to setting the equivalent command line flags. - [Remove effect of `#[no_link]` attribute on name resolution][92034] This may expose new names, leading to conflicts with preexisting names in a given namespace and a compilation failure. - [Cargo will document libraries before binaries.][cargo/10172] - [Respect doc=false in dependencies, not just the root crate][cargo/10201] - [Weaken guarantee around advancing underlying iterators in zip][83791] - [Make split_inclusive() on an empty slice yield an empty output][89825] - [Update std::env::temp_dir to use GetTempPath2 on Windows when available.][89999] - [unreachable! was updated to match other formatting macro behavior on Rust 2021][92137] Internal Changes ---------------- These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools. - [Fix many cases of normalization-related ICEs][91255] - [Replace dominators algorithm with simple Lengauer-Tarjan][85013] - [Store liveness in interval sets for region inference][90637] - [Remove `in_band_lifetimes` from the compiler and standard library, in preparation for removing this unstable feature.][91867] [91867]: rust-lang/rust#91867 [83744]: rust-lang/rust#83744 [83791]: rust-lang/rust#83791 [85013]: rust-lang/rust#85013 [89825]: rust-lang/rust#89825 [89999]: rust-lang/rust#89999 [90128]: rust-lang/rust#90128 [90207]: rust-lang/rust#90207 [90521]: rust-lang/rust#90521 [90586]: rust-lang/rust#90586 [90637]: rust-lang/rust#90637 [90833]: rust-lang/rust#90833 [90846]: rust-lang/rust#90846 [91003]: rust-lang/rust#91003 [91172]: rust-lang/rust#91172 [91255]: rust-lang/rust#91255 [91284]: rust-lang/rust#91284 [91535]: rust-lang/rust#91535 [91593]: rust-lang/rust#91593 [91728]: rust-lang/rust#91728 [91878]: rust-lang/rust#91878 [91896]: rust-lang/rust#91896 [91926]: rust-lang/rust#91926 [91984]: rust-lang/rust#91984 [92020]: rust-lang/rust#92020 [92034]: rust-lang/rust#92034 [92483]: rust-lang/rust#92483 [cargo/10088]: rust-lang/cargo#10088 [cargo/10133]: rust-lang/cargo#10133 [cargo/10145]: rust-lang/cargo#10145 [cargo/10152]: rust-lang/cargo#10152 [cargo/10165]: rust-lang/cargo#10165 [cargo/10172]: rust-lang/cargo#10172 [cargo/10201]: rust-lang/cargo#10201 [cargo/10269]: rust-lang/cargo#10269 [cstr_from_bytes]: https://doc.rust-lang.org/stable/std/ffi/struct.CStr.html#method.from_bytes_with_nul_unchecked [muninit_ptr]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.as_ptr [muninit_init]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.assume_init [muninit_init_ref]: https://doc.rust-lang.org/stable/std/mem/union.MaybeUninit.html#method.assume_init_ref [unwindsafe_once]: https://doc.rust-lang.org/stable/std/sync/struct.Once.html#impl-UnwindSafe [refunwindsafe_once]: https://doc.rust-lang.org/stable/std/sync/struct.Once.html#impl-RefUnwindSafe [tryfrom_ref_arr]: https://doc.rust-lang.org/stable/std/convert/trait.TryFrom.html#impl-TryFrom%3C%26%27_%20mut%20%5BT%5D%3E [lowercase]: https://doc.rust-lang.org/stable/std/char/struct.ToLowercase.html#impl-DoubleEndedIterator [uppercase]: https://doc.rust-lang.org/stable/std/char/struct.ToUppercase.html#impl-DoubleEndedIterator [try_from_char_err]: https://doc.rust-lang.org/stable/std/char/struct.TryFromCharError.html [available_parallelism]: https://doc.rust-lang.org/stable/std/thread/fn.available_parallelism.html [result-copied]: https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.copied [result-cloned]: https://doc.rust-lang.org/stable/std/result/enum.Result.html#method.cloned [asm]: https://doc.rust-lang.org/stable/core/arch/macro.asm.html [global_asm]: https://doc.rust-lang.org/stable/core/arch/macro.global_asm.html [is_break]: https://doc.rust-lang.org/stable/std/ops/enum.ControlFlow.html#method.is_break [is_continue]: https://doc.rust-lang.org/stable/std/ops/enum.ControlFlow.html#method.is_continue [try_from_char_u8]: https://doc.rust-lang.org/stable/std/primitive.char.html#impl-TryFrom%3Cchar%3E [zip]: https://doc.rust-lang.org/stable/std/iter/fn.zip.html [is_power_of_two8]: https://doc.rust-lang.org/stable/core/num/struct.NonZeroU8.html#method.is_power_of_two [is_power_of_two16]: https://doc.rust-lang.org/stable/core/num/struct.NonZeroU16.html#method.is_power_of_two [is_power_of_two32]: https://doc.rust-lang.org/stable/core/num/struct.NonZeroU32.html#method.is_power_of_two [is_power_of_two64]: https://doc.rust-lang.org/stable/core/num/struct.NonZeroU64.html#method.is_power_of_two [is_power_of_two128]: https://doc.rust-lang.org/stable/core/num/struct.NonZeroU128.html#method.is_power_of_two [stdarch/1266]: rust-lang/stdarch#1266
The current guarantee (introduced in #52279) is too strong as it prevents adapters from exploiting knowledge about the iterator length and using counted loops for example because they would stop calling
next()
before it ever returnedNone
. Additionally several nested zip iterators already fail to uphold this.This does not yet remove any of the specialization code that tries (and sometimes fails) to uphold the guarantee for
next()
because removing it would also affect
next_back()
in more surprising ways.The intent is to be able to remove for example this branch
rust/library/core/src/iter/adapters/zip.rs
Lines 234 to 243 in 36bcf40
or this test
rust/library/core/tests/iter/adapters/zip.rs
Lines 177 to 188 in 36bcf40
Solves #82303 by declaring it a non-issue.