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Sign upFix `extend` from assuming a fused iterator. #150
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This looks reasonable, but can you elaborate on the commit message / PR description how exactly this fixes the rustc issue? |
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I updated the comment, but the exact details are quite complex (and dealing with parts of librustc I know little about). |
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Sure, looks good, thanks! |
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@bors-servo r+ |
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Fix `extend` from assuming a fused iterator. Iterators may resume after returning None. I think it is generally expected that `extend` should stop on the first None. Fixes #147. Specifically, in rustc, there are some situations where an iterator of Results are collected into a `Result<SmallVec, _>` (such as [here](https://github.com/rust-lang/rust/blob/c1c60d292e2dd2deff7084208274f9a02f750d43/src/librustc/ty/relate.rs#L139-L142) which ends up in [this call to `collect`](https://github.com/rust-lang/rust/blob/c1c60d292e2dd2deff7084208274f9a02f750d43/src/librustc/ty/context.rs#L3002)). The [Result adapter](https://github.com/rust-lang/rust/blob/c1c60d292e2dd2deff7084208274f9a02f750d43/src/libcore/result.rs#L1245-L1258) returns None for the first `Err` in the sequence. However, it is possible for an iterator to have additional elements after the first Err. With this bug, SmallVec was falling through to the slow-path `for` loop, and resuming the iterator grabbing too many elements. I believe this was having some bad interactions with the type interner where the additional elements after the `Err` were getting processed (via a `map()`) and interned when they shouldn't be (or otherwise having some side effects from the `map`). <!-- Reviewable:start --> --- This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/rust-smallvec/150) <!-- Reviewable:end -->
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Publish 0.6.10. This incorporates #144, #152, #150, and #151, which should all be minor version updates according to semver. <!-- Reviewable:start --> --- This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/rust-smallvec/153) <!-- Reviewable:end -->
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ehuss commentedJun 6, 2019
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edited
Iterators may resume after returning None. I think it is generally expected that
extendshould stop on the first None.Fixes #147. Specifically, in rustc, there are some situations where an iterator of Results are collected into a
Result<SmallVec, _>(such as here which ends up in this call tocollect). The Result adapter returns None for the firstErrin the sequence. However, it is possible for an iterator to have additional elements after the first Err. With this bug, SmallVec was falling through to the slow-pathforloop, and resuming the iterator grabbing too many elements. I believe this was having some bad interactions with the type interner where the additional elements after theErrwere getting processed (via amap()) and interned when they shouldn't be (or otherwise having some side effects from themap).This change is