std: Add Vec::from_iter comment #22394
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r? @gankro (rust_highfive has picked a reviewer for you, use r? to override) |
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@bors: r+ 42053 rollup y'all |
| // optimization below (eliding bound/growth checks) means that we | ||
| // actually run the iterator twice. To ensure the "moral equivalent" we | ||
| // do a `fuse()` operation to ensure that the iterator continues to | ||
| // return `None` after seeing the first `None`. | ||
| let mut i = iterator.fuse(); |
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FWIW, I'm not super keen on this fuse approach. In general it adds a check to every next call, to protect against a case that is basically "unspecified behaviour" anyway (the iterator being shorter than its stated size_hint).
I'd prefer adding a check after the loop:
// the iterator ran out of elements before filling the whole vector, so we're done.
if vector.len() < vector.capacity() { return vector }since that is guaranteed to be only a single check and outside the inner loop, even if the optimiser can't remove it.
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Good idea! I have updated and added an appropriate comment as well.
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… r=brson Requested by Niko in rust-lang#22200 (and is good to have anyway)
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… r=brson Requested by Niko in rust-lang#22200 (and is good to have anyway)
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It appears this landed in a rollup but then wasn't auto-closed. re-r? @huonw |
| let mut i = iterator.fuse(); | ||
| for element in i.by_ref().take(vector.capacity()) { | ||
| // This equivalent crucially runs the iterator precisely once. Below we | ||
| // actually in theory run the iteartor twice (one without bounds checks |
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r=me with the speeling fix. |
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Requested by Niko in rust-lang#22200 (and is good to have anyway)
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Requested by Niko in rust-lang#22200 (and is good to have anyway)
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Requested by Niko in #22200 (and is good to have anyway)