string_extract_if: initial implementation - #154583

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Tracking issue: #154318

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GrigorenkoPV marked this pull request as ready for review March 30, 2026 22:55
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Comment threadlibrary/alloc/src/string/extract_if.rs
/// During the iteration, the underlying vector's consists of:
/// - A valid UTF-8 prefix (`valid_prefix.len()` bytes)
/// of characters that we iterated over and didn't extract.
/// - A middle portion of `bytes_removed` initialized bytes that might not be valid UTF-8.

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Since we only ever remove a full char, how can we break the UTF-8 property? AFAICT, the Vec should always be initialized and a valid String. The removal operations this performs should correspond directly to calling String::drain(start..end) after a sequence of false returns. That is a tiny bit less efficient since it'll re-check the start/end characters are at a UTF-8 boundary, but that check is O(1) so it shouldn't be that much slower.

I think the current implementation doesn't actually optimize to copy larger chunks only when needed, it looks like we copy each char (likely via call to memmove) which seems likely to be pretty inefficient?

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☔ The latest upstream changes (presumably #159113) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

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Comment threadlibrary/alloc/src/string/extract_if.rs
 /// During the iteration, the underlying vector's consists of:
 /// - A valid UTF-8 prefix (`valid_prefix.len()` bytes)
 /// of characters that we iterated over and didn't extract.
 /// - A middle portion of `bytes_removed` initialized bytes that might not be valid UTF-8.
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Since we only ever remove a full char, how can we break the UTF-8 property? AFAICT, the Vec should always be initialized and a valid String. The removal operations this performs should correspond directly to calling String::drain(start..end) after a sequence of false returns. That is a tiny bit less efficient since it'll re-check the start/end characters are at a UTF-8 boundary, but that check is O(1) so it shouldn't be that much slower.
I think the current implementation doesn't actually optimize to copy larger chunks only when needed, it looks like we copy each char (likely via call to memmove) which seems likely to be pretty inefficient?
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Comment threadlibrary/alloc/src/string/extract_if.rs
 /// During the iteration, the underlying vector's consists of:
 /// - A valid UTF-8 prefix (`valid_prefix.len()` bytes)
 /// of characters that we iterated over and didn't extract.
 /// - A middle portion of `bytes_removed` initialized bytes that might not be valid UTF-8.
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Since we only ever remove a full char, how can we break the UTF-8 property? AFAICT, the Vec should always be initialized and a valid String. The removal operations this performs should correspond directly to calling String::drain(start..end) after a sequence of false returns. That is a tiny bit less efficient since it'll re-check the start/end characters are at a UTF-8 boundary, but that check is O(1) so it shouldn't be that much slower.
I think the current implementation doesn't actually optimize to copy larger chunks only when needed, it looks like we copy each char (likely via call to memmove) which seems likely to be pretty inefficient?
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Comment threadlibrary/alloc/src/string/extract_if.rs
 /// During the iteration, the underlying vector's consists of:
 /// - A valid UTF-8 prefix (`valid_prefix.len()` bytes)
 /// of characters that we iterated over and didn't extract.
 /// - A middle portion of `bytes_removed` initialized bytes that might not be valid UTF-8.
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Since we only ever remove a full char, how can we break the UTF-8 property? AFAICT, the Vec should always be initialized and a valid String. The removal operations this performs should correspond directly to calling String::drain(start..end) after a sequence of false returns. That is a tiny bit less efficient since it'll re-check the start/end characters are at a UTF-8 boundary, but that check is O(1) so it shouldn't be that much slower.
I think the current implementation doesn't actually optimize to copy larger chunks only when needed, it looks like we copy each char (likely via call to memmove) which seems likely to be pretty inefficient?
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Comment threadlibrary/alloc/src/string/extract_if.rs
 /// During the iteration, the underlying vector's consists of:
 /// - A valid UTF-8 prefix (`valid_prefix.len()` bytes)
 /// of characters that we iterated over and didn't extract.
 /// - A middle portion of `bytes_removed` initialized bytes that might not be valid UTF-8.
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Since we only ever remove a full char, how can we break the UTF-8 property? AFAICT, the Vec should always be initialized and a valid String. The removal operations this performs should correspond directly to calling String::drain(start..end) after a sequence of false returns. That is a tiny bit less efficient since it'll re-check the start/end characters are at a UTF-8 boundary, but that check is O(1) so it shouldn't be that much slower.
I think the current implementation doesn't actually optimize to copy larger chunks only when needed, it looks like we copy each char (likely via call to memmove) which seems likely to be pretty inefficient?
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Comment threadlibrary/alloc/src/string/extract_if.rs
 /// During the iteration, the underlying vector's consists of:
 /// - A valid UTF-8 prefix (`valid_prefix.len()` bytes)
 /// of characters that we iterated over and didn't extract.
 /// - A middle portion of `bytes_removed` initialized bytes that might not be valid UTF-8.
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Since we only ever remove a full char, how can we break the UTF-8 property? AFAICT, the Vec should always be initialized and a valid String. The removal operations this performs should correspond directly to calling String::drain(start..end) after a sequence of false returns. That is a tiny bit less efficient since it'll re-check the start/end characters are at a UTF-8 boundary, but that check is O(1) so it shouldn't be that much slower.
I think the current implementation doesn't actually optimize to copy larger chunks only when needed, it looks like we copy each char (likely via call to memmove) which seems likely to be pretty inefficient?
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Comment threadlibrary/alloc/src/string/extract_if.rs
 /// During the iteration, the underlying vector's consists of:
 /// - A valid UTF-8 prefix (`valid_prefix.len()` bytes)
 /// of characters that we iterated over and didn't extract.
 /// - A middle portion of `bytes_removed` initialized bytes that might not be valid UTF-8.
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Since we only ever remove a full char, how can we break the UTF-8 property? AFAICT, the Vec should always be initialized and a valid String. The removal operations this performs should correspond directly to calling String::drain(start..end) after a sequence of false returns. That is a tiny bit less efficient since it'll re-check the start/end characters are at a UTF-8 boundary, but that check is O(1) so it shouldn't be that much slower.
I think the current implementation doesn't actually optimize to copy larger chunks only when needed, it looks like we copy each char (likely via call to memmove) which seems likely to be pretty inefficient?
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Comment threadlibrary/alloc/src/string/extract_if.rs
 /// During the iteration, the underlying vector's consists of:
 /// - A valid UTF-8 prefix (`valid_prefix.len()` bytes)
 /// of characters that we iterated over and didn't extract.
 /// - A middle portion of `bytes_removed` initialized bytes that might not be valid UTF-8.
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Since we only ever remove a full char, how can we break the UTF-8 property? AFAICT, the Vec should always be initialized and a valid String. The removal operations this performs should correspond directly to calling String::drain(start..end) after a sequence of false returns. That is a tiny bit less efficient since it'll re-check the start/end characters are at a UTF-8 boundary, but that check is O(1) so it shouldn't be that much slower.
I think the current implementation doesn't actually optimize to copy larger chunks only when needed, it looks like we copy each char (likely via call to memmove) which seems likely to be pretty inefficient?
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