Stop hashing a usize when hashing [u8; 1]
#86131
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Jun 8, 2021
| /// | ||
| /// For example, the hash of the array `[x, y, z]` may or may not be the same as | ||
| /// the hash of the tuple `(x, y, z)`, and the hash of the array `[x, y, z]` may | ||
| /// or may not be the same as the hash of the vector `vec![x, y, z]`. |
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This may be in conflict with requirements of the Borrow trait which is implemented for arrays and slices.
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Here's a playground. Shows the problem and a workaround.
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🤦 Thanks, @petrochenkov. I'd checked That's definitely fatal to this change. I'll make a new PR so there's actually a test failure if the implementation is changed. Edit: New PR up at #86140 |
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This changes
Hashfor arrays to be implemented viaHash::hash_sliceinstead of via<[T]>::hash. The latter calls the former, but only after also hashing the length, which is unnecessary overhead for arrays where it's always the same.I was inspired to make this PR because I changed an old project from having a
HashMap<(usize, usize), _>to having aHashMap<[usize; 2], _>, and was surprised to see the perf worsen from 3.62s to 4.30s.(Yes, that project really is dominated by hash table lookups that badly.)