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[stdlib] Do not allocate when creating a Set from a generic Sequence which happens to be another Set #62158

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Merged
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Dec 1, 2022
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[stdlib] Do not allocate when creating a Set from a generic Sequence which happens to be another Set #62158

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9 changes: 4 additions & 5 deletions stdlib/public/core/Set.swift
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Original file line number Diff line number Diff line change
Expand Up @@ -671,13 +671,12 @@ extension Set: SetAlgebra {
@inlinable
public init<Source: Sequence>(_ sequence: __owned Source)
where Source.Element == Element {
self.init(minimumCapacity: sequence.underestimatedCount)
if let s = sequence as? Set<Element> {
// If this sequence is actually a native `Set`, then we can quickly
// adopt its native buffer and let COW handle uniquing only
// if necessary.
self._variant = s._variant
// If this sequence is actually a `Set`, then we can quickly
// adopt its storage and let COW handle uniquing only if necessary.
self = s
} else {
self.init(minimumCapacity: sequence.underestimatedCount)
for item in sequence {
insert(item)
}
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