perf: skip builder allocation for empty source in HashMap.from#26280
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perf: skip builder allocation for empty source in HashMap.from#26280He-Pin wants to merge 1 commit into
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Motivation: HashMap.from did not check source.knownSize == 0 before allocating a builder, unlike HashSet.from which has this optimization. For empty sources, this unnecessarily allocates a HashMapBuilder, calls ensureUnaliased, and then returns HashMap.empty. Modification: Add case _ if source.knownSize == 0 => empty[K, V] to HashMap.from, matching the pattern in HashSet.from. Result: HashMap.from now returns the singleton empty map directly for empty sources, avoiding unnecessary builder allocation.
SolalPirelli
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Jun 9, 2026
SolalPirelli
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SolalPirelli
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Given that this matches HashSet better I think we should take it https://github.com/scala/scala3/blob/main/library/src/scala/collection/immutable/HashSet.scala#L1963
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Description
Skip builder allocation for empty source in HashMap.from.
Problem
HashMap.from did not check source.knownSize == 0 before allocating a builder, unlike HashSet.from which has this optimization. For empty sources, this unnecessarily allocates a HashMapBuilder, calls ensureUnaliased, and then returns HashMap.empty.
Solution
Add case _ if source.knownSize == 0 => empty[K, V] to HashMap.from, matching the pattern in HashSet.from.
Result
HashMap.from now returns the singleton empty map directly for empty sources, avoiding unnecessary builder allocation.
LLM Policy
LLM-based tools were used extensively in this contribution. The code was reviewed and tested locally before submission.