chore: Optimize Source.from iterable.

[He-Pin]

Optimize it when the size is little.

[He-Pin]

fixed: #2558

[mdedetrich]

lgtm

[Copilot]

Pull request overview

This PR optimizes Source.from methods in both Scala and Java APIs to use more efficient specialized sources (Source.empty and Source.single) when the input iterable is known to be small (0 or 1 elements), avoiding the overhead of creating an IterableSource for these trivial cases.

• Scala implementation uses pattern matching on knownSize to detect empty and single-element iterables
• Java implementation uses Collection type checks with isEmpty() and size() == 1 conditions
• Test adjustments ensure materialization semantics are preserved in edge case scenarios

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
stream/src/main/scala/org/apache/pekko/stream/scaladsl/Source.scala	Adds optimization to Source.apply to use empty or single sources when iterable has 0 or 1 elements based on knownSize
stream/src/main/scala/org/apache/pekko/stream/javadsl/Source.scala	Adds optimization to Source.from to use empty() or single() when Java Collection has 0 or 1 elements
stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/SourceSpec.scala	Adds test case covering the optimized paths for empty, single, and multiple element sources
stream-tests/src/test/scala/org/apache/pekko/stream/scaladsl/FlowFlatMapPrefixSpec.scala	Wraps empty list sources with lazySource to preserve materialization timing semantics in edge case tests
stream-tests/src/test/java/org/apache/pekko/stream/javadsl/SourceTest.java	Adds test case for Java Source.from covering empty, single, and multiple element iterables

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[He-Pin]

The current code will miss the x :: Nil case, so I upstream the changes to scala
scala/scala#11188

[mdedetrich]

The current code will miss the x :: Nil case, so I upstream the changes to scala scala/scala#11188

You will also have to do Scala 2.12 and also Scala 3 (aka dotty)

[He-Pin]

@mdedetrich No such method in Scala 2.12:) and Scala 3 will sync the stdlib from 2.13.x