Drop non-existent classpath entries before parsing - #468

Raised in review: doesn't the test create the file that's now being filtered out? (RewriteRunTest.kt#L1027-L1084)

It doesn't. Two separate things are handed to KotlinParser, and the filter only touches one of them:

• Source paths — kotlinPaths, passed to kp.parse(kotlinPaths, baseDir, ctx). src/main/kotlin/com/foo/A.kt is here. The filter is not on this stream.
• Classpath roots — dependencyPaths, built from sourceSet.getRuntimeClasspath() + getCompileClasspath(). The filter is only here.

To confirm rather than argue it, I instrumented the predicate to log every entry it keeps and drops, and ran kotlinSourceGradle9:

DROPPED  <PROJECT>/build/classes/java/main
DROPPED  <PROJECT>/build/classes/java/test
DROPPED  <PROJECT>/build/classes/kotlin/test
DROPPED  <PROJECT>/build/resources/main
DROPPED  <PROJECT>/build/resources/test
KEPT     <PROJECT>/build/classes/kotlin/main
KEPT     .../org.jetbrains.kotlin/kotlin-stdlib/2.2.0/kotlin-stdlib-2.2.0.jar
KEPT     .../org.jetbrains/annotations/13.0/annotations-13.0.jar

A.kt's compiled output, build/classes/kotlin/main, is on the classpath and is kept — it exists because compileKotlin ran. What gets dropped is the five output directories that were never created because their compile tasks were NO-SOURCE: the test project has no Java sources, no resources, and no test source set. Those are the roots KotlinCoreEnvironment.findExistingRoot chokes on.

Files::exists is evaluated when the stream is consumed inside rewriteRun, i.e. after the compile tasks have run, so anything that exists by then survives.

One thing worth calling out explicitly: the dropped test entries show this loop runs over every source set, so the filter applies more broadly than the single failing scenario. I think that's still sound — a path that doesn't exist on disk contributes no type information to any parser — but it is a wider blast radius than "fix the one broken test", so it's worth a second opinion.