[LL] Use soft references in FileStructureCache

- We can also soft-reference the `KtFile` key. Chances are, if the
  `KtFile` can be garbage-collected, we do not need a `FileStructure`
  instance for it either.

^KT-65978 fixed

analysis/low-level-api-fir/src/org/jetbrains/kotlin/analysis/low/level/api/fir/file/structure/FileStructureCache.kt

@@ -5,19 +5,26 @@
 
 package org.jetbrains.kotlin.analysis.low.level.api.fir.file.structure
 
+import com.intellij.util.containers.ContainerUtil
 import org.jetbrains.kotlin.analysis.low.level.api.fir.LLFirModuleResolveComponents
 import org.jetbrains.kotlin.psi.KtFile
-import java.util.concurrent.ConcurrentHashMap
+import java.util.concurrent.ConcurrentMap
 
 /**
- * Belongs to a [org.jetbrains.kotlin.analysis.low.level.api.fir.api.LLFirResolveSession]
+ * Caches [FileStructure] instances for an [LLFirResolveSession][org.jetbrains.kotlin.analysis.low.level.api.fir.api.LLFirResolveSession].
  */
 internal class FileStructureCache(private val moduleResolveComponents: LLFirModuleResolveComponents) {
-    private val cache = ConcurrentHashMap<KtFile, FileStructure>()
+    /**
+     * File structure elements can be rebuilt at any time and do not need to be unique (like FIR symbols), so they can be soft-referenced
+     * from this cache to reduce memory consumption. Any `analyze` call in a file causes file structure elements to be built, so during
+     * operations which cause a lot of files to be analyzed (such as Find Usages), a session might accumulate a lot of file structure
+     * elements.
+     */
+    private val cache: ConcurrentMap<KtFile, FileStructure> = ContainerUtil.createConcurrentSoftKeySoftValueMap()
 
     fun getFileStructure(ktFile: KtFile): FileStructure = cache.computeIfAbsent(ktFile) {
         FileStructure.build(ktFile, moduleResolveComponents)
     }
 
     fun getCachedFileStructure(ktFile: KtFile): FileStructure? = cache[ktFile]
-}
+}