KT-4519 Show first line with content of block comment when collapsed (#928)

idea/src/org/jetbrains/kotlin/idea/KotlinFoldingBuilder.kt

@@ -97,12 +97,28 @@ class KotlinFoldingBuilder : CustomFoldingBuilder(), DumbAware {
     }
 
     override fun getLanguagePlaceholderText(node: ASTNode, range: TextRange): String = when {
-        node.elementType == KtTokens.BLOCK_COMMENT -> "/.../"
-        node.elementType == KDocTokens.KDOC -> "/**...*/"
+        node.elementType == KtTokens.BLOCK_COMMENT -> "/${getFirstLineOfComment(node)}.../"
+        node.elementType == KDocTokens.KDOC -> "/**${getFirstLineOfComment(node)}...*/"
         node.psi is KtImportList -> "..."
         else ->  "{...}"
     }
 
+    private fun getFirstLineOfComment(node: ASTNode): String {
+        val targetCommentLine = node.text.split("\n").firstOrNull {
+            getCommentContents(it).isNotEmpty()
+        } ?: return ""
+        return " ${getCommentContents(targetCommentLine)} "
+    }
+
+    private fun getCommentContents(line: String): String {
+        return line.trim()
+                .replace("/**", "")
+                .replace("/*", "")
+                .replace("*/", "")
+                .replace("*", "")
+                .trim()
+    }
+
     override fun isRegionCollapsedByDefault(node: ASTNode): Boolean {
         val settings = JavaCodeFoldingSettings.getInstance()
 

idea/testData/folding/checkCollapse/headerKDoc.kt

@@ -1,9 +1,9 @@
-<fold text='/**...*/' expand='~false~'>/**
+<fold text='/** Some header kdoc ...*/' expand='~false~'>/**
  * Some header kdoc
  */</fold>
 package some
 
-<fold text='/**...*/' expand='true'>/**
+<fold text='/** Other ...*/' expand='true'>/**
  * Other
  */</fold>
 

idea/testData/folding/checkCollapse/headerMultilineComment.kt

@@ -1,10 +1,10 @@
-<fold text='/.../' expand='~false~'>/*
+<fold text='/ Some header .../' expand='~false~'>/*
  * Some header
  */</fold>
 
 val a = 12
 
-<fold text='/.../' expand='true'>/*
+<fold text='/ Other .../' expand='true'>/*
  * Other
  */</fold>
 

idea/testData/folding/noCollapse/kdocComments.kt

@@ -1,9 +1,9 @@
-<fold text='/**...*/'>/**
+<fold text='/** Kdoc licence ...*/'>/**
  * Kdoc licence
  */</fold>
 package some
 
-<fold text='/**...*/'>/**
+<fold text='/** Some Kdoc comment ...*/'>/**
  * Some Kdoc comment
  */</fold>
 class A

idea/testData/folding/noCollapse/multilineComments.kt

@@ -1,4 +1,5 @@
-<fold text='/.../'>/*
+<fold text='/ Some multiline comment .../'>/*
  * Some multiline comment
+ * And more comment
  */</fold>
 class A