Completions for requests just before string (#22894)

fixes #22514

Author: NPCRUS

compiler/src/dotty/tools/dotc/interactive/Completion.scala

@@ -97,6 +97,8 @@ object Completion:
       else if sel.isGiven && sel.bound.span.contains(pos.span) then Mode.ImportOrExport
       else Mode.None // import scala.{util => u@@}
     case GenericImportOrExport(_) => Mode.ImportOrExport | Mode.Scope // import TrieMa@@
+    case untpd.InterpolatedString(_, untpd.Literal(Constants.Constant(_: String)) :: _) :: _ =>
+      Mode.Term | Mode.Scope
     case untpd.Literal(Constants.Constant(_: String)) :: _ => Mode.Term | Mode.Scope // literal completions
     case (ref: untpd.RefTree) :: _ =>
       val maybeSelectMembers = if ref.isInstanceOf[untpd.Select] then Mode.Member else Mode.Scope
@@ -171,6 +173,14 @@ object Completion:
         case (importOrExport: untpd.ImportOrExport) :: _ => Some(importOrExport)
         case _ => None
 
+  private object StringContextApplication:
+    def unapply(path: List[tpd.Tree]): Option[tpd.Apply] = 
+      path match
+        case tpd.Select(qual @ tpd.Apply(tpd.Select(tpd.Select(_, StdNames.nme.StringContext), _), _), _) :: _ =>
+          Some(qual)
+        case _ => None
+      
+
   /** Inspect `path` to determine the offset where the completion result should be inserted. */
   def completionOffset(untpdPath: List[untpd.Tree]): Int =
     untpdPath match
@@ -221,7 +231,10 @@ object Completion:
       // Ignore synthetic select from `This` because in code it was `Ident`
       // See example in dotty.tools.languageserver.CompletionTest.syntheticThis
       case tpd.Select(qual @ tpd.This(_), _) :: _ if qual.span.isSynthetic      => completer.scopeCompletions
-      case tpd.Select(qual, _) :: _               if qual.typeOpt.hasSimpleKind => completer.selectionCompletions(qual)
+      case StringContextApplication(qual) =>
+        completer.scopeCompletions ++ completer.selectionCompletions(qual) 
+      case tpd.Select(qual, _) :: _               if qual.typeOpt.hasSimpleKind => 
+        completer.selectionCompletions(qual)
       case tpd.Select(qual, _) :: _                                             => Map.empty
       case (tree: tpd.ImportOrExport) :: _                                      => completer.directMemberCompletions(tree.expr)
       case _                                                                    => completer.scopeCompletions

presentation-compiler/test/dotty/tools/pc/tests/completion/CompletionStringContextSuite.scala

@@ -0,0 +1,79 @@
+package dotty.tools.pc.tests.completion
+
+import dotty.tools.pc.base.BaseCompletionSuite
+import org.junit.Test
+import dotty.tools.dotc.core.Types.ThisType.raw
+
+class CompletionStringContextSuite extends BaseCompletionSuite:
+  @Test
+  def inScopeSymbol = check(
+    """
+    |object M:
+    | val VersionRegex = "".r
+    | VersionRe@@"1234"
+    """.stripMargin,
+    "|VersionRegex: Regex".stripMargin 
+  )
+
+  @Test
+  def workspaceSymbol = check(
+    """
+    |object M:
+    | ListBuf@@"1234" 
+    """.stripMargin,
+    """
+    |ListBuffer[A](elems: A*): ListBuffer[A] - scala.collection.mutable
+    |new ListBuffer[A]: ListBuffer[A] - scala.collection.mutable
+    |ListBuffer - scala.collection.mutable
+    |""".stripMargin 
+  )
+
+  @Test
+  def providedSymbol = check(
+    """
+    |object M:
+    | ra@@"1234"
+    """.stripMargin,
+    "|raw(args: Any*): String".stripMargin 
+  )
+
+  // bellow are tests of edits
+  @Test
+  def editTest1 = checkEdit(
+    """
+    |object M:
+    | ra@@"1234"
+    """.stripMargin,
+    """
+    |object M:
+    | raw"1234"
+    |""".stripMargin 
+  )
+
+  @Test
+  def editTest2 = checkEdit(
+    """
+    |object M:
+    | printl@@"1234"
+    """.stripMargin,
+    """
+    |object M:
+    | println"1234"
+    |""".stripMargin,
+    assertSingleItem = false 
+  )
+
+  @Test
+  def editTest3 = checkEdit(
+    """
+    |object M:
+    | def select(s: String): String = s
+    | selec@@"1234"
+    """.stripMargin,
+    """
+    |object M:
+    | def select(s: String): String = s
+    | select"1234"
+    |""".stripMargin,
+    assertSingleItem = false 
+  )