Draft: additional completions for using clause (scala#23647)

fix for  scala#22939

---------

Co-authored-by: Piotr Fałdrowicz <pfl@touk.pl>

Author: vder

presentation-compiler/src/main/dotty/tools/pc/completions/CompletionAffix.scala

@@ -56,6 +56,7 @@ case class CompletionAffix(
   private def loopPrefix(prefixes: List[PrefixKind]): String =
     prefixes match
       case PrefixKind.New :: tail => "new " + loopPrefix(tail)
+      case PrefixKind.Using :: tail => "using " + loopPrefix(tail)
       case _ => ""
 
   /**
@@ -87,7 +88,7 @@ enum SuffixKind:
   case Brace, Bracket, Template, NoSuffix
 
 enum PrefixKind:
-  case New
+  case New, Using
 
 type Suffix = Affix[SuffixKind]
 type Prefix = Affix[PrefixKind]

presentation-compiler/src/main/dotty/tools/pc/completions/Completions.scala

@@ -193,13 +193,24 @@ class Completions(
     )
   end isAbstractType
 
-  private def findSuffix(symbol: Symbol): CompletionAffix =
+  private def findSuffix(symbol: Symbol, adjustedPath: List[untpd.Tree]): CompletionAffix =
     CompletionAffix.empty
       .chain { suffix => // for [] suffix
         if shouldAddSuffix && symbol.info.typeParams.nonEmpty then
           suffix.withNewSuffixSnippet(Affix(SuffixKind.Bracket))
         else suffix
       }
+      .chain{ suffix =>
+        adjustedPath match
+            case (ident: Ident) :: (app@Apply(_, List(arg))) :: _  =>
+              app.symbol.info match
+                case mt@MethodType(termNames) if app.symbol.paramSymss.last.exists(_.is(Given)) &&
+                  !text.substring(app.fun.span.start, arg.span.end).contains("using") =>
+                  suffix.withNewPrefix(Affix(PrefixKind.Using))
+                case _ => suffix    
+            case _ => suffix
+        
+      }
       .chain { suffix => // for () suffix
         if shouldAddSuffix && symbol.is(Flags.Method) then
           val paramss = getParams(symbol)
@@ -271,7 +282,7 @@ class Completions(
       val existsApply = extraMethodDenots.exists(_.symbol.name == nme.apply)
 
       extraMethodDenots.map { methodDenot =>
-        val suffix = findSuffix(methodDenot.symbol)
+        val suffix = findSuffix(methodDenot.symbol, adjustedPath)
         val affix = if methodDenot.symbol.isConstructor && existsApply then
           adjustedPath match
             case (select @ Select(qual, _)) :: _ =>
@@ -293,7 +304,7 @@ class Completions(
 
     if skipOriginalDenot then extraCompletionValues
     else
-      val suffix = findSuffix(denot.symbol)
+      val suffix = findSuffix(denot.symbol, adjustedPath)
       val name = undoBacktick(label)
       val denotCompletionValue = toCompletionValue(name, denot, suffix)
       denotCompletionValue :: extraCompletionValues

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

@@ -238,6 +238,65 @@ class CompletionArgSuite extends BaseCompletionSuite:
       ""
     )
 
+  @Test def `using` =
+    checkEdit(
+      s"""|def hello(using String): Unit = ???
+          |@main def main1(): Unit =
+          |  val str = "hello"
+          |  hello(st@@)
+          |""".stripMargin,
+          s"""|def hello(using String): Unit = ???
+              |@main def main1(): Unit =
+              |  val str = "hello"
+              |  hello(using str)
+              |""".stripMargin,
+          assertSingleItem = false)
+
+  @Test def `using2` =
+    checkEdit(
+      s"""|def hello(using String): Unit = ???
+          |@main def main1(): Unit =
+          |  val str = "hello"
+          |  hello(using st@@)
+          |""".stripMargin,
+      s"""|def hello(using String): Unit = ???
+          |@main def main1(): Unit =
+          |  val str = "hello"
+          |  hello(using str)
+          |""".stripMargin,
+          assertSingleItem = false)
+
+  @Test def `using3` =
+    checkEdit(
+      s"""|def hello(using String, Int): Unit = ???
+          |@main def main1(): Unit =
+          |  val str = "hello"
+          |  val int = 4
+          |  hello(str, in@@)
+          |""".stripMargin,
+      s"""|def hello(using String, Int): Unit = ???
+          |@main def main1(): Unit =
+          |  val str = "hello"
+          |  val int = 4
+          |  hello(str, int)
+          |""".stripMargin,
+          assertSingleItem = false)
+
+  @Test def `using4` =
+    checkEdit(
+      s"""|def hello(name: String)(using String): Unit = ???
+          |@main def main1(): Unit =
+          |  val str = "hello"
+          |  hello("name")(str@@)
+          |""".stripMargin,
+      s"""|def hello(name: String)(using String): Unit = ???
+          |@main def main1(): Unit =
+          |  val str = "hello"
+          |  hello("name")(using str)
+          |""".stripMargin,
+          assertSingleItem = false
+          )
+
   @Test def `default-args` =
     check(
       s"""|object Main {