Handle language feature imports

Author: som-snytt

compiler/src/dotty/tools/dotc/typer/ImportInfo.scala

@@ -207,7 +207,9 @@ class ImportInfo(symf: Context ?=> Symbol,
     def test(prefix: TermName, feature: TermName): Option[Boolean] =
       untpd.languageImport(qualifier) match
         case Some(`prefix`) =>
-          if forwardMapping.contains(feature) then Some(true)
+          if forwardMapping.contains(feature) then
+            ctx.usages.use(this, selectors(selectorOf(feature)))
+            Some(true)
           else if excluded.contains(feature) then Some(false)
           else None
         case _ => None

tests/neg/unused-imports.check

@@ -1,19 +1,23 @@
--- [E007] Type Mismatch Error: tests/neg/unused-imports.scala:8:15 -----------------------------------------------------
-8 |  def i: Int = "42" // error sanity check
-  |               ^^^^
-  |               Found:    ("42" : String)
-  |               Required: Int
-  |
-  | longer explanation available when compiling with `-explain`
--- Error: tests/neg/unused-imports.scala:3:24 --------------------------------------------------------------------------
-3 |import scala.concurrent.* // error
+-- [E007] Type Mismatch Error: tests/neg/unused-imports.scala:11:15 ----------------------------------------------------
+11 |  def i: Int = "42" // error sanity check
+   |               ^^^^
+   |               Found:    ("42" : String)
+   |               Required: Int
+   |
+   | longer explanation available when compiling with `-explain`
+-- Error: tests/neg/unused-imports.scala:5:16 --------------------------------------------------------------------------
+5 |import language.postfixOps // error
+  |                ^^^^^^^^^^
+  |                Unused import
+-- Error: tests/neg/unused-imports.scala:6:24 --------------------------------------------------------------------------
+6 |import scala.concurrent.* // error
   |                        ^
   |                        Unused import
--- Error: tests/neg/unused-imports.scala:9:77 --------------------------------------------------------------------------
-9 |  import scala.collection.mutable.{HashMap => GoodMap, Seq => _, ListBuffer, Buffer, Set => OK} // error // error
-  |                                                                             ^^^^^^
-  |                                                                             Unused import
--- Error: tests/neg/unused-imports.scala:9:43 --------------------------------------------------------------------------
-9 |  import scala.collection.mutable.{HashMap => GoodMap, Seq => _, ListBuffer, Buffer, Set => OK} // error // error
-  |                                   ^^^^^^^^^^^^^^^^^^
-  |                                   Unused import
+-- Error: tests/neg/unused-imports.scala:12:43 -------------------------------------------------------------------------
+12 |  import scala.collection.mutable.{HashMap as GoodMap, Seq as _, ListBuffer, Buffer, Set as OK} // error // error
+   |                                   ^^^^^^^^^^^^^^^^^^
+   |                                   Unused import
+-- Error: tests/neg/unused-imports.scala:12:77 -------------------------------------------------------------------------
+12 |  import scala.collection.mutable.{HashMap as GoodMap, Seq as _, ListBuffer, Buffer, Set as OK} // error // error
+   |                                                                             ^^^^^^
+   |                                                                             Unused import

tests/neg/unused-imports.scala

@@ -1,6 +1,8 @@
-// scalac: -Wunused:imports -Werror -Ydebug
+// scalac: -Wunused:imports -Werror -Ydebug -feature
 
 //import language.future
+import language.implicitConversions
+import language.postfixOps // error
 import scala.concurrent.* // error
 import scala.concurrent.ExecutionContext.Implicits.*
 
@@ -51,3 +53,12 @@ object Givens:
   given cOrdering: Ordering[C] with
     override def compare(c0: C, c1: C) = 0
   val greeting = "we love Givens"
+
+class A:
+  def f(b: B): Unit = b.f(this)
+
+object A:
+  implicit val a2b: Conversion[A, B] = _ => B()
+
+class B:
+  def f(b: B): Unit = ()