Add a hint for using global language import with REPL (#22220)

Fix #16250

Author: noti0na1

Diff for: compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3706,7 +3706,15 @@ object Parsers {
           in.languageImportContext = in.languageImportContext.importContext(imp, NoSymbol)
           for case ImportSelector(id @ Ident(imported), EmptyTree, _) <- selectors do
             if Feature.handleGlobalLanguageImport(prefix, imported) && !outermost then
-              syntaxError(em"this language import is only allowed at the toplevel", id.span)
+              val desc =
+                if ctx.mode.is(Mode.Interactive) then
+                  "not allowed in the REPL"
+                else "only allowed at the toplevel"
+              val hint =
+                if ctx.mode.is(Mode.Interactive) then
+                  f"\nTo use this language feature, include the flag `-language:$prefix.$imported` when starting the REPL"
+                else ""
+              syntaxError(em"this language import is $desc$hint", id.span)
             if allSourceVersionNames.contains(imported) && prefix.isEmpty then
               if !outermost then
                 syntaxError(em"source version import is only allowed at the toplevel", id.span)

Diff for: compiler/test/dotty/tools/repl/ReplCompilerTests.scala

@@ -493,6 +493,24 @@ class ReplCompilerTests extends ReplTest:
     assertTrue(all.head.startsWith("-- [E103] Syntax Error"))
     assertTrue(all.exists(_.trim().startsWith("|  Illegal start of statement: this modifier is not allowed here")))
 
+  @Test def `i16250a`: Unit = initially:
+    val hints = List(
+      "this language import is not allowed in the REPL",
+      "To use this language feature, include the flag `-language:experimental.captureChecking` when starting the REPL"
+    )
+    run("import language.experimental.captureChecking")
+    val all = lines()
+    assertTrue(hints.forall(hint => all.exists(_.contains(hint))))
+
+  @Test def `i16250b`: Unit = initially:
+    val hints = List(
+      "this language import is not allowed in the REPL",
+      "To use this language feature, include the flag `-language:experimental.pureFunctions` when starting the REPL"
+    )
+    run("import language.experimental.pureFunctions")
+    val all = lines()
+    assertTrue(hints.forall(hint => all.exists(_.contains(hint))))
+
 object ReplCompilerTests:
 
   private val pattern = Pattern.compile("\\r[\\n]?|\\n");