fix: take into account preface for repl commands

Author: ckipp01

compiler/src/dotty/tools/repl/ReplDriver.scala

@@ -220,7 +220,15 @@ class ReplDriver(settings: Array[String],
     }
 
     if expr.startsWith(":") then
-      ParseResult.commands.map(command => makeCandidate(command._1))
+      ParseResult.commands.collect {
+        // If expr is only : then we return the commands with : since jline
+        // correctly handles them
+        case command if expr == ":" => makeCandidate(command._1)
+        // However if there is more than just the : we filter by it and then
+        // drop the : since jline will correctly complete it but you'll end up
+        // with ::import for example instead of :import
+        case command if command._1.startsWith(expr) => makeCandidate(command._1.drop(1))
+      }
     else
       given state: State = newRun(state0)
       compiler

compiler/test/dotty/tools/repl/TabcompleteTests.scala

@@ -207,4 +207,12 @@ class TabcompleteTests extends ReplTest {
       tabComplete(":")
     )
   }
+
+  @Test def commandPreface = initially {
+    // This looks odd, but if we return :doc here it will result in ::doc in the REPL
+    assertEquals(
+      List("doc"),
+      tabComplete(":d")
+    )
+  }
 }