feat(repl): autocomplete repl commands

This adds in completion for the available commands in the repl just like
there is for Scala 2. For example you can do `:@@` which will give you
back all the commands.

Author: ckipp01

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

@@ -127,7 +127,7 @@ object ParseResult {
     stats
   }
 
-  private val commands: List[(String, String => ParseResult)] = List(
+  private[repl] val commands: List[(String, String => ParseResult)] = List(
     Quit.command -> (_ => Quit),
     Quit.alias -> (_ => Quit),
     Help.command -> (_  => Help),

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

@@ -205,7 +205,7 @@ class ReplDriver(settings: Array[String],
       label
 
   /** Extract possible completions at the index of `cursor` in `expr` */
-  protected final def completions(cursor: Int, expr: String, state0: State): List[Candidate] = {
+  protected final def completions(cursor: Int, expr: String, state0: State): List[Candidate] =
     def makeCandidate(label: String) = {
 
       new Candidate(
@@ -218,20 +218,24 @@ class ReplDriver(settings: Array[String],
         /* complete = */ false  // if true adds space when completing
       )
     }
-    implicit val state = newRun(state0)
-    compiler
-      .typeCheck(expr, errorsAllowed = true)
-      .map { tree =>
-        val file = SourceFile.virtual("<completions>", expr, maybeIncomplete = true)
-        val unit = CompilationUnit(file)(using state.context)
-        unit.tpdTree = tree
-        given Context = state.context.fresh.setCompilationUnit(unit)
-        val srcPos = SourcePosition(file, Span(cursor))
-        val (_, completions) = Completion.completions(srcPos)
-        completions.map(_.label).distinct.map(makeCandidate)
-      }
-      .getOrElse(Nil)
-  }
+
+    if expr.startsWith(":") then
+      ParseResult.commands.map(command => makeCandidate(command._1))
+    else
+      given state: State = newRun(state0)
+      compiler
+        .typeCheck(expr, errorsAllowed = true)
+        .map { tree =>
+          val file = SourceFile.virtual("<completions>", expr, maybeIncomplete = true)
+          val unit = CompilationUnit(file)(using state.context)
+          unit.tpdTree = tree
+          given Context = state.context.fresh.setCompilationUnit(unit)
+          val srcPos = SourcePosition(file, Span(cursor))
+          val (_, completions) = Completion.completions(srcPos)
+          completions.map(_.label).distinct.map(makeCandidate)
+        }
+        .getOrElse(Nil)
+  end completions 
 
   private def interpret(res: ParseResult)(implicit state: State): State = {
     res match {

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

@@ -191,4 +191,20 @@ class TabcompleteTests extends ReplTest {
                      |Foo.`bac"""stripMargin))
   }
 
+  @Test def commands = initially {
+    assertEquals(
+      List(
+        ":doc",
+        ":exit",
+        ":help",
+        ":imports",
+        ":load",
+        ":quit",
+        ":reset",
+        ":settings",
+        ":type"
+      ),
+      tabComplete(":")
+    )
+  }
 }