Erase unused arguments as if they did not exist in the program

* Addapted all tests
* Moved unused argument checks to postyper (for top down traversal with Unused ctx mode).
* Removed UnusedArgLift and revert to PhantomArgLift
* Disallow user written unused defs and non parameter unused val (temporary)

Author: nicolasstucki

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -48,12 +48,10 @@ class Compiler {
       List(new Pickler),            // Generate TASTY info
       List(new LinkAll),            // Reload compilation units from TASTY for library code (if needed)
       List(new FirstTransform,      // Some transformations to put trees into a canonical form
-           new UnusedChecks,        // Check that unused terms are not used
            new CheckReentrant,      // Internal use only: Check that compiled program has no data races involving global vars
            new ElimJavaPackages),   // Eliminate syntactic references to Java packages
       List(new CheckStatic,         // Check restrictions that apply to @static members
            new UnusedRefs,          // Removes all calls and references to unused values
-           new UnusedArgLift,       // Extracts the evaluation of unused arguments placing them before the call.
            new ElimRepeated,        // Rewrite vararg parameters and arguments
            new NormalizeFlags,      // Rewrite some definition flags
            new ExtensionMethods,    // Expand methods of value classes with extension methods
@@ -71,7 +69,8 @@ class Compiler {
            new ShortcutImplicits,   // Allow implicit functions without creating closures
            new CrossCastAnd,        // Normalize selections involving intersection types.
            new Splitter),           // Expand selections involving union types into conditionals
-      List(new UnusedDecls,         // Removes all unused defs and vals decls (except for parameters)
+      List(new PhantomArgLift,      // Extracts the evaluation of phantom arguments placing them before the call
+           new UnusedDecls,         // Removes all unused defs and vals decls (except for parameters)
            new VCInlineMethods,     // Inlines calls to value class methods
            new SeqLiterals,         // Express vararg arguments as arrays
            new InterceptedMethods,  // Special handling of `==`, `|=`, `getClass` methods

compiler/src/dotty/tools/dotc/core/Mode.scala

@@ -99,4 +99,9 @@ object Mode {
 
   /** We are in the IDE */
   val Interactive = newMode(20, "Interactive")
+
+  /** We are currently in code that will not be used at runtime.
+   *  It can be in an argument to an unused parameter or a type selection.
+   */
+  val Unused = newMode(21, "Unused")
 }