fix comment, remove redundant code

Author: mikhail-m1

src/librustc_mir/dataflow/drop_flag_effects.rs

@@ -8,8 +8,6 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-use syntax_pos::DUMMY_SP;
-
 use rustc::mir::{self, Mir, Location};
 use rustc::ty::{self, TyCtxt};
 use util::elaborate_drops::DropFlagState;
@@ -187,23 +185,13 @@ pub(crate) fn drop_flag_effects_for_location<'a, 'gcx, 'tcx, F>(
     where F: FnMut(MovePathIndex, DropFlagState)
 {
     let move_data = &ctxt.move_data;
-    let param_env = ctxt.param_env;
     debug!("drop_flag_effects_for_location({:?})", loc);
 
     // first, move out of the RHS
     for mi in &move_data.loc_map[loc] {
         let path = mi.move_path_index(move_data);
         debug!("moving out of path {:?}", move_data.move_paths[path]);
 
-        // don't move out of non-Copy things
-        let lvalue = &move_data.move_paths[path].lvalue;
-        let ty = lvalue.ty(mir, tcx).to_ty(tcx);
-        let gcx = tcx.global_tcx();
-        let erased_ty = gcx.lift(&tcx.erase_regions(&ty)).unwrap();
-        if !erased_ty.moves_by_default(gcx, param_env, DUMMY_SP) {
-            continue;
-        }
-
         on_all_children_bits(tcx, mir, move_data,
                              path,
                              |mpi| callback(mpi, DropFlagState::Absent))
@@ -231,13 +219,8 @@ pub(crate) fn drop_flag_effects_for_location<'a, 'gcx, 'tcx, F>(
                     }
                 }
             }
-            mir::StatementKind::StorageDead(local) => {
-                on_lookup_result_bits(tcx, mir, move_data,
-                                      move_data.rev_lookup.find(&mir::Lvalue::Local(local)),
-                                      |mpi| callback(mpi, DropFlagState::Absent))
-
-            }
             mir::StatementKind::StorageLive(_) |
+            mir::StatementKind::StorageDead(_) |
             mir::StatementKind::InlineAsm { .. } |
             mir::StatementKind::EndRegion(_) |
             mir::StatementKind::Validate(..) |

src/librustc_mir/dataflow/impls/mod.rs

@@ -457,7 +457,10 @@ impl<'a, 'gcx, 'tcx> BitDenotation for MovingOutStatements<'a, 'gcx, 'tcx> {
         let rev_lookup = &move_data.rev_lookup;
 
         match stmt.kind {
-            // skip move out for StorageDead
+            // this analysis only tries to find moves explicitly
+            // written by the user, so we ignore the move-outs
+            // created by `StorageDead` and at the beginning
+            // of a function.
             mir::StatementKind::StorageDead(_) => {}
             _ => {
                 debug!("stmt {:?} at loc {:?} moves out of move_indexes {:?}",