Elide storage markers when elaborating deref projections

Author: tmiasko

compiler/rustc_middle/src/mir/patch.rs

@@ -68,22 +68,25 @@ impl<'tcx> MirPatch<'tcx> {
         Location { block: bb, statement_index: offset }
     }
 
-    pub fn new_local_with_info(
+    pub fn new_internal_with_info(
         &mut self,
         ty: Ty<'tcx>,
         span: Span,
         local_info: Option<Box<LocalInfo<'tcx>>>,
     ) -> Local {
         let index = self.next_local;
         self.next_local += 1;
-        let mut new_decl = LocalDecl::new(ty, span);
+        let mut new_decl = LocalDecl::new(ty, span).internal();
         new_decl.local_info = local_info;
         self.new_locals.push(new_decl);
         Local::new(index as usize)
     }
 
     pub fn new_temp(&mut self, ty: Ty<'tcx>, span: Span) -> Local {
-        self.new_local_with_info(ty, span, None)
+        let index = self.next_local;
+        self.next_local += 1;
+        self.new_locals.push(LocalDecl::new(ty, span));
+        Local::new(index as usize)
     }
 
     pub fn new_internal(&mut self, ty: Ty<'tcx>, span: Span) -> Local {

compiler/rustc_mir_transform/src/deref_separator.rs

@@ -28,8 +28,6 @@ impl<'tcx> MutVisitor<'tcx> for DerefChecker<'tcx> {
             let mut last_len = 0;
             let mut last_deref_idx = 0;
 
-            let mut prev_temp: Option<Local> = None;
-
             for (idx, elem) in place.projection[0..].iter().enumerate() {
                 if *elem == ProjectionElem::Deref {
                     last_deref_idx = idx;
@@ -39,14 +37,12 @@ impl<'tcx> MutVisitor<'tcx> for DerefChecker<'tcx> {
             for (idx, (p_ref, p_elem)) in place.iter_projections().enumerate() {
                 if !p_ref.projection.is_empty() && p_elem == ProjectionElem::Deref {
                     let ty = p_ref.ty(&self.local_decls, self.tcx).ty;
-                    let temp = self.patcher.new_local_with_info(
+                    let temp = self.patcher.new_internal_with_info(
                         ty,
                         self.local_decls[p_ref.local].source_info.span,
                         Some(Box::new(LocalInfo::DerefTemp)),
                     );
 
-                    self.patcher.add_statement(loc, StatementKind::StorageLive(temp));
-
                     // We are adding current p_ref's projections to our
                     // temp value, excluding projections we already covered.
                     let deref_place = Place::from(place_local)
@@ -66,22 +62,8 @@ impl<'tcx> MutVisitor<'tcx> for DerefChecker<'tcx> {
                             Place::from(temp).project_deeper(&place.projection[idx..], self.tcx);
                         *place = temp_place;
                     }
-
-                    // We are destroying the previous temp since it's no longer used.
-                    if let Some(prev_temp) = prev_temp {
-                        self.patcher.add_statement(loc, StatementKind::StorageDead(prev_temp));
-                    }
-
-                    prev_temp = Some(temp);
                 }
             }
-
-            // Since we won't be able to reach final temp, we destroy it outside the loop.
-            if let Some(prev_temp) = prev_temp {
-                let last_loc =
-                    Location { block: loc.block, statement_index: loc.statement_index + 1 };
-                self.patcher.add_statement(last_loc, StatementKind::StorageDead(prev_temp));
-            }
         }
     }
 }