librustc: Add a new nop statement to the MIR.

This is useful when passes want to remove statements without affecting
`Location`s.

src/librustc/mir/repr.rs

@@ -187,6 +187,14 @@ impl<'tcx> Mir<'tcx> {
         self.var_decls.len() +
         self.temp_decls.len() + 1
     }
+
+    /// Changes a statement to a nop. This is both faster than deleting instructions and avoids
+    /// invalidating statement indices in `Location`s.
+    pub fn make_statement_nop(&mut self, location: Location) {
+        let block = &mut self[location.block];
+        debug_assert!(location.statement_index < block.statements.len());
+        block.statements[location.statement_index].make_nop()
+    }
 }
 
 impl<'tcx> Index<BasicBlock> for Mir<'tcx> {
@@ -686,6 +694,14 @@ pub struct Statement<'tcx> {
     pub kind: StatementKind<'tcx>,
 }
 
+impl<'tcx> Statement<'tcx> {
+    /// Changes a statement to a nop. This is both faster than deleting instructions and avoids
+    /// invalidating statement indices in `Location`s.
+    pub fn make_nop(&mut self) {
+        self.kind = StatementKind::Nop
+    }
+}
+
 #[derive(Clone, Debug, RustcEncodable, RustcDecodable)]
 pub enum StatementKind<'tcx> {
     /// Write the RHS Rvalue to the LHS Lvalue.
@@ -699,6 +715,9 @@ pub enum StatementKind<'tcx> {
 
     /// End the current live range for the storage of the local.
     StorageDead(Lvalue<'tcx>),
+
+    /// No-op. Useful for deleting instructions without affecting statement indices.
+    Nop,
 }
 
 impl<'tcx> Debug for Statement<'tcx> {
@@ -711,6 +730,7 @@ impl<'tcx> Debug for Statement<'tcx> {
             SetDiscriminant{lvalue: ref lv, variant_index: index} => {
                 write!(fmt, "discriminant({:?}) = {:?}", lv, index)
             }
+            Nop => write!(fmt, "nop"),
         }
     }
 }

src/librustc/mir/visit.rs

@@ -346,6 +346,7 @@ macro_rules! make_mir_visitor {
                     StatementKind::StorageDead(ref $($mutability)* lvalue) => {
                         self.visit_lvalue(lvalue, LvalueContext::StorageDead, location);
                     }
+                    StatementKind::Nop => {}
                 }
             }
 

src/librustc_borrowck/borrowck/mir/dataflow/impls.rs

@@ -455,7 +455,8 @@ impl<'a, 'tcx> BitDenotation for MovingOutStatements<'a, 'tcx> {
                                      });
             }
             repr::StatementKind::StorageLive(_) |
-            repr::StatementKind::StorageDead(_) => {}
+            repr::StatementKind::StorageDead(_) |
+            repr::StatementKind::Nop => {}
         }
     }
 

src/librustc_borrowck/borrowck/mir/dataflow/sanity_check.rs

@@ -105,7 +105,8 @@ fn each_block<'a, 'tcx, O>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
                 (lvalue, rvalue)
             }
             repr::StatementKind::StorageLive(_) |
-            repr::StatementKind::StorageDead(_) => continue,
+            repr::StatementKind::StorageDead(_) |
+            repr::StatementKind::Nop => continue,
             repr::StatementKind::SetDiscriminant{ .. } =>
                 span_bug!(stmt.source_info.span,
                           "sanity_check should run before Deaggregator inserts SetDiscriminant"),

src/librustc_borrowck/borrowck/mir/mod.rs

@@ -389,7 +389,8 @@ fn drop_flag_effects_for_location<'a, 'tcx, F>(
                                        |moi| callback(moi, DropFlagState::Present))
             }
             repr::StatementKind::StorageLive(_) |
-            repr::StatementKind::StorageDead(_) => {}
+            repr::StatementKind::StorageDead(_) |
+            repr::StatementKind::Nop => {}
         },
         None => {
             debug!("drop_flag_effects: replace {:?}", block.terminator());

src/librustc_mir/transform/qualify_consts.rs

@@ -910,7 +910,8 @@ impl<'a, 'tcx> Visitor<'tcx> for Qualifier<'a, 'tcx, 'tcx> {
                 }
                 StatementKind::SetDiscriminant { .. } |
                 StatementKind::StorageLive(_) |
-                StatementKind::StorageDead(_) => {}
+                StatementKind::StorageDead(_) |
+                StatementKind::Nop => {}
             }
         });
     }

src/librustc_mir/transform/type_check.rs

@@ -385,6 +385,7 @@ impl<'a, 'gcx, 'tcx> TypeChecker<'a, 'gcx, 'tcx> {
                     }
                 }
             }
+            StatementKind::Nop => {}
         }
     }
 

src/librustc_trans/mir/constant.rs

@@ -292,7 +292,8 @@ impl<'a, 'tcx> MirConstContext<'a, 'tcx> {
                         }
                     }
                     mir::StatementKind::StorageLive(_) |
-                    mir::StatementKind::StorageDead(_) => {}
+                    mir::StatementKind::StorageDead(_) |
+                    mir::StatementKind::Nop => {}
                     mir::StatementKind::SetDiscriminant{ .. } => {
                         span_bug!(span, "SetDiscriminant should not appear in constants?");
                     }

src/librustc_trans/mir/statement.rs

@@ -78,6 +78,7 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
             mir::StatementKind::StorageDead(ref lvalue) => {
                 self.trans_storage_liveness(bcx, lvalue, base::Lifetime::End)
             }
+            mir::StatementKind::Nop => bcx,
         }
     }