Fix rust-lang#43457 (and avoid some unneeded lookups to boot, woo).

src/librustc_mir/hair/cx/block.rs

@@ -22,10 +22,7 @@ impl<'tcx> Mirror<'tcx> for &'tcx hir::Block {
         // We have to eagerly translate the "spine" of the statements
         // in order to get the lexical scoping correctly.
         let stmts = mirror_stmts(cx, self.id, &*self.stmts);
-        let opt_def_id = cx.tcx.hir.opt_local_def_id(self.id);
-        let opt_destruction_extent = opt_def_id.and_then(|def_id| {
-            cx.tcx.region_maps(def_id).opt_destruction_extent(self.id)
-        });
+        let opt_destruction_extent = cx.region_maps.opt_destruction_extent(self.id);
         Block {
             targeted_by_break: self.targeted_by_break,
             extent: CodeExtent::Misc(self.id),
@@ -42,11 +39,8 @@ fn mirror_stmts<'a, 'gcx, 'tcx>(cx: &mut Cx<'a, 'gcx, 'tcx>,
                                 stmts: &'tcx [hir::Stmt])
                                 -> Vec<StmtRef<'tcx>> {
     let mut result = vec![];
-    let opt_def_id = cx.tcx.hir.opt_local_def_id(block_id);
     for (index, stmt) in stmts.iter().enumerate() {
-        let opt_dxn_ext = opt_def_id.and_then(|def_id| {
-            cx.tcx.region_maps(def_id).opt_destruction_extent(stmt.node.id())
-        });
+        let opt_dxn_ext = cx.region_maps.opt_destruction_extent(stmt.node.id());
         match stmt.node {
             hir::StmtExpr(ref expr, id) |
             hir::StmtSemi(ref expr, id) => {