supplement for the missing or incomplete comments

Author: dingxiangfei2009

compiler/rustc_mir_build/src/build/block.rs

@@ -119,10 +119,13 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                 } => {
                     let ignores_expr_result = matches!(pattern.kind, PatKind::Wild);
                     this.block_context.push(BlockFrame::Statement { ignores_expr_result });
+
+                    // Lower the `else` block first because its parent scope is actually
+                    // enclosing the rest of the `let .. else ..` parts.
+                    let else_block_span = this.thir[*else_block].span;
                     // This place is not really used because this destination place
                     // should never be used to take values at the end of the failure
                     // block.
-                    let else_block_span = this.thir[*else_block].span;
                     let dummy_place = this.temp(this.tcx.types.never, else_block_span);
                     let failure_entry = this.cfg.start_new_block();
                     let failure_block;

compiler/rustc_typeck/src/check/region.rs

@@ -128,16 +128,24 @@ fn resolve_block<'tcx>(visitor: &mut RegionResolutionVisitor<'tcx>, blk: &'tcx h
             match statement.kind {
                 hir::StmtKind::Local(hir::Local { els: Some(els), .. }) => {
                     // Let-else has a special lexical structure for variables.
+                    // First we take a checkpoint of the current scope context here.
                     let mut prev_cx = visitor.cx;
+
                     visitor.enter_scope(Scope {
                         id: blk.hir_id.local_id,
                         data: ScopeData::Remainder(FirstStatementIndex::new(i)),
                     });
                     visitor.cx.var_parent = visitor.cx.parent;
                     visitor.visit_stmt(statement);
+                    // We need to back out temporarily to the last enclosing scope
+                    // for the `else` block, so that even the temporaries receiving
+                    // extended lifetime will be dropped inside this block.
+                    // We are visiting the `else` block in this order so that
+                    // the sequence of visits agree with the order in the default
+                    // `hir::intravisit` visitor.
                     mem::swap(&mut prev_cx, &mut visitor.cx);
-                    // We need to back out temporarily and
                     visitor.visit_block(els);
+                    // From now on, we continue normally.
                     visitor.cx = prev_cx;
                 }
                 hir::StmtKind::Local(..) | hir::StmtKind::Item(..) => {