Add comments based on code review feedback

Author: eholk

compiler/rustc_typeck/src/check/generator_interior.rs

@@ -371,6 +371,14 @@ impl<'a, 'tcx> Visitor<'tcx> for InteriorVisitor<'a, 'tcx> {
 
         debug!("is_borrowed_temporary: {:?}", self.drop_ranges.is_borrowed_temporary(expr));
 
+        // Typically, the value produced by an expression is consumed by its parent in some way,
+        // so we only have to check if the parent contains a yield (note that the parent may, for
+        // example, store the value into a local variable, but then we already consider local
+        // variables to be live across their scope).
+        //
+        // However, in the case of temporary values, we are going to store the value into a
+        // temporary on the stack that is live for the current temporary scope and then return a
+        // reference to it. That value may be live across the entire temporary scope.
         let scope = if self.drop_ranges.is_borrowed_temporary(expr) {
             self.region_scope_tree.temporary_scope(expr.hir_id.local_id)
         } else {

compiler/rustc_typeck/src/expr_use_visitor.rs

@@ -51,8 +51,8 @@ pub trait Delegate<'tcx> {
     /// The value found at `place` is being copied.
     /// `diag_expr_id` is the id used for diagnostics (see `consume` for more details).
     fn copy(&mut self, place_with_id: &PlaceWithHirId<'tcx>, diag_expr_id: hir::HirId) {
-        // In most cases, treating a copy as a borrow is the right thing, so we forward
-        // this to the borrow callback by default.
+        // In most cases, copying data from `x` is equivalent to doing `*&x`, so by default
+        // we treat a copy of `x` as a borrow of `x`.
         self.borrow(place_with_id, diag_expr_id, ty::BorrowKind::ImmBorrow, false)
     }