Add some code comments

compiler/rustc_ast/src/ast.rs

@@ -1760,9 +1760,13 @@ pub enum CaptureBy {
         /// The span of the `move` keyword.
         move_kw: Span,
     },
-    /// `move` keyword was not specified.
+    /// `move` or `use` keywords were not specified.
     Ref,
     /// `use |x| y + x`.
+    ///
+    /// Note that if you have a regular closure like `|| x.use`, this will *not* result
+    /// in a `Use` capture. Instead, the `ExprUseVisitor` will look at the type
+    /// of `x` and treat `x.use` as either a copy/clone/move as appropriate.
     Use {
         /// The span of the `use` keyword.
         use_kw: Span,

compiler/rustc_hir_typeck/src/expr_use_visitor.rs

@@ -327,6 +327,12 @@ impl<'tcx, Cx: TypeInformationCtxt<'tcx>, D: Delegate<'tcx>> ExprUseVisitor<'tcx
     pub fn consume_clone_or_copy(&self, place_with_id: &PlaceWithHirId<'tcx>, diag_expr_id: HirId) {
         debug!("delegate_consume_or_clone(place_with_id={:?})", place_with_id);
 
+        // `x.use` will do one of the following
+        // * if it implements `Copy`, it will be a copy
+        // * if it implements `UseCloned`, it will be a call to `clone`
+        // * otherwise, it is a move
+        //
+        // we do a conservative approximation of this, treating it as a move unless we know that it implements copy or `UseCloned`
         if self.cx.type_is_copy_modulo_regions(place_with_id.place.ty()) {
             self.delegate.borrow_mut().copy(place_with_id, diag_expr_id);
         } else if self.cx.type_is_use_cloned_modulo_regions(place_with_id.place.ty()) {