spurious "borrow might be used .. when [variable] is dropped and runs the destructor" #70919
Description
use std::rc::Rc; use std::cell::{RefCell, RefMut}; use std::mem::drop; fn huh(x: Rc<RefCell<i32>>) { let mut rc: Rc<RefCell<i32>> = x.clone(); let mut inner: RefMut<'_, i32> = rc.borrow_mut(); loop { drop(inner); drop(rc); rc = x.clone(); inner = rc.borrow_mut(); } }produces:
error[E0505]: cannot move out of `rc` because it is borrowed --> src/lib.rs:9:14 | 6 | let mut inner: RefMut<'_, i32> = rc.borrow_mut(); | -- borrow of `rc` occurs here ... 9 | drop(rc); | ^^ move out of `rc` occurs here ... 12 | inner = rc.borrow_mut(); | ----- borrow might be used here, when `inner` is dropped and runs the destructor for type `std::cell::RefMut<'_, i32>` error[E0506]: cannot assign to `rc` because it is borrowed --> src/lib.rs:11:9 | 6 | let mut inner: RefMut<'_, i32> = rc.borrow_mut(); | -- borrow of `rc` occurs here ... 11 | rc = x.clone(); | ^^ assignment to borrowed `rc` occurs here 12 | inner = rc.borrow_mut(); | ----- borrow might be used here, when `inner` is dropped and runs the destructor for type `std::cell::RefMut<'_, i32>` (Same error with -Zpolonius.)
The error messages are wrong: the assignment to inner can never run a destructor, since the previous value of inner was dropped earlier on.
And AFAICT this code is sound and ought to be accepted.