rust - How can I work around a RefCell issue? -

i have struct 2 c pointers , 1 rust hashmap.

struct mystruct {     p1: *mut ...,     p2: *mut ...,     hm: box<hashmap<...>> } 

my struct gets handled rc<refcell<mystruct>> , have c function gets called this:

c_call(my_struct.borrow().p1, my_struct.borrow().p2); 

c has rust callback gets called during execution of c_call requires my_struct.borrow_mut(), my_struct borrowed c_call needs p1 , p2, refcell<t> borrowed.

the problem c_call can't changed , needs immutable access p1 , p2 , borrow_mut of my_struct.

here's mcve:

use std::cell::refcell; use std::collections::hashmap; use std::mem::uninitialized; use std::os::raw::c_void; use std::rc::rc;  struct mystruct {     p1: *mut c_void,     p2: *mut c_void,     hm: box<hashmap<string, string>> }  // c_call can't mutate hm because my_struct borrowed // c_call can't changed fn c_call(_p1: *mut c_void, _p2: *mut c_void, my_struct: rc<refcell<mystruct>>) {     my_struct.borrow_mut().hm.insert("hey".to_string(), "you".to_string()); }  // call receives rc<refcell<mystruct>> , need call c_call fn call(my_struct: rc<refcell<mystruct>>) {     c_call(my_struct.borrow().p1, my_struct.borrow().p2, my_struct.clone()); }  fn main() {     unsafe {         let my_struct = mystruct {             p1: uninitialized::<*mut c_void>(), // irrelevant             p2: uninitialized::<*mut c_void>(),             hm: box::new(hashmap::new())         };          let my_struct = rc::new(refcell::new(my_struct));          call(my_struct);     } } 

(playpen)

how can work around issue?

your issue calling borrow() in arguments of c_call call borrow object until call finishes. if change to

let (p1, p2) = {     let x = my_struct.borrow();     (x.p1, x.p2) }; c_call(p1, p2, my_struct.clone()); 

then borrow ends before c_call call, c_call can borrow_mut object, too.