Mutably borrow one struct field while borrowing another in a closure

Question

I have a struct containing two fields and I want to modify one field (mutable borrow) using another field (immutable borrow), but I get an error from the borrow checker.

For instance, the following code:

struct Struct {
    field1: Vec<i32>,
    field2: Vec<i32>,
}

fn main() {
    let mut strct = Struct {
        field1: vec![1, 2, 3],
        field2: vec![2, 3, 4],
    };

    strct.field1.retain(|v| !strct.field2.contains(v));

    println!("{:?}", strct.field1);
}

gives the following error:

error[E0502]: cannot borrow `strct.field1` as mutable because it is also borrowed as immutable
  --> src/main.rs:12:5
   |
12 |     strct.field1.retain(|v| !strct.field2.contains(v));
   |     ^^^^^^^^^^^^^------^---^^-----^^^^^^^^^^^^^^^^^^^^
   |     |            |      |    |
   |     |            |      |    first borrow occurs due to use of `strct` in closure
   |     |            |      immutable borrow occurs here
   |     |            immutable borrow later used by call
   |     mutable borrow occurs here

What are the Rust ways of updating one field using another from within a closure?

Answer 1

Usually the borrow checker can distinguish between the different fields of a structure, but this doesn't work within closures (lambdas).

Instead, borrow the second field outside the closure:

let field2 = &strct.field2;
strct.field1.retain(|v| !field2.contains(v));