We interchange one field of the structure when borrowing another in closing

Question

I have a structure that contains two fields, and I want to change one field (mutable borrowing) using another field (immutable borrowing), but I get an error message from a borrower check.

For example, 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` as immutable because `strct.field1` is also borrowed as mutable --> src/main.rs:12:25 | 12 | strct.field1.retain(|v| !strct.field2.contains(v)); | ------------ ^^^ ----- - mutable borrow ends here | | | | | | | borrow occurs due to use of `strct` in closure | | immutable borrow occurs here | mutable borrow occurs here 

What are the ways Rust can update one field using another from a closure?

Answer 1

Typically, the borrower verification tool can distinguish between different fields of the structure, but this does not work within the closure (lambdas).

Instead, borrow the second field outside the closure:

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