Problemataic borrowing

weird_borrowing_error.rs

struct NoLifetime {}
struct WithLifetime <'a> {
    pub field: &'a i32
}
fn main() {
    let mut some_val = NoLifetime{};
    borrow_mut_function(&mut some_val);
    borrow_mut_function(&mut some_val);//Borrowing as mutable for the second time.
    
    let num = 5;
    let mut life_val = WithLifetime{field:&num};
    borrow_lifetime(&mut life_val);
    borrow_lifetime(&mut life_val);//Borrowing as mutable for the second time.
    let num_again = borrow_lifetime(&mut life_val); //Borrow, assign lifetime result
    borrow_lifetime(&mut life_val);//Compiler:  cannot borrow `life_val` as mutable more than 
}
fn borrow_mut_function(val_in: &mut NoLifetime) -> String {
    "abc".to_string()
}
fn borrow_lifetime<'a>(val_in: &'a mut WithLifetime) -> &'a i32 {
    val_in.field
}