winksaville / exper-mut-borrowing-in-a-loop

Mutable borrowing in a loop

The compiler error didn't help me solve a bug I had. I have a struct which maintains some data and its producing some strings as I call a read_line method. The obvious thing was to have read_line return a &str to the mutable string in the struct. But I couldn't do that now I have a mutable and immutable reference to the same struct.

So this code:

    let mut data = Data::new(gen_data(ln));
    while let Some(line) = data.read_line() {
        println!("{}: {}", data.line_number, line);
    }

Would cause this error:

$ cargo run
   Compiling exper-mut-borrowing-in-loop v0.1.0 (/home/wink/prgs/rust/myrepos/exper-mut-borrowing-in-loop)
error[E0502]: cannot borrow `data.line_number` as immutable because it is also borrowed as mutable
  --> src/main.rs:91:28
   |
92 |     while let Some(line) = data.read_line() {
   |                            ---- mutable borrow occurs here
93 |         println!("{}: {}", data.line_number, line);
   |                            ^^^^^^^^^^^^^^^^  ---- mutable borrow later used here
   |                            |
   |                            immutable borrow occurs here
   |
   = note: this error originates in the macro `$crate::format_args_nl` which comes from the expansion of the macro `println` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0502`.
error: could not compile `exper-mut-borrowing-in-loop` (bin "exper-mut-borrowing-in-loop") due to 1 previous error

But if I just don't use the struct in the loop

    let mut data = Data::new(gen_data(ln));
    while data.read_line().is_some() {
        println!("{}: {}", data.line_number, data.line);
    }

or like this

    let mut data = Data::new(gen_data(ln));
    while let Some(_) = data.read_line() {
        println!("{}: {}", data.line_number, data.line);
    }

There is no problem.

It seems to me the problem is the Some(line) more than using data.read_line() or data.line_number. Also the compiler says that line is a mutable borrow in the println! but I think an Option<&str> is immutable, so I'm confused.

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