在Rust中结束可变借用的选项有哪些？

Question

I'm strugglin' with the borrow checker - wonder o' wonder.

While I found a solution by adding a block, I am curious if there are other ways to end a mutable borrow so the next statement can access a binding afterwards.

Here's what I did so far:

let mut canvas: Canvas = Canvas {
    width: 5,
    height: 5,
    array: vec!['x'; 5*5],
};

{
    let mut renderer: CanvasRenderer = CanvasRenderer::new(&mut canvas);

    renderer.render_point('x', 3, 3);
}

println!("The Value in the array is: {}", canvas.array[9]);

I wrap a block around the binding of a CanvasRenderer object and after mutating the canvas and the scope ends, the CanvasRenderer dies and my mutable borrowed canvas is available to be read or whatever.

This works - but now I'd like to see other solutions!

I heard about drop(stuff) but it did not work as I thought it should.

Answer 1

There is no other way; using blocks is the way to do it. Before Rust 2018 (available in Rust 1.31) all borrows are lexical, that is, they always correspond to some lexical scope. The only scope which is larger than a single statement is that of a block, so blocks are your only tool to limit borrow scopes.

drop() would not work for two reasons: first, because it would require non-lexical scope which is not supported before Rust 2018, and second, it cannot be a general-purpose tool for managing borrows: for example, it wouldn't be able to end an immutable borrow simply because immutable references are Copy and can't be "dropped".

See also:

• Moved variable still borrowing after calling `drop`?
• What are non-lexical lifetimes?