Mutable borrow in a loop

Question

I have the following code:

struct Baz {
    x: usize,
    y: usize,
}

struct Bar {
    baz: Baz,
}

impl Bar {
    fn get_baz_mut(&mut self) -> &         


        

Answer 1

It does not work because returning a borrowed value extends the borrow to the end of the function.

See here for some useful details.

This works with non-lexical lifetimes with the 1.27 nightly version:

#![feature(nll)]

struct Baz {
    x: usize,
    y: usize,
}

// ...

The non-lexical lifetimes RFC explains the actual working of lifetimes:

Problems arise however when you have a reference that spans multiple statements. In that case, the compiler requires the lifetime to be the innermost expression (which is often a block) that encloses both statements, and that is typically much bigger than is really necessary or desired

rustc nightly 1.28

As pointed out by @pnkfelix, the non-lexical lifetimes implementation starting from nightly 1.28 no longer compiles the above code.

There is however a long-term plan to (re)-enable a more powerful NLL analysis.