Why is this borrow still “active”?

13

This is a simplified example of something I've encountered:

trait Bla<'a> {
    fn create(a: &'a Foo) -> Self;
    fn consume(self) -> u8;
}

struct Foo;
impl Foo {
    fn take(&mut self, u: u8) {}
}

struct Bar {
    foo: Foo,
}

impl Bar {
    fn foobar<'a, 'b, B: Bla<'b>>(&'a mut self)
    where 'a: 'b {
        let u = {
            // immutable borrow occurs here
            // type annotation requires that `self.foo` is borrowed for `'b`
            let foo: &'b Foo = &self.foo;
            let b = B::create(foo);
            b.consume()
        };

        // error[E0502]: cannot borrow `self.foo` as mutable because it is also borrowed as immutable
        self.foo.take(u);
    }
}

Why is self.foo still considered to be borrowed even after exiting the block it was borrowed on, with everything that used the borrow also dropped?

rust

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asked Apr 18 at 9:38

Tom FrankTom Frank

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