Hmm, came across this (more general) idea:

fn recycle_vec<A, B>(mut v: Vec<A>) -> Vec<B> {
    const {
        use std::alloc::Layout;

        let la = Layout::new::<A>();
        let lb = Layout::new::<B>();

        assert!(la.size() == lb.size(), "Size of types must match");
        assert!(la.align() == lb.align(), "Alignment of types must match");
    }

    v.clear();

    // SAFETY: vector cleared, no elements affected, layout equal
    unsafe { std::mem::transmute(v) }
}

Artificial usage example:

    let mut vec = Vec::with_capacity(10);

    loop {
        let s = String::from("a b c");
        vec.extend(s.split(' '));

        vec = recycle_vec(vec);
    }

What do you think?

Maybe an extension trait will beneficial: There are other candidates, like HashMap etc. This problem occurs occasionally in different forms, for example: Please help me let me keep my allocation

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