Unsoundness in Polonius involving impl Trait

The following code was created based on #160979 with trial and error. It compiles in nightly and segfaults. It doesn't compile with -Zpolonius=off.

type Payload = Box<i32>;

trait Produce<'a> {
    fn produce(self) -> &'a &'a Payload;
}
impl<'a> Produce<'a> for &'a &'a Payload {
    fn produce(self) -> &'a &'a Payload {
        self
    }
}

trait Outlives<'a, 'b> {
    fn shorten(x: impl Produce<'a> + 'a) -> impl Produce<'a> + 'b;
}
impl<'a: 'b, 'b> Outlives<'a, 'b> for () {
    fn shorten(x: impl Produce<'a> + 'a) -> impl Produce<'a> + 'b {
        x
    }
}

fn make_producer<'a, 'b>(payload: &'a &'b Payload) -> impl Produce<'a> + 'b + use<'a, 'b>
where
    (): Outlives<'a, 'b>,
{
    <()>::shorten(payload)
}

fn main() {
    let wrong: &&Payload;
    let a = Box::new(1);
    {
        let mut r: &Payload = &a;
        let x: &&Payload = &r;
        wrong = make_producer(x).produce();
        r = &*Box::new(Box::new(2));
    }
    println!("{wrong}");
}

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Reproducible on the playground with version 1.99.0-nightly (2026-08-12 c98d0cb27cc63afdd626)

Simplified a bit:

type Payload = Box<i32>;

trait Produce<'a> {
    fn produce(self) -> &'a Payload;
}
impl<'a> Produce<'a> for &'a Payload {
    fn produce(self) -> &'a Payload {
        self
    }
}

trait Outlives<'a, 'b> {
    fn shorten(x: impl Produce<'a> + 'a) -> impl Produce<'a> + 'b;
}
impl<'a: 'b, 'b> Outlives<'a, 'b> for () {
    fn shorten(x: impl Produce<'a> + 'a) -> impl Produce<'a> + 'b {
        x
    }
}

fn make_producer<'a, 'b>(payload: &'a Payload) -> impl Produce<'a> + 'b + use<'a, 'b>
where
    (): Outlives<'a, 'b>,
{
    <()>::shorten(payload)
}

fn main() {
    let wrong: &Payload;
    {
        let x: Box<Payload> = Box::new(Box::new(1));
        let r: &Payload = &*x;
        wrong = make_producer(r).produce();
    }
    println!("{wrong}");
}

Surprisingly, annotating the type in the above code as let wrong: &'static Payload still results in code that compiles. Despite this, using a turbofish like make_producer::<'static, '_> or make_producer::<'_, 'static> results in a compile error.

There are a few more ways to cause unsoundness found at #t-types/polonius > Reporting a few Polonius bugs @ 💬 (listed as "bug 2" through "bug 7"), which might or might not have the same root cause.

Rollup merge of rust-lang#164168 - jackh726:alias-liveness-unsoundnes…

Don't test a type argument for alias liveness

Rollup merge of rust-lang#164168 - jackh726:alias-liveness-unsoundnes…

Unrolled build for #164168 in rollup 165023

Rollup merge of #164168 - jackh726:alias-liveness-unsoundness-tests, …

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Unsoundness in Polonius involving impl Trait #161038

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Unsoundness in Polonius involving impl Trait #161038

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theemathas commented on Aug 13, 2026

theemathas commented on Aug 13, 2026

theemathas commented on Aug 21, 2026

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