Tracking Issue for Reborrow trait lang experiment

This is a tracking issue for the lang experiment into a Reborrow trait and its dual CoerceShared trait.
The feature gate for the issue is #![feature(reborrow)].

For more information, see rust-lang/goals#399.

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.
Discussion comments will get marked as off-topic or deleted.
Repeated discussions on the tracking issue may lead to the tracking issue getting locked.

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Unresolved Questions

The implementation of the Reborrow trait is likely something like this:

struct Example<'a, 'b> {
  a: A<'a>,
  b: B<'b>
}

fn reborrow(e: Example) {
  let Example { a, b } = e;
  reborrow(Example { a, b });
}

This allows the compiler to see the individual field usages and, eventually, enables the correct lifetimes to be "used as exclusive". But the question is; where does this bottom out? Is any field that implements Reborrow (but not Copy, if Reborrow is blanket-implemented for Copy types) a point where the recursion into structs stops? That stops composing of reborrowable types to a depth of 2. Is a PhantomReborrowable<'a> marker type needed? If so, then what is the variance of the lifetime there?

Implementation history

👍 for a lang experiment here.

I'm happy to champion this.

Rollup merge of rust-lang#145726 - aapoalas:reborrow-lang-experiment,…

Is supporting Reborrow/CoerceShared for method receivers without UFCS just a matter of doing the implementation work? I was just playing with the implementation on current nightly and noticed that I need UFCS to make code like this work:

struct Foo<'a>(PhantomData<&'a ()>);
#[derive(Clone, Copy)]
struct FooRef<'a>(PhantomData<&'a ()>);

impl FooRef<'_> {
    fn eat_foo_ref(self) {
    }
}

impl Foo<'_> {
    fn eat_foo(self) {
    }
}

impl<'a> Reborrow for Foo<'a> {}

impl<'a> CoerceShared<FooRef<'a>> for Foo<'a> {}

fn main() {
    let x = Foo(PhantomData);
    
    let _works = (<Foo>::eat_foo(x), <FooRef>::eat_foo_ref(x));
    // let _coerce_shared_does_not_work = x.eat_foo_ref();
    // let _reborrow_does_not_work = (x.eat_foo(), <FooRef>::eat_foo_ref(x));
}

But I cannot find any mention of this not working.

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=a573cbc6ecc099044f89f98b0b958f0f

Is supporting Reborrow/CoerceShared for method receivers without UFCS just a matter of doing the implementation work? I was just playing with the implementation on current nightly and noticed that I need UFCS to make code like this work:

We haven't really gotten around to looking at this side of the equation yet - so far the compiler only inserts Reborrow/CoerceShared at assignment locations, but the method call location is not one. This ties in with #44874 and might (or probably will) affect the method resolution algorithm, which is a bit of a ... complication. As such, unfortunately for now this is unsupported.

If you're interested in looking into this, I'd be happy to help (with what meager knowledge I have)!

fix: UserRef soundness

Rollup merge of rust-lang#156338 - P8L1:derive-reborrow-coerce-shared…

Unrolled build for #156338

Rollup merge of #151753 - aapoalas:aapoalas/reborrow-and-coerce-share…

#145612 (comment)

Is supporting Reborrow/CoerceShared for method receivers without UFCS just a matter of doing the implementation work? […]

My PR #163494 partially fixes this—it ensures that a method call on a Reborrow type performs a reborrow.

Rollup merge of rust-lang#163519 - Jules-Bertholet:no-reborrow-destru…

Rollup merge of rust-lang#151753 - aapoalas:aapoalas/reborrow-and-coe…

Rollup merge of rust-lang#163519 - Jules-Bertholet:no-reborrow-destru…

Rollup merge of #151753 - aapoalas:aapoalas/reborrow-and-coerce-share…

Unrolled build for #163519 in rollup 163583

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Tracking Issue for Reborrow trait lang experiment #145612

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Tracking Issue for Reborrow trait lang experiment #145612

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joshtriplett commented on Aug 20, 2025

tmandry commented on Aug 21, 2025

28 remaining items

tvsfx commented on Jul 8, 2026

aapoalas commented on Jul 8, 2026

Jules-Bertholet commented on Sep 29, 2026

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