Implementing Reborrow prevents normally moving a value of that type.

@aapoalas

I tried this code:

#![feature(reborrow)]

use std::marker::Reborrow;

struct Thing<'a>(&'a ());

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

fn foo<'a>(x: Thing<'a>) -> Thing<'a> {
    x
}

I expected this code to compile, since it's just an identity function. Instead, I got the following error:

    Checking foo v0.1.0 (/Users/timch/foo)
error[E0515]: cannot return reference to temporary value
  --> src/lib.rs:9:39
   |
 9 |   fn foo<'a>(x: Thing<'a>) -> Thing<'a> {
   |  _______________________________________^
10 | |     x
11 | | }
   | |_^ returns a reference to data owned by the current function

error[E0515]: cannot return value referencing function parameter `x`
  --> src/lib.rs:9:39
   |
 9 |   fn foo<'a>(x: Thing<'a>) -> Thing<'a> {
   |  _______________________________________^
10 | |     x
   | |     - `x` is borrowed here
11 | | }
   | |_^ returns a value referencing data owned by the current function

For more information about this error, try `rustc --explain E0515`.
error: could not compile `foo` (lib) due to 2 previous errors

cc @aapoalas

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rustc --version --verbose:

rustc 1.97.0-dev
binary: rustc
commit-hash: unknown
commit-date: unknown
host: aarch64-apple-darwin
release: 1.97.0-dev
LLVM version: 22.1.4

Compiled from the main branch at commit 63b1dfc.

Just ran into a variant of this: the same issue occurs when creating and returning Thing inside a function (playground)

#![feature(reborrow)]

use std::marker::Reborrow;

struct Thing<'a>(&'a ());

// Compiles when this line is commented out
impl<'a> Reborrow for Thing<'a> {}

fn make_thing(x: &'static ()) -> Thing<'static> {
    Thing(x)
}

fn make_up_thing() -> Thing<'static> {
    Thing(&())
}

Compiler error

error[E0515]: cannot return value referencing temporary value
  --> src/lib.rs:9:49
   |
 9 |   fn make_thing(x: &'static ()) -> Thing<'static> {
   |  _________________________________________________^
10 | |     Thing(x)
   | |     -------- temporary value created here
11 | | }
   | |_^ returns a value referencing data owned by the current function

error[E0515]: cannot return reference to temporary value
  --> src/lib.rs:9:49
   |
 9 |   fn make_thing(x: &'static ()) -> Thing<'static> {
   |  _________________________________________________^
10 | |     Thing(x)
11 | | }
   | |_^ returns a reference to data owned by the current function

error[E0515]: cannot return value referencing temporary value
  --> src/lib.rs:13:38
   |
13 |   fn make_up_thing() -> Thing<'static> {
   |  ______________________________________^
14 | |     Thing(&())
   | |     ---------- temporary value created here
15 | | }
   | |_^ returns a value referencing data owned by the current function

error[E0515]: cannot return reference to temporary value
  --> src/lib.rs:13:38
   |
13 |   fn make_up_thing() -> Thing<'static> {
   |  ______________________________________^
14 | |     Thing(&())
15 | | }
   | |_^ returns a reference to data owned by the current function

fix: UserRef soundness

This looks like to have been fixed by #159103.

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Implementing `Reborrow` prevents normally moving a value of that type. #156310

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Implementing Reborrow prevents normally moving a value of that type. #156310

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tvsfx commented on Jul 21, 2026

sleepycatcoding commented on Sep 6, 2026

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Implementing `Reborrow` prevents normally moving a value of that type. #156310