rust-lang / rust-lang/rust

Incorrect borrow checker error

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A-borrow-checker C-discussion
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Description

Whe I try to compile this code:

#![feature(ptr_metadata)]

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

pub trait Device {
}

pub struct DeviceMetadata {
    metadata: std::ptr::DynMetadata<dyn Device>,
}

impl DeviceMetadata {
    pub fn new(device: &dyn Device) -> Self {
        let device = device as *const dyn Device;
        //let device = device as *const dyn Device;
        Self {
            metadata: std::ptr::metadata(device),
        }
    }
}

fn main() {
}

I get error:

error[E0521]: borrowed data escapes outside of associated function
  --> src/main.rs:17:23
   |
13 |     pub fn new(device: &dyn Device) -> Self {
   |                ------  - let's call the lifetime of this reference `'1`
   |                |
   |                `device` is a reference that is only valid in the associated function body
...
17 |             metadata: std::ptr::metadata(device),
   |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^
   |                       |
   |                       `device` escapes the associated function body here
   |                       argument requires that `'1` must outlive `'static`

For more information about this error, try `rustc --explain E0521`.

When I uncomment line 15, it works. device is pointer in both situations, so it should work in both cases.

Meta

Tested with nightly here: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024

Also at my computer:

rustc 1.91.0-nightly (6ba0ce409 2025-08-21)
binary: rustc
commit-hash: 6ba0ce40941eee1ca02e9ba49c791ada5158747a
commit-date: 2025-08-21
host: x86_64-unknown-linux-gnu
release: 1.91.0-nightly
LLVM version: 21.1.0

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Research direction

Reproduce the example in src/main.rs or the linked Rust Playground with the reported nightly compiler. Compare the diagnostics for the two pointer-cast forms and read the guidance from rustc --explain E0521. Done means the behavior is explained or corrected and covered by an appropriate compiler regression test.

Written by the indexing model from the issue text.

Assessment

Tech stack
rust
Domain
compilers
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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