Borrowed value does not live long enough beta regression

[marmeladema]

Code

I tried this code:

use std::collections::BTreeMap;

struct Context<'e> {
    _data: &'e [u8],
}

pub struct Rule<'s> {
    _rule: Box<dyn for<'e> Fn(&'e Context<'e>) -> bool + Sync + Send + 's>,
}

pub struct Scheme {}

pub struct Parser<'s> {
    _scheme: &'s Scheme,
}

impl<'s> Parser<'s> {
    pub fn new(_scheme: &'s Scheme) -> Self {
        Self { _scheme }
    }
}

#[derive(Default)]
pub struct Config<'s> {
    _data: BTreeMap<String, Rule<'s>>,
}

impl<'s> Config<'s> {
    pub fn update(&mut self, _: Parser<'s>) {}
}

pub fn main() {
    let mut configuration = Config::default();
    let scheme = Scheme {};
    let parser = Parser::new(&scheme);

    configuration.update(parser);
}

I expected to see this happen: it compiles successfully

Instead, this happened:

error[[E0597]](https://doc.rust-lang.org/beta/error-index.html#E0597): `scheme` does not live long enough
  --> src/main.rs:35:30
   |
35 |     let parser = Parser::new(&scheme);
   |                              ^^^^^^^ borrowed value does not live long enough
...
38 | }
   | -
   | |
   | `scheme` dropped here while still borrowed
   | borrow might be used here, when `configuration` is dropped and runs the destructor for type `Config<'_>`
   |
   = note: values in a scope are dropped in the opposite order they are defined

For more information about this error, try `rustc --explain E0597`.

Beta: https://play.rust-lang.org/?version=beta&mode=debug&edition=2021&gist=3f293aff3e5dc16ad303940d4613514a
Stable: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3f293aff3e5dc16ad303940d4613514a

Version it worked on

It most recently worked on: 1.63.0 (but probably before too)

$ rustc --version --verbose
rustc 1.63.0 (4b91a6ea7 2022-08-08)
binary: rustc
commit-hash: 4b91a6ea7258a947e59c6522cd5898e7c0a6a88f
commit-date: 2022-08-08
host: x86_64-unknown-linux-gnu
release: 1.63.0
LLVM version: 14.0.5

Version with regression

rustc --version --verbose:

$ rustc +beta --version --verbose
rustc 1.64.0-beta.6 (25912c097 2022-09-09)
binary: rustc
commit-hash: 25912c09710a2de2a97b4904e9b528b73439927d
commit-date: 2022-09-09
host: x86_64-unknown-linux-gnu
release: 1.64.0-beta.6
LLVM version: 14.0.6

This might be an acceptable breakage but we faced this regression in production code so I figured it might be worthwhile creating a bug report for this in case it was an unnoticed regression.

@rustbot modify labels: +regression-from-stable-to-beta -regression-untriaged

[marmeladema]

Creating scheme before configuration solves the issue but it is still a regression between stable and beta (and nightly)

[lukas-code]

This was caused by #99413.
See also #100544.

[marmeladema]

Thank you for your fast reply @lukas-code!
Its unfortunate that a fix like this lead to a backward incompatible change.
Error reporting is already awesome but a note in the error message saying this was previously accepted but isn't anymore would have been awesome 👍

[jackh726]

Closing as duplicate of #100544