Overly restrictive lifetime in `std::panic::Location::file`

[ijchen]

The problem

The signature of std::panic::Location::file is currently:

pub struct Location<'a> {
    file: &'a str,
    // ...
}

impl<'a> Location<'a> {
    pub const fn file(&self) -> &str
}

which, with lifetimes expanded, is:

pub const fn file<'b>(self: &'b Location<'a>) -> &'b str

This makes the following impossible:

let location: Location<'static> = unimplemented!();

let file: &'static str = location.file();
//        ------------   ^^^^^^^^ borrowed value does not live long enough
//        |
//        type annotation requires that `location` is borrowed for `'static`

The solution

As far as I can tell, this should be a trivial fix - literally just adding the 'a lifetime to the return type:

impl<'a> Location<'a> {
    pub const fn file(&self) -> &'a str
}

Unfortunately, I also believe this is technically a breaking change (playground):

fn uh_oh<'x>() {
    let _: for<'a> fn(&'a Location<'x>) -> &'a str = Location::<'x>::file;
}

error: lifetime may not live long enough
  --> src/lib.rs:22:12
   |
21 | fn uh_oh<'x>() {
   |          -- lifetime `'x` defined here
22 |     let _: for<'a> fn(&'a Location<'x>) -> &'a str = Location::<'x>::file;
   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type annotation requires that `'x` must outlive `'static`

Looking for mentor

I would love to get more experience contributing to rust, so if someone would be willing to guide me through the process, I'd love to implement the fix myself, and go through the whole merge + review process to get more comfortable contributing.

I imagine the first step would be to consult the library or library API team on the path forward considering potential breakage ("crater run"? deprecate and make a file2?), then implementation, review, and merge - but I'm new to this, and would appreciate a mentor to guide me through the process 😄

[traviscross]

The caller method has the signature:

impl<'a> Location<'a> {
    pub const fn caller() -> &'static Location<'static>;
}

This presumably could have used the 'a lifetime here also but we went with 'static instead.

Anyone know, what are the tradeoffs here, and should file return 'a or 'static (as caller does)?

[traviscross]

This was discussed in the libs-api call today. It resulted in research into the question of "why does Location have a lifetime at all?". That research didn't yield a lot of satisfying answers. Anyone here know how that came to be?

Overall, people were happy to see this change happen, and @the8472 volunteered to help mentor this.

[the8472]

I would love to get more experience contributing to rust, so if someone would be willing to guide me through the process, I'd love to implement the fix myself, and go through the whole merge + review process to get more comfortable contributing.

General project setup instructions are in the rustc dev guide: https://rustc-dev-guide.rust-lang.org/building/quickstart.html
std-specific information can be found in https://std-dev-guide.rust-lang.org/

This change in particular is an API change, so our API stability rules apply.
Due to that the <'a> on the struct itself can't be removed.
But we can probably change the signature of file(), though this will require a crater run to check ecosystem impact. And then an FCP to get team approval for the API change.

If you have more questions you can ask on our zulip channel or in the PR.

This was discussed in the libs-api call today. It resulted in research into the question of "why does Location have a lifetime at all?". That research didn't yield a lot of satisfying answers. Anyone here know how that came to be?

Investigating this (digging through git history and associated issues) might be helpful to provide a stronger justification, but shouldn't be necessary if the team is on board.
It's possibly just a historic artifact from how things were implemented at some point and how the implementation evolved to also support caller().

[ijchen]

Thanks for the update T-libs-api, and @the8472 thank you very much for offering to mentor me! I will spend the next few days setting up rustc, reviewing the linked std-specific resources, digging into the origins of the lifetime, and then make a PR and do a crater run to check impact. Thanks all!