Impl Error for Box<T: Error>

[GuillaumeGomez]

Fixes #30349

[rust-highfive]

r? @alexcrichton

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[alexcrichton]

I'd like to run crater over this to make sure it doesn't have too much intended breakage first.

[GuillaumeGomez]

Please do! :)

[alexcrichton]

Crater reports one regressions which is spurious, so I think this is good to go.

@bors: r+ 31f7f3a835141605c579d00471ce1bb331f2f192

[alexcrichton]

@bors: r-

er actually, can you define this for T: ?Sized instead of just T?

[GuillaumeGomez]

I didn't because of a problem. Hold on, I try again to see why.

[GuillaumeGomez]

That's the error I get by adding ?Sized:

#[stable(feature = "box_error", since = "1.7.0")]
impl<T: Error + ?Sized> Error for Box<T> {
    fn description(&self) -> &str {
        Error::description(&**self)
    }

    fn cause(&self) -> Option<&Error> {
        Error::cause(&**self)
    }
}

[alexcrichton]

Hm right, that's what I remember getting in the past. I'm gonna hold off on r+'ing for now and tag this with T-libs to get some more discussion.

I'm a little worried that Box<Error> won't implement Error (which one might reasonably expect), but it does seem valuable that Box<MyCustomError> can be used with try! to return a Box<Error> (due to the From impl).

Curious on others' thoughts as well though

[alexcrichton]

cc @rust-lang/libs

[bors]

☔ The latest upstream changes (presumably #31124) made this pull request unmergeable. Please resolve the merge conflicts.

[GuillaumeGomez]

Any news on this?

[alexcrichton]

Unfortunately we haven't had time yet in libs triage to come back around to this, but it's still on the agenda.

[alexcrichton]

The libs team discussed this during triage today and the conclusion was to merge. We may want to ensure that Box<Error> implements Error somehow in the future, but in the meantime it seems like that shouldn't necessarily prevent this sort of impl from existing anyway. Thanks again for the PR @GuillaumeGomez!

@bors: r+ f9f2981

[bors]

⌛ Testing commit f9f2981 with merge 3c9442f...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-mac-ios-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt