Tracking Issue for `io_error_downcast`

[NobodyXu]

Feature gate: #![feature(io_error_downcast)]

This is a tracking issue for new API std::io::Error::downcast.

Existing APIs requires two separate calls to obtain the raw os error or the inner error and they both return Option.

Thus, users would have to first call Error::get_ref to check that we indeed has an inner error and checked that it is the type we expected.

Users cannot use std::io::Error::into_inner becuase takes the error by value and returns Option<Box<dyn Error + Send + Sync>> instead of Result<Box<dyn Error + Send + Sync>, Self>.

They also cannot workaround this issue by calling Error::raw_os_error, since the io::Error can also be constructed using impl From<ErrorKind> for Error or using other internal methods only accessible by the std library.

This new feature io_error_downcast solved this issue by providing new API std::io::Error::downcast to downcast the inner error easily without unwrap or expect.

Public API

// std::io::Error

impl Error {
    fn downcast<E: std::error::Error + Send + Sync + 'static>(self) -> Result<E, Self>;
}

Steps / History

• API Change Proposal: Add new API try_downcast_inner to std::io::Error libs-team#57
• Implementation: Add new unstable API downcast to std::io::Error #98387
• Update std::io::Error::downcast return type and doc: Update std::io::Error::downcast return type #120117
• Final comment period (FCP)¹
• Stabilization PR Stablise io_error_downcast #124076

Unresolved Questions

• None yet.

Footnotes

1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

[NobodyXu]

@rust-lang/libs-api I'd like to push for stabilisation of this feature.

The only thing I'm not sure about is the return valueResult<Box<E>, Self>
Returning a Box here might expose too much of an internal detail, in case the internals of io::Error change in future to use something like a thin box.

[NobodyXu]

Emmm it seems I can't at libs-api team...

[NobodyXu]

r? @thomcc

[NobodyXu]

"r?" doesn't seem to work for me...
Is that because I don't have the permission to do so?

[NobodyXu]

@yaahc I'd want to stablise the feature and I'd like your feedback.
IMHO there's nothing blocking the stablisation.

[NobodyXu]

@yaahc Pinging

[NobodyXu]

Stablisation Report

Implementation History

• std::io::Error::downcast implemented in Add new unstable API downcast to std::io::Error #98387

API Summary

A summary of the API is included in the description of this issue.

Experience Report

n/a

[NobodyXu]

@yaahc seems to be on away from github
@joshtriplett Can we have a fcp poll for this?

[NobodyXu]

r? libs-api

[NobodyXu]

Hmmm, sorry for the @ mentions, I didn't know how to proper request for stablisation.