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.