Macro adhocerr::wrap

macro_rules! wrap {
    ($msg:literal) => { ... };
    ($fmt:literal, $($arg:tt)*) => { ... };
}

Thinly wrap an error by defining a hidden error type and returning a closure to construct it

Examples

Wrap an error without changing its size or allocating:

use adhocerr::wrap;

fn record_success() -> Result<(), impl Error + 'static> {
    std::fs::write(".success", "true").map_err(wrap!("Failed to save results of script"))
}

Which expands to:

fn record_success() -> Result<(), impl Error + 'static> {
    std::fs::write(".success", "true").map_err({
        #[derive(Debug)]
        struct WrappedError<E> {
            source: E,
        }

        impl<E> std::error::Error for WrappedError<E>
        where
            E: std::error::Error + 'static,
        {
            fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
                Some(&self.source)
            }
        }

        impl<E> core::fmt::Display for WrappedError<E>
        where
            E: std::error::Error + 'static,
        {
            fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
                f.write_str("Failed to save results of script")
            }
        }

        |source| WrappedError { source }
    })
}

Wrapping an error with an runtime generated String:

use adhocerr::wrap;

fn record_success(file: &Path) -> Result<(), impl Error + 'static> {
    std::fs::write(file, "true").map_err(wrap!(
        "Failed to save results of script to file: {}",
        file.display()
    ))
}

Which expands to:

fn record_success(file: &Path) -> Result<(), impl Error + 'static> {
    std::fs::write(file, "true").map_err(|source| {
        adhocerr::private::format_wrap_err(
            source,
            format_args!(
                "Failed to save results of script to file: {}",
                file.display()
            ),
        )
    })
}