Macro write_err_unchecked_source 

macro_rules! write_err_unchecked_source {
    ($writer:expr, $string:literal $(, $args:expr)*; $source:expr) => { ... };
}

Formats error.

Unlike write_err doesn’t statically check that $source implements std::error::Error. This should be used only in rare generic situations. Specifically, in case of a generic MyError<Error>, this macro should be used when formatting an error message for the generic type Error.

If std feature is OFF appends error source (delimited by : ). We do this because e.source() is only available in std builds, without this macro the error source is lost for no-std builds.