Macro write_err 

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

Formats 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.

Note: will not compile on std if $source doesn’t implement std::error::Error. This is intentional because if it doesn’t the macro MUST NOT be used. However this check is very rarely imprecise so if you know what you’re doing you can use the unchecked version.