macro_rules! whatever {
    ($fmt:literal$(, $($arg:expr),* $(,)?)?) => { ... };
    ($source:expr, $fmt:literal$(, $($arg:expr),* $(,)?)*) => { ... };
}Expand description
Propagates an error, adding formatted context.
- whatever!("msg")returns- Err(format_err!(...)).
- whatever!(source, "msg {x}", x)unwraps- sourceor returns with context.