Macro coercible_errors 

macro_rules! coercible_errors {
    () => { ... };
    ($error: ty) => { ... };
    ($error: ty, $coercible_with: ident, $coerced_error: ident, $coerced_result: ident) => { ... };
}

Sets up coercible_errors for a previously defined error type.

It re-exports the types Never and OkResult, and defines three new traits and types CoercibleWith, CoercedError and CoercedResult.