What is the most idiomatic way to merge two error types?

Question

I have a type Foo whose methods may "raise" errors of an associated type Foo::Err.

pub trait Foo {
    type Err;
    
             


        

Answer 1

Typically you don't do a "merge", but instead use nested errors, like this.

enum IntError {
    Overflow,
    Underflow
}

enum StrError {
    TooLong,
    TooShort,
}

enum GenericError {
    Int(IntError),
    Str(StrError),
}

impl From for GenericError {
    fn from(e: IntError) -> Self {
        GenericError::Int(e)
    }
}

impl From for GenericError {
    fn from(e: StrError) -> Self {
        GenericError::Str(e)
    }
}