Support optional Error::source (#426)

[tyranron]

Resolves #426

Synopsis

See #426 (comment):

I’d like to be able to use derive_more with optional sources, like:

#[derive(derive_more::Error, derive_more::Display, Debug)]
#[display("it’s an error")]
struct MyErr {
    source: Option<Box<dyn std::error::Error + Send + Sync + 'static>>,
}

Solution

The way, the thiserror it does, is by checking the type name to contain Option. Relying on type could be fragile, as the call site could introduce its own type named Option in the scope where macro is called. Ideally, we need to aid this case with type machinery.

Checklist

• Documentation is updated
• Tests are added/updated
• CHANGELOG entry is added