Allow missing sections

[cramertj]

Fix #6

Unfortunately, I wasn't able to come up with a simple way to match zero or one instances of a pattern. Because of this, it's possible to write a few pathological errors that will cause infinite recursion rather than a more helpful description.

Example:

#[test]
fn smoke_test_9() {
    error_chain! {
        types {
            Error, ErrorKind, ChainErr, Result;
        }
        types {
            Error, ErrorKind, ChainErr, Result;
        }

        foreign_links { }

        errors {
            HttpStatus(e: u32) {
                description("http request returned an unsuccessful status code")
                display("http request returned an unsuccessful status code: {}", e)
            }
        }
    }
}

Because there are multiple types declarations, they are both combined into a single types declaration which looks like types { Error, ErrorKind, ChainErr, Result; Error, ErrorKind, ChainErr, Result; }, which doesn't match any of the existing rules except the one that produced it.

It is necessary for there to be a rule like this that matches zero or one type blocks with zero or one entry sections, so I don't see how to avoid matching on more than one type block or more than one entry section.

Perhaps an RFC for a macro_rules ? is in order? Do you know why this was excluded to begin with? It seems like a natural extension of * and +.

[brson]

Thanks @cramertj ! It's the end of the day here and I'm beat, but I'll look at this tomorrow (really).

[cramertj]

@brson Sounds good! No rush.

[cramertj]

Ping @brson

[brson]

Thanks! I'll get a build out shortly.