Custom Source implementation when using derive macro

[philliard3]

Used existing macro code as a basis for implementing a source option for the derive macro. The idea is to allow users to derive their own Source using the provided trait, which was previously missing from derive.

This work was started based on the need expressed in issue #222 . Please let me know if anything seems incorrect or anything more needs to be done.

[maciejhirsz]

Better late than never, eh? :)

[edko99]

Would it please be possible to implement Source for ropey::Rope?
I've tried, but got stuck because ropey::RopeSlice takes a lifetime parameter, so I cannot assign it to the Slice associated type.

[maciejhirsz]

@edko99 I think so. Since I made the trait GATs got stabilizied, so I reckon we could change:

pub trait Source {
    type Slice: ?Sized + PartialEq + Eq + Debug;

    ...
}

To:

pub trait Source {
    type Slice<'source>: PartialEq + Eq + Debug;
}

The bigger issue is that Logos expects to be able to access continuous chunks of memory (there are hidden methods that are used just by the macro for that purpose), but I think it should still be doable by getting the first non-zero sized chunk of the RopeSlice.