an easy way to impl Default

[tshepang]

No description provided.

[nikomatsakis]

I was going to close this PR, but actually @tshepang maybe you want to replace it with one that adds a comment for future folks to explain why the impl is there?

[tshepang]

Why don't we want Default for this case (a question for my curiosity and for the upcoming PR)?

As an aside, I should have seen #49.

[nikomatsakis]

@tshepang thanks! unfortunately this has a merge conflict... and, actually, with @csmoe's changes in #127, we actually probably can derive(Default) now

[tshepang]

ok, will wait for integration into rustc to test the derive(Default)

[nikomatsakis]

I think @tshepang we should be able to derive-default now, with perhaps some minor changes.

[tshepang]

Is that more like it?

[tshepang]

Ok, it still results in a failing rustc build (with these changes)...

error[E0599]: no function or associated item named `default` found for type `polonius_engine::facts::AllFacts<borrow_check::nll::facts::RustcFacts>` in the current scope
   --> src/librustc_mir/borrow_check/nll/mod.rs:175:24                       
    |                                                                            
175 |         Some(AllFacts::default())                                                      
    |                        ^^^^^^^ function or associated item not found in `polonius_engine::facts::AllFacts<borrow_check::nll::facts::RustcFacts>`
    |                                                                  
    = note: the method `default` exists but the following trait bounds were not satisfied:
            `polonius_engine::facts::AllFacts<borrow_check::nll::facts::RustcFacts> : std::default::Default`

I will let this one rest.