Relx call to xref should take into account ignore_xref about relx

[acw224]

Some update:
erlang/rebar3#2580
#874

[ferd]

We've communicated by e-mail before on this. In short, we think this is a good idea and something that does indeed need fixing, and greatly appreciate the PRs.

The one bit we're not sure of is the division of responsibility between Relx and Rebar3 in terms of policies and components. The one bit we think needs changing is that we're passing the warning through the Relx state, which sort of piggy-backs on implementation details.

What we think should happen instead is this:

1. extend relx state to accept in a filter function for xref. Make it default to fun(_) -> true end if unspecified
2. keep the relx state weaving the way you build it here, but apply the filter before printing the way it was already printing (within relx). We can at this point merge the Relx PR fine independently, which speaks to the isolation we get
3. adjust the rebar3 PR to take the adjusted filtering function you already wrote, but wrap it in fun(Warn) -> YourCode end and set it in the relx state. You should use a closure to carry whatever rebar3 state you need without relx knowing a thing about the details
4. The xref filtering is now configured by rebar3 knowing its config state but handled by Relx as if nothing special took place. We're essentially using this to tunnel rebar3's config into the relx state to apply it to the xref warnings, rather than tunneling the xref warnings back to rebar3.

Do you think this make sense? It sounds like it shouldn't be too much of a rewrite since many of the components can be kept and shifted around instead.

[acw224]

Thanks for the feedback.

I was able to carry the required modifications pretty easily, except for one little detail. The method filter_xref_results in rebar3
https://github.com/erlang/rebar3/blob/3169392c7d73e6b849fb3117fb76177478c85f38/src/rebar_prv_xref.erl#L183 is a bit more involved than being just a predicate on list.

Therefore, I found it a bit easier to pass the whole filter method as a fun(list()) -> list(), which default to the identity.

[acw224]

On Relx side #882
On rebar3 side erlang/rebar3#2611

[tsloughter]

Resolved on the relx side in #882