Methods take an insanely long time to compile

[tomaka]

This commit takes an insane amount of time to compile. Before the commit, the crate takes about 15 seconds to compile. After the commit, I stopped after around 15 minutes. EDIT: estimates would be about 2 hours.

I've isolated the problem to this code. These are two methods implemented on tuples. They should expand to roughly something like this:

let &(first, B, C, D, E, F, G, H) = self;
let iter = first.buffers_list();
let iter = iter.chain(B.buffers_list());
let iter = iter.chain(C.buffers_list());
let iter = iter.chain(D.buffers_list());
let iter = iter.chain(E.buffers_list());
let iter = iter.chain(F.buffers_list());
let iter = iter.chain(G.buffers_list());
let iter = iter.chain(H.buffers_list());
Box::new(iter)

If I reduce the maximum size of the tuple here to only 16 elements, the item-bodies checking pass takes about 11 seconds. With 17 elements, it takes about 18 seconds. With 18 elements, it takes about 33 seconds. With 19 elements, about 60 seconds.

cc @eddyb

[eddyb]

I think this has to do with the depth of the nested Chain type.
cc @arielb1 @nikomatsakis Is it likely that the new trait system will solve this?

[durka]

Is this #22204?

[nikomatsakis]

Nominating for triage. Seems likely to be related to #22204, haven't had time to investigate.

[nikomatsakis]

Another issue that is basically the same: #38528

[nikomatsakis]

Going to close in favor of #38528, I'll copy the example there.