@nikomatsakis @arielb1 found ONE (heisen? not really) ICE related to trait objects.
Other places cause the same thing.
I used RUST_LOG=rustc::middle::astencode.
You can grep for 'Encoding inlined item'.
Looks like: DEBUG:rustc::middle::astencode: > Encoding inlined item: middle::expr_use_visitor::ExprUseVisitor<'d, 't, 'tcx, TYPER>::walk_local (Ok(6594124))
...
DEBUG:rustc::middle::astencode: Encoding side tables for id 1337
error: internal compiler error: cannot encode region variables

Then I look for that node using:
make RUSTFLAGS="--pretty expanded,identified" | grep -C50 1337