Increasingly slow compilation as more levels of async are added

This is a somewhat minimal testcase extracted from a crate I'm working on. I was using Rust 1.45 and it compiled in a reasonable amount of time, then I upgraded to Rust 1.46 and suddenly compilation seemed to hang forever. Based on this more minimal test case I'm guessing it's not actually hanging forever, just taking a very long time.

I've found that the slowness seems tied to the depth of the async call chain. In the example code I have a long call chain: handle_req_1 is called by handle_req_2 is called by handle_req_3, etc. Here's the compilation timing I've observed when changing handle_req_final to directly call one of the handle_req_N functions:

handle_req_1 -> 0m12s
handle_req_2 -> 0m28s
handle_req_3 -> 1m06s
handle_req_4 -> 2m18s
handle_req_5 -> 5m01s

These times only include building the rust146hang lib, not the dependencies.

(Caveat: unscientific timings, these are not averaged over multiple runs or anything.)