Slow coherence checking and type checking.

I was working on creating a crate and encountered some unexpectedly long compile times.
I don't have a minimal reproduction, but I have expanded my macros, and copied the code into a playground.

It occurs on all channels. It is still pretty bad even with the parallel front-end enabled. According to time-passes, the biggest culprits were coherence checking and type checking.

time:   6.038; rss:   80MB ->  272MB ( +192MB)  coherence_checking
time:   6.084; rss:   80MB ->  274MB ( +195MB)  type_check_crate

My use case is clearly atypical, so I will understand if you consider this frivolous and close it. (Although, when I searched to see if my issue had already been reported, I did notice submitted situations that were arguably even less likely to be encountered by anyone.)

Start with the linked Rust Playground example and reproduce the compile-time and memory behavior across compiler channels, using time-passes to inspect coherence checking and type_check_crate. Reduce the expanded macro case if possible and compare it with parallel front-end enabled; done means identifying a reproducible cause or a focused minimal reproduction for the slow checks.