Compiler performance returning nested closures

[197g]

The following takes approximately one year to compile:

fn main() {
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||();
}

(This estimate was derived from observing that compile time scales seemingly exponentially in the depth of the instantiated closure. At level 23 it took about 25 seconds, and at 22 only 13. Extrapolating this doubling-per-depth leads us to about a year at level 43 which is depicted above).

[tesuji]

Flamegraph: https://gist.github.com/lzutao/3e900642d43ec9bad186c74b3028902d

[jonas-schievink]

fn main() {
||||||||||||||||||||||||||||||||||||||||||||();
}

Yields:

+-----------------------------------------------+-----------+-----------------+----------+------------+------------+--------------+-----------------------+
| Item                                          | Self time | % of total time | Time     | Item count | Cache hits | Blocked time | Incremental load time |
+-----------------------------------------------+-----------+-----------------+----------+------------+------------+--------------+-----------------------+
| mir_borrowck                                  | 8.64s     | 98.672          | 21.61s   | 23         | 0          | 0.00ns       | 0.00ns                |

cc @matthewjasper