add "gen" to list of matches in 'rust_mangle' #2857

Job	Run time
docs	23s
minimal	16s
msrv	48s
check-cfg	21s
quickchecking	19s
rustfmt-clippy	31s
test-expectations (ubuntu-latest)	48s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	1m 59s
test-expectations (macos-latest)	47s
test-book	9s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	1m 44s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	1m 54s
test-no-headers	33s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	1m 55s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	2m 11s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	2m 12s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	2m 12s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	2m 23s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	4m 3s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	4m 9s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	4m 0s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	4m 1s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	5m 4s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	4m 57s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	5m 6s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	4m 56s
test (ubuntu-latest, 16.0, 0, 0, 1)	2m 40s
test (macos-latest, 16.0, 0, 0, 0)	1m 57s
success	1s
	1h 2m 19s

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