feat: add custom attributes to `fn` items #2949

Job	Run time
msrv	22s
minimal	10s
docs	17s
check-cfg	22s
quickchecking	14s
rustfmt-clippy	24s
test (ubuntu-latest, 9.0, 0, 0, 0, 0)	44s
test-expectations (ubuntu-latest)	43s
test (ubuntu-latest, 9.0, 0, 0, 1, 0)	43s
test-book	13s
test-expectations (macos-latest)	53s
test (ubuntu-latest, 9.0, 0, 1, 0, 0)	50s
test (ubuntu-latest, 9.0, 0, 1, 1, 0)	43s
test-no-headers	20s
test (ubuntu-latest, 9.0, 1, 0, 0, 0)	51s
test (ubuntu-latest, 9.0, 1, 0, 1, 0)	56s
test (ubuntu-latest, 9.0, 1, 1, 0, 0)	56s
test (ubuntu-latest, 9.0, 1, 1, 1, 0)	48s
test (ubuntu-latest, 16.0, 0, 0, 0, 0)	45s
test (ubuntu-latest, 16.0, 0, 0, 1, 0)	46s
test (ubuntu-latest, 16.0, 0, 1, 0, 0)	47s
test (ubuntu-latest, 16.0, 0, 1, 1, 0)	45s
test (ubuntu-latest, 16.0, 1, 0, 0, 0)	44s
test (ubuntu-latest, 16.0, 1, 0, 1, 0)	46s
test (ubuntu-latest, 16.0, 1, 1, 0, 0)	44s
test (ubuntu-latest, 16.0, 1, 1, 1, 0)	44s
test (ubuntu-latest, 16.0, 0, 0, 1)	46s
test (macos-latest, 16.0, 0, 0, 0)	48s
success	1s
	18m 5s

Provide feedback