Add `is_safe` calls to generated return value #996

Job	Run time
benchcomp-tests	4m 19s
documentation	2m 19s
perf	31m 38s
regression (macos-13)	1m 23s
write-json-symtab-regression	18m 13s
regression (ubuntu-20.04)	1m 10s
regression (ubuntu-22.04)	1m 14s
regression (macos-14)	1m 27s
	1h 1m 43s

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