fix: better compatibility with older AST (#175) #677

Job	Run time
doctest	1m 55s
feature-checks	3m 59s
docs	1m 17s
clippy	1m 25s
fmt	15s
test stable (ubuntu-latest)	1m 29s
test stable --all-features (ubuntu-latest)	2m 40s
test 1.70 (ubuntu-latest)	1m 8s
test stable (macos-latest)	1m 50s
test stable --all-features (macos-latest)	3m 19s
test 1.70 (macos-latest)	1m 30s
test stable (windows-latest)	3m 50s
test stable --all-features (windows-latest)	5m 38s
test 1.70 (windows-latest)	2m 46s
	33m 1s

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