--- a/rust/hg-core/Cargo.toml	Fri Sep 30 09:05:48 2022 -0600
+++ b/rust/hg-core/Cargo.toml	Mon Oct 10 14:48:39 2022 +0100
@@ -35,6 +35,9 @@
 memmap2 = { version = "0.5.3", features = ["stable_deref_trait"] }
 zstd = "0.5.3"
 format-bytes = "0.3.0"
+# once_cell 1.15 uses edition 2021, while the heptapod CI 
+# uses an old version of Cargo that doesn't support it.
+once_cell = "=1.14.0"
 
 # We don't use the `miniz-oxide` backend to not change rhg benchmarks and until
 # we have a clearer view of which backend is the fastest.