diff --git a/.cargo/config.toml b/.cargo/config.toml
index 1089e6cd..4c64d1c8 100644
--- a/.cargo/config.toml
+++ b/.cargo/config.toml
@@ -1,2 +1,2 @@
-[build]
-rustflags = ["--cfg=ruma_unstable_exhaustive_types"]
+[env]
+RUMA_UNSTABLE_EXHAUSTIVE_TYPES = "1"