diff --git a/compiler/rustc_abi/src/callconv.rs b/compiler/rustc_abi/src/callconv.rs
index 7ad7088b3089..bb959d2f2e8e 100644
--- a/compiler/rustc_abi/src/callconv.rs
+++ b/compiler/rustc_abi/src/callconv.rs
@@ -35,6 +35,7 @@ pub fn unit(self) -> Option<Reg> {
     /// Try to combine two `HomogeneousAggregate`s, e.g. from two fields in
     /// the same `struct`. Only succeeds if only one of them has any data,
     /// or both units are identical.
+    #[cfg(feature = "nightly")]
     fn merge(self, other: HomogeneousAggregate) -> Result<HomogeneousAggregate, Heterogeneous> {
         match (self, other) {
             (x, HomogeneousAggregate::NoData) | (HomogeneousAggregate::NoData, x) => Ok(x),