diff --git a/config/makefiles/rust.mk b/config/makefiles/rust.mk
--- a/config/makefiles/rust.mk
+++ b/config/makefiles/rust.mk
@@ -56,17 +56,21 @@ endif
 # These flags are passed via `cargo rustc` and only apply to the final rustc
 # invocation (i.e., only the top-level crate, not its dependencies).
 cargo_rustc_flags = $(CARGO_RUSTCFLAGS)
 ifndef DEVELOPER_OPTIONS
 ifndef MOZ_DEBUG_RUST
 # Enable link-time optimization for release builds, but not when linking
 # gkrust_gtest.
 ifeq (,$(findstring gkrust_gtest,$(RUST_LIBRARY_FILE)))
+# Pass -Clto for older versions of rust, and CARGO_PROFILE_RELEASE_LTO=true
+# for newer ones that support it. Combining the latter with -Clto works, so
+# set both everywhere.
 cargo_rustc_flags += -Clto
+export CARGO_PROFILE_RELEASE_LTO=true
 endif
 endif
 endif

 ifdef CARGO_INCREMENTAL
 export CARGO_INCREMENTAL
 endif