diff --git a/Cargo.toml b/Cargo.toml
index 9deb2ac564e9a..107ee46ea3909 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -135,7 +135,7 @@ cargo-args = ["-Zbuild-std=core"]
 rustc-std-workspace-core = { version = "1.0.0", optional = true }
 
 [features]
-default = ["std"]
+default = ["const-extern-fn", "std"]
 std = []
 rustc-dep-of-std = ["rustc-std-workspace-core"]
 extra_traits = []
diff --git a/ci/run.sh b/ci/run.sh
index 22b356a6425b1..5ac4070d928aa 100755
--- a/ci/run.sh
+++ b/ci/run.sh
@@ -9,6 +9,8 @@ mirrors_url="https://ci-mirrors.rust-lang.org/libc"
 
 target="$1"
 
+export RUST_BACKTRACE="${RUST_BACKTRACE:-1}"
+
 # If we're going to run tests inside of a qemu image, then we don't need any of
 # the scripts below. Instead, download the image, prepare a filesystem which has
 # the current state of this repository, and then run the image.
@@ -78,6 +80,20 @@ if [ -n "${QEMU:-}" ]; then
     exec grep -E "^(PASSED)|(test result: ok)" "${CARGO_TARGET_DIR}/out.log"
 fi
 
+cmd="cargo test --target $target ${LIBC_CI_ZBUILD_STD+"-Zbuild-std"}"
+
+# Run tests in the `libc` crate
+case "$target" in
+    # FIXME(android): unit tests fail to start on Android
+    # FIXME(s390x): unit tests fail to locate glibc
+    *android*) ;;
+    *s390x*) ;;
+    *) $cmd
+esac
+
+# Everything else is in `libc-test`
+cmd="$cmd --manifest-path libc-test/Cargo.toml"
+
 if [ "$target" = "s390x-unknown-linux-gnu" ]; then
     # FIXME: s390x-unknown-linux-gnu often fails to test due to timeout,
     # so we retry this N times.
@@ -86,31 +102,17 @@ if [ "$target" = "s390x-unknown-linux-gnu" ]; then
     passed=0
     until [ $n -ge $N ]; do
         if [ "$passed" = "0" ]; then
-            if cargo test \
-                --no-default-features \
-                --manifest-path libc-test/Cargo.toml \
-                --target "$target" \
-                ${LIBC_CI_ZBUILD_STD+"-Zbuild-std"}
-             then
+            if $cmd --no-default-features; then
                 passed=$((passed+1))
                 continue
             fi
         elif [ "$passed" = "1" ]; then
-            if cargo test \
-                --manifest-path libc-test/Cargo.toml \
-                --target "$target" \
-                ${LIBC_CI_ZBUILD_STD+"-Zbuild-std"}
-            then
+            if $cmd; then
                 passed=$((passed+1))
                 continue
             fi
         elif [ "$passed" = "2" ]; then
-            if cargo test \
-                --features extra_traits \
-                --manifest-path libc-test/Cargo.toml \
-                --target "$target" \
-                ${LIBC_CI_ZBUILD_STD+"-Zbuild-std"}
-            then
+            if $cmd --features extra_traits; then
                 break
             fi
         fi
@@ -118,20 +120,7 @@ if [ "$target" = "s390x-unknown-linux-gnu" ]; then
         sleep 1
     done
 else
-    cargo test \
-        --no-default-features \
-        --manifest-path libc-test/Cargo.toml \
-        --target "$target" \
-        ${LIBC_CI_ZBUILD_STD+"-Zbuild-std"}
-
-    cargo test \
-        --manifest-path libc-test/Cargo.toml \
-        --target "$target" \
-        ${LIBC_CI_ZBUILD_STD+"-Zbuild-std"}
-
-    RUST_BACKTRACE=1 cargo test \
-        --features extra_traits \
-        --manifest-path libc-test/Cargo.toml \
-        --target "$target" \
-        ${LIBC_CI_ZBUILD_STD+"-Zbuild-std"}
+    $cmd --no-default-features
+    $cmd
+    $cmd --features extra_traits
 fi
diff --git a/tests/const_fn.rs b/tests/const_fn.rs
new file mode 100644
index 0000000000000..d9b41b8073c70
--- /dev/null
+++ b/tests/const_fn.rs
@@ -0,0 +1,3 @@
+#[cfg(target_os = "linux")]
+const _FOO: libc::c_uint = unsafe { libc::CMSG_SPACE(1) };
+//^ if CMSG_SPACE is not const, this will fail to compile