run cargo fmt -- --check on beta

library/compiler-builtins/libm/.travis.yml

@@ -15,6 +15,8 @@ matrix:
     - env: TARGET=powerpc64-unknown-linux-gnu
     - env: TARGET=powerpc64le-unknown-linux-gnu
     - env: TARGET=x86_64-unknown-linux-gnu
+    - env: TARGET=cargo-fmt
+      rust: beta
 
 before_install: set -e
 
@@ -22,8 +24,6 @@ install:
   - bash ci/install.sh
 
 script:
-  # code must be already formatted
-  - cargo fmt -- --check
   - bash ci/script.sh
 
 after_script: set +e

library/compiler-builtins/libm/ci/install.sh

@@ -1,12 +1,15 @@
 set -euxo pipefail
 
 main() {
+    if [ $TARGET = cargo-fmt ]; then
+        rustup component add rustfmt-preview
+        return
+    fi
+
     if ! hash cross >/dev/null 2>&1; then
         cargo install cross
     fi
 
-    rustup component add rustfmt-preview
-
     rustup target add x86_64-unknown-linux-musl
 
     if [ $TARGET != x86_64-unknown-linux-gnu ]; then

library/compiler-builtins/libm/ci/script.sh

@@ -1,6 +1,11 @@
 set -euxo pipefail
 
 main() {
+    if [ $TARGET = cargo-fmt ]; then
+        cargo fmt -- --check
+        return
+    fi
+
     # quick check
     cargo check