force PR code to be formatted

.travis.yml

@@ -22,6 +22,8 @@ install:
   - bash ci/install.sh
 
 script:
+  # code must be already formatted
+  - cargo fmt -- --check
   - bash ci/script.sh
 
 after_script: set +e

ci/install.sh

@@ -5,6 +5,8 @@ main() {
         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

ci/script.sh

@@ -10,10 +10,8 @@ main() {
     # generate tests
     cargo run --package test-generator --target x86_64-unknown-linux-musl
 
-    if cargo fmt --version >/dev/null 2>&1; then
-        # nicer syntax error messages (if any)
-        cargo fmt
-    fi
+    # nicer syntax error messages (if any)
+    cargo fmt
 
     # run tests
     cross test --target $TARGET --release