Install rustfmt as a separate Travis install step.

- Add a check-rustfmt.sh script which checks if the right version of
  rustfmt is installed.
- Run check-rustfmt.sh --install as an install step under travis_wait.

This is to work around the issue where cargo takes forever to build
rustfmt, causing Travis to terminate the build because it hasn't
produced any output for 10 minutes.

.travis.yml

@@ -10,6 +10,7 @@ addons:
             - python3-pip
 install:
     - pip3 install --user --upgrade mypy flake8
+    - travis_wait ./check-rustfmt.sh --install
 script: ./test-all.sh
 cache:
     cargo: true

check-rustfmt.sh

@@ -0,0 +1,35 @@
+#!/bin/bash
+#
+# Usage: check-rustfmt.sh [--install]
+#
+# Check that the desired version of rustfmt is installed.
+#
+# Rustfmt is still immature enough that its formatting decisions can change
+# between versions. This makes it difficult to enforce a certain style in a
+# test script since not all developers will upgrade rustfmt at the same time.
+# To work around this, we only verify formatting when a specific version of
+# rustfmt is installed.
+#
+# Exits 0 if the right version of rustfmt is installed, 1 otherwise.
+#
+# With the --install option, also tries to install the right version.
+
+# This version should always be bumped to the newest version available.
+VERS="0.8.3"
+
+if cargo install --list | grep -q "^rustfmt v$VERS"; then
+    exit 0
+fi
+
+if [ "$1" != "--install" ]; then
+    echo "********************************************************************"
+    echo "*  Please install rustfmt v$VERS to verify formatting.             *"
+    echo "*  If a newer version of rustfmt is available, update this script. *"
+    echo "********************************************************************"
+    echo "$0 --install"
+    sleep 1
+    exit 1
+fi
+
+echo "Installing rustfmt v$VERS."
+cargo install --force --vers="$VERS" rustfmt

cranelift/test-all.sh

@@ -22,28 +22,9 @@ function banner() {
 }
 
 # Run rustfmt if we have it.
-#
-# Rustfmt is still immature enough that its formatting decisions can change
-# between versions. This makes it difficult to enforce a certain style in a
-# test script since not all developers will upgrade rustfmt at the same time.
-# To work around this, we only verify formatting when a specific version of
-# rustfmt is installed.
-#
-# This version should always be bumped to the newest version available.
-RUSTFMT_VERSION="0.8.3"
-
-if cargo install --list | grep -q "^rustfmt v$RUSTFMT_VERSION"; then
+if $topdir/check-rustfmt.sh; then
     banner "Rust formatting"
     $topdir/format-all.sh --write-mode=diff
-elif [ -n "$TRAVIS" ]; then
-    # We're running under Travis CI.
-    # Install rustfmt, it will be cached for the next build.
-    echo "Installing rustfmt v$RUSTFMT_VERSION."
-    cargo install --force --vers="$RUSTFMT_VERSION" rustfmt
-    $topdir/format-all.sh --write-mode=diff
-else
-    echo "Please install rustfmt v$RUSTFMT_VERSION to verify formatting."
-    echo "If a newer version of rustfmt is available, update this script."
 fi
 
 # Check if any Python files have changed since we last checked them.