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.

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