Clean up cache after new rustc version

Copied from crates.io repo

Author: jtgeibel

.travis.yml

@@ -9,6 +9,7 @@ rust:
   - nightly
 
 before_install:
+  - script/ci/cargo-clean-on-new-rustc-version.sh
   - rustup component add rustfmt
   - rustup component add clippy
 

script/ci/cargo-clean-on-new-rustc-version.sh

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+set -e
+
+stamp_file=target/rustc_version_stamp
+current_version=$(rustc --version)
+
+if [ -f $stamp_file ]; then
+    # Compare the current version against the previous version
+    if echo "$current_version" | cmp -s $stamp_file -; then
+        echo "Version of rustc hasn't changed, keeping the cache intact"
+    else
+        echo "The version of rustc has changed, running cargo clean"
+        cargo clean
+    fi
+else
+    echo "There is no existing version stamp, keeping the cache intact"
+fi
+
+# Save the version stamp for next time
+mkdir -p target/
+echo $current_version > $stamp_file